Author Archives: sdavis

By David W. Price, Douglas G. Sutherland, Jay Rathert, John McCormack and Barry Saville

Author’s Note:The Process Watch series explores key concepts about process control—defect inspection, metrology and data analytics—for the semiconductor industry. This article is the third in a series on process control strategies for automotive semiconductor devices. For this article, we are pleased to include insights from our colleagues at KLA-Tencor, John McCormack and Barry Saville. 

Semiconductors continue to grow in importance in the automotive supply chain, requiring IC manufacturers to adapt their processes to produce chips that meet automotive quality standards. The first article in this series1 focused on the fact that the same types of IC manufacturing defects that cause yield loss also cause poor chip reliability and can lead to premature failures in the field. To achieve the high reliability required in automotive ICs, additional effort must be taken to ensure that sources of defects are eliminated in the manufacturing process. The second article in this series2 outlined strategies, such as frequent tool monitoring and a continuous improvement program, that reduce the number of defects added at each step in the IC manufacturing process. This article explores how to drive tool monitoring to a higher level of performance in order to help automotive IC manufacturers achieve chip failure rates below the parts per billion level.

As a reminder, tool monitoring is the established best practice for isolating the source of random defectivity contributed by the fab’s process tools. During tool monitoring, a bare wafer is inspected to establish its baseline defectivity, run through a specific process tool (or chamber), and then inspected again. Any defects that were added to the wafer must have come from that specific process tool. This method can reveal the cleanest “golden” tools in the fab, as well as the “dog” tools that contribute the most defects and require corrective action. With plots of historical defect data from the process tools, goals and milestones for continuous improvement can be implemented.

When semiconductor fabs design their tool monitoring strategy, they must decide on the minimum size of defects that they want to detect and monitor. If historical test results have shown that smaller defects do not impact yield, then fabs will run their inspection tools at a lower sensitivity so that they no longer detect these smaller defects. By doing this, they can focus only on the larger yield-killer defects, avoiding distraction from the smaller “nuisance” defects. This approach works for a consumer fab that is only trying to optimize yield, but what about the automotive fab? Recall that yield and reliability issues are caused by the same defects types – yield and reliability defects differ only in their size and/or where they land on the device pattern.2 Therefore, a tool monitoring strategy that leaves the fab blind to smaller defects may be missing the very defects that will be responsible for future reliability issues.

Moreover, it’s important to understand that defects that seem small and inconsequential at one process layer may have a dramatic impact later in the process flow – their impact can be exacerbated by the subsequent process steps. The two SEM images in figure 1 were taken at exactly the same location on the same wafer, but at different steps of the manufacturing process. The image on the left shows a single, small defect that was found on the wafer after a deposition layer. This defect was previously thought to be a nuisance defect with no negative effect on the die pattern or chip performance. The image on the right shows that same deposition defect after metal 1 pattern formation. The presumed nuisance defect has altered the quality of the metal line printed several process steps later. This chip might pass electrical wafer sort, but this type of metal deformity could easily become a reliability issue in the field when activated by automotive environmental stressors.

Figure 1. The left image shows small particle created at a deposition layer. The right image shows the exact same location on the wafer after the metal 1 pattern formation. The metal line defect was caused by the small particle at the prior deposition layer. This type of deformity in the metal line could easily become a reliability issue in the field.

So how does an automotive IC fab determine the smallest defect size that will pose a reliability risk? To start, it is important to understand the impact of different defect sizes on reliability. Consider, for example, the different magnitudes of a line open defect shown in figure 2. A chip that has a pattern structure with a full line open will likely fail at electrical wafer sort and thus does not pose any reliability risk. A chip with a 50% line open – a line that is pinched or otherwise restricted to ~50% of its cross-sectional area – will likely pass electrical wafer sort but poses a significant reliability risk in the field. If this chip is used in a car, environmental conditions such as heat, humidity and vibrations, can cause degradation of this defect to a full line open, resulting in chip failure.

Figure 2. The image on the left shows a full line open, while the right image shows a ~50% line open. The chip on the left will fail at sort (assuming there is no redundancy). The chip on the right may pass electrical wafer sort but is a reliability risk in the field.

As a next step, it is important to understand how different size defects affect a chip’s pattern integrity. More specifically, what is the smallest defect that will result in a line open? What is the smallest defect that will result in a 50% line open?

Figure 3 shows the results of a Monte Carlo simulation that models the impact of different size defects introduced at a BEOL film deposition step. Minimum defect size is plotted on the vertical axis against varying metal layer pitch dimensions. This data corresponds to the metal 1 spacing for the 7nm, 10nm, 14nm and 28nm design nodes, respectively.

The green data points correspond to the smallest defects that will cause a full line open and the orange data points correspond to the smallest defects that will produce a 50% line open (i.e., a potential reliability failure). In each case the smallest defect that will cause a potential reliability failure is 50-75% of the smallest defect that will cause a full line open.

Figure 3. The green data points show the minimum defect size required to cause a full line open at the minimum metal pitch. The orange data points show the minimum defect size needed to cause a 50% line open. The x-axis is the metal 1 spacing for the 7nm (far left data point), 10nm, 14nm and 28nm (far right data point) design nodes.

These modeling results imply that to control for, and reduce, the number of reliability defects present in the process, fabs need to capture smaller defects. Therefore, they require higher sensitivity inspections than what is required for yield optimization. In general, detection of reliability defects requires an inspection sensitivity that is one node ahead of the current design node plan for yield alone. Simply put, a fab’s previous standards for reducing defectivity to optimize yield will not be sufficient to optimize reliability.

Increasing the sensitivities of the tool monitoring inspection recipes, or in some cases, using a more capable inspection system, will find smaller defects and possibly reveal previously hidden signatures of defectivity, as in Figure 4 below. While these signatures may have had a tolerable impact on yield in a consumer fab, they represent an unacceptable risk to reliability for automotive fabs pursuing continuous improvement and Zero Defect standards.

Figure 4: Hidden defect signatures that may impact reliability are often revealed with appropriate tool monitoring sensitivity. Zero Defect standards require corrective action on the process tool contributing these defects.

There are several important unpatterned wafer defect inspection factors for a fab to consider when creating a strategy to improve tool monitoring inspection sensitivity to find the small, reliability-related defects contributed by process tools. First, it is important to recognize that in a mature fab where yields are already high, there is rarely a single process layer or module that will be the “silver bullet” to reducing defectivity adequately to meet reliability improvement goals. Rather, it is sum of small gains across many layers that produce the desired gains in reliability. Because yield and the associated reliability improvements are cumulative across layers, reliability gains achieved through process tool monitoring using unpatterned wafer inspection are best demonstrated using a multi-layer regression model:

Yield = f(Ys)+f(SFS1)+f(SFS2)+ f(SFS3)+ ….. f(SFSN) + error

  • Ys = systematic yield loss (not particles related)
  • SFSx = cumulative Sursfcan unpatterned wafer inspection detected particles for many layers
  • Error = Yield loss mechanisms not detected by Surfscan

This implies that reliability improvements require a fab’s commitment to continuous improvement in defectivity levels across all processes and process modules.

Second, the fab should consider the quality of the bare wafer used for process tool monitoring. Recycling bare wafers increases the surface roughness with each cycle, an attribute known as haze. This haze level is fundamentally noise that affects the inspection system’s ability to differentiate the signal of smaller defects. Variability in haze across the population of test wafers acts as a limit to overall inspection recipe capability, requiring normalization, calibration and haze limits to reduce the impact of this noise source on defect sensitivity.

Next, the fab should ensure that the monitor step closely mimics the process that a production, patterned wafer follows. Small time-saving deviations in the monitor wafer flow to short cut the process may inadvertently skip the causal mechanism of defectivity. Furthermore, an over-reliance on mechanical handling checks alone bypasses the process completely and misses the critical contribution the process plays in particle generation.

When increasing the inspection recipe sensitivity, the fab must co-optimize both the “pre” and “post” inspection together. Often cycling the bare wafer through a process step can “decorate” small pre-existing defects on the wafer that were initially below the detection threshold. Once decorated, the defects now appear bigger and are more easily detected. In an unoptimized “post” inspection, these decorated defects can look like “adders,” leading to a false alarm and inadvertent process tool down time. Optimizing the inspections together maximizes the sensitivity and increases the confidence in the excursion alarms while avoiding time-consuming false alarms.

Lastly, it is important to review and classify the defects found during unpatterned inspection to correlate their relevance to the defects found at the equivalent patterned wafer process step. Only then can the fab be confident that the source of the defects has been isolated and appropriate corrective action has been taken.

To meet the high reliability demands of the automotive industry, IC manufacturers will need to go beyond simply monitoring and controlling the number of yield limiting defects on the wafer. They will need to improve the sensitivity of their tool monitoring inspections to one node smaller than what would historically be considered relevant. Only with this extra sensitivity can they detect and eliminate defects that would otherwise escape the fab and cause premature reliability failures. Additionally, when implementing a tool monitoring strategy, fabs need to carefully consider multiple factors, such as monitor wafer recycling, pre and post inspection sensitivity and the importance of a fab-wide continuous improvement program. With so much riding on automotive semiconductor reliability, increased sensitivity to smaller defects is an essential part of an optimal Zero Defect continuous improvement program.

About the Authors:

Dr. David W. Price and Jay Rathert are Senior Directors at KLA-Tencor Corp. Dr. Douglas Sutherland is a Principal Scientist at KLA-Tencor Corp. Over the last 15 years, they have worked directly with over 50 semiconductor IC manufacturers to help them optimize their overall process control strategy for a variety of specific markets, including implementation of strategies for automotive reliability, legacy fab cost and risk optimization, and advanced design rule time-to-market. The Process Watch series of articles attempts to summarize some of the universal lessons they have observed through these engagements.

John McCormack is a Senior Director at KLA-Tencor. Barry Saville is Consulting Engineer at KLA-Tencor. John and Barry both have over 25 years of experience in yield improvement and defectivity reduction, working with many IC manufacturers around the world.

References:

  1. Price, Sutherland and Rathert, “Process Watch: The (Automotive) Problem With Semiconductors,” Solid State Technology, January 2018.
  2. Price, Sutherland and Rathert, “Process Watch: Baseline Yield Predicts Baseline Reliability,” Solid State Technology, March 2018.

A lithographic method for TSV alignment to embedded targets was evaluated using in-line stepper self metrology, with TIS correction.

BY WARREN W. FLACK, Veeco Instruments, Plainview, NY and JOHN SLABBEKOORN, imec, Leuven, Belgium

Demand for consumer product related devices including backside illuminated image sensors, interposers and 3D memory is driving advanced packaging using through silicon via (TSV) [1]. The various process flows for TSV processing (via first, via middle and via last) affect the relative levels of integration required at the foundry and OSAT manufacturing locations. Via last provides distinct advantages for process integration, including minimizing the impact on back end of line (BEOL) processing, and does not require a TSV reveal for the wafer thinning process. Scaling the diameter of the TSV significantly improves the system performance and cost. Current via last diameters are approximately 30μm with advanced TSV designs at 5 μm [2].

Lithography is one of the critical factors affecting overall device performance and yield for via last TSV fabrication [2]. One of the unique lithography requirements for via last patterning is the need for back-to-front side wafer alignment. With smaller TSV diameters, the back-to- front overlay becomes a critical parameter because via landing pads on the first level metal must be large enough to include both TSV critical dimension (CD) and overlay variations, as shown in FIGURE 1. Reducing the size of via landing pads provide significant advantages for device design and final chip size. This study evaluates 5μm TSVs with overlay performance of ≤ 750nm.

Alignment, illumination and metrology

Lithography was performed using an advanced packaging 1X stepper with a 0.16 numerical aperture (NA) Wynne Dyson lens. This stepper has a dual side alignment (DSA) system which uses infrared (IR) illumination to view metal targets through a thinned silicon wafer [3]. For the purposes of this study and its results, the wafer device side is referred to as the “front side” and the silicon side is referred to as the “back side.” The side facing up on the lithography tool is the back side of the TSV wafer, as shown in FIGURE 2.

The top IR illumination method for viewing embedded alignment targets, shown in Fig. 2, provides practical advantages for integration with stepper lithography. Since the illumination and imaging are directed from the top, this method does not interfere with the design of the wafer chuck, and does not constrain alignment target positioning on the wafer. The top IR alignment method illuminates the alignment target from the back side using an IR wavelength capable of transmitting through silicon (shown as light green in FIGURE 2) and the process films (shown in blue). In this configuration the target (shown in orange) needs to be made from an IR reflective material such as metal for optimal contrast. The alignment sequence requires that the wafer move in the Z axis in order to shift alignment focus from the wafer surface to the embedded target.

Back-to-front side registration was measured using a metrology package on the lithography tool which uses the DSA alignment system. This stepper self metrology package (DSA-SSM) includes routines to diagnose and compensate for measurement error from having features at different heights. For each measurement site the optical metrology system needs to move the focus in Z between the resist feature and the embedded feature. Therefore angular differences between the Z axis of motion, the optical axis of the alignment camera, and the wafer normal will contribute to measurement error for the tool [3]. The quality of the wafer stage motion is also very important because a significant pitch and roll signature would result in a location dependent error for embedded feature measurement, which would complicate the analysis.

If the measurement operation is repeatable and consistent across the wafer, then a constant error coming from the measurement tool, commonly referred to as tool induced shift (TIS), can be characterized using the method of TIS calibration, which incorporates measurements at 0 and 180 degree orientations. The TIS error—or calibration—is calculated by dividing the sum of offsets for the two orientations by two [4]. While the TIS calibration is effective for many types of measurements for planar metrology, for embedded feature metrology, the quality of measurement and calibration also depend on the quality and repeatability of wafer positioning, including tilt. In previous studies, the registration data obtained from the current method were self consistent and proved to be an effective inspection method [3, 5]. However given the dependencies affecting TIS calibration for embedded feature metrology, it is desirable to confirm the registration result using an alternate metrology method [5]. In order to independently verify the DSA-SSM, overlay data dedicated electrical structures were designed and placed on the test chip.

Electrical verification of TSV alignment is performed after complete processing of the test chip and relies on the landing position of a TSV on a fork-to-fork test structure in the embedded metal 1 (damascene metal). When the TSV processing is complete the copper filled TSV will make contact with metal 1. The TSV creates a short between the two sets of metal forks, allowing measurement of two resistance values which can be translated into edge measurements. For the case of ideal TSV alignment, the two resistances are equal. The measurement resolution of the electrical structure is limited by the pitch of the fork branches. In this study resolution is enhanced by creating structures with four different fork pitches. A similar fork-to-fork structure rotated 90 degrees is used for the Y alignment. Using this approach both overlay error and size of the TSV in both X and Y can be electrically determined [6].

Experimental methods

This study scrutinizes image placement performance by examining DSA optical metrology repeatability after TSV lithography, and then comparing this optical registration data with final electrical registration data.

The TSV-last process begins with a 300mm device wafer with metal 1, temporarily bonded to a carrier for mechanical support as shown in FIGURE 3. The back side of the silicon device wafer (light green) is thinned by grinding and then polished smooth by chemical mechanical planarization (CMP). The TSV is imaged in photoresist (red) and etched through the thinned silicon layer. FIGURE 3 depicts the complete process flow including the TSV, STI and PMD etch, TSV fill, redis- tribution layer (RDL) and de-bonding from carrier. The aligned TSV structure must land completely on the metal 1 pad (dark blue).

TSV lithography is done with a stepper equipped with DSA. The photoresist is a gh-line novolac based positive- tone material requiring 1250mJ/cm2 exposure dose with a thickness of 7.5μm [5]. The TSV diameter is 5μm, and the silicon thickness is 50μm. TSV etching of the silicon is performed by Bosch etching [7]. Tight control of lithography and TSV etching is required to insure that vias land completely on metal 1 pads, as shown in FIGURE 1.

Acceptable features for DSA-SSM metrology need to fit the via process requirements for integration. Since the TSV etch process is very sensitive to pattern size and density, the TSV layer is restricted to one size of via, and the DSA-SSM measurement structure is constructed using this shape. The design of the DSA-SSM measurement structure uses a cluster of 5μm vias with unique grouping and clocked rotation to avoid confusion with adjacent TSV device patterns during alignment.

FIGURE 4 shows two different focus offsets of DSA camera images of the overlay structure. For this structure, the reference metal 1 feature (outlined by the blue ring) and the resist pattern feature (outlined by the red ring) are not in the same focal plane. For a silicon thickness of 50μm, focusing on one feature will render the other feature out of focus, requiring each feature to have its own focus offset, which is specified in the metrology measurement recipe.

Optical registration process control

This study leveraged a sampling plan of 23 lithography fields with 5 measurements per field, resulting in a total of 115 measurements per wafer. Since the full wafer layout contains 262 fields, this sampling plan provides a good statistical sample for monitoring linear grid and intrafield parameters.

In the initial run, the overlay settings were optimized using the DSA-SSM metrology feedback and then the parameters were fixed to investigate overlay stability over a nine-week period. Trend charts for mean and 3σ for seven TSV lots are shown in FIGURE 5. Each measurement lot consists of 8 wafers, with 115 measure- ments per wafer, and all data is corrected for TIS on a per lot basis using measurements of a single wafer at 0 and 180 degree orientations [3]. The lot 3σ is consistently less than 600nm over the nine-week period. There appears to be a consistent small Y mean error (blue diamond) that could be adjusted to improve subsequent overlay results. With a Y mean correction applied, the registration data shows mean plus 3σ ≤ 600nm.

Validating TSV alignment and in-line optical metrology

Two TSV last test chip wafers were completely processed to the stage that they can be electrically measured. TABLE 1 shows the registration numbers confirming a good match between the two metrology methods. It is important to note that an extra translation step is performed between the optical and the electrical measurement: the TSV etch.

In this analysis the TSV etch is assumed to be perfectly vertical. From the data we can conclude that the TSV etch is indeed vertical enough not to interfere with the overlay data. Otherwise this would show as translation or scaling effects between the two metrology methods.

Conclusions

The lithographic method for TSV alignment to embedded targets was evaluated using in-line stepper self metrology, with TIS correction. Registration data was collected over a nine-week period to characterize the stability of TSV alignment. With corrections applied, the registration data demonstrates mean plus 3σ ≤ 600nm. The in-line optical registration data was then correlated to detailed electrical measurements performed on the same wafers at the end of the process to provide independent assessment of the accuracy of the optical data. Good correlation between optical and electrical data confirms the accuracy of the in-line optical metrology method, and also confirms that the TSV etch through 50μm thick silicon is vertical.

References

1. Vardaman, J. et. al., TechSearch International: Advanced Packaging Update, July 2016.
2. Van Huylenbroeck, S. et. al., “Small Pitch High Aspect Ratio Via Last TSV Module”, The 66th Electronic Components and Technology Conference, Los Vegas, NV, May 2016.
3. Flack, W. et. al., “Optimization of Through Si Via Last Lithography for 3D Packaging”, Twelfth International Wafer- Level Packaging Conference, San Jose, CA, October 2015.
4. Preil, M. et. al, “Improving the Accuracy of Overlay Measurements through Reduction of Tool and Wafer Induced Shifts”, Metrology, Inspection, and Process Control for Microlithography Proceedings, SPIE 3050, 1997.
5. Flack, W. et. al., “Verification of Back-to-Front Side Alignment for Advanced Packaging”, Ninth Interna- tional Wafer-Level Packaging Conference, Santa Clara, CA, November. 2012.
6. Flack, W. et.al., “Overlay Performance of Through Si Via Last Lithography for 3D Packaging”, 18th Electronics Packaging Technology Conference, Singapore, December 2016
7. Slabbekoorn, J. et. al, “Bosch Process Characterization For Donut TSV’s” Eleventh International Wafer-Level Packaging Conference, Santa Clara, CA, November 2014.

Layout schema generation generates random, realistic, DRC-clean layout patterns of the new design technology for use in test vehicles.

BY WAEL ELMANHAWY and JOE KWAN, Mentor Graphics, Beaverton, OR

Predicting and improving yield in the early stages of technology development is one of the main reasons we create test macros on test masks. Identifying potential manufacturing failures during the early technology development phase lets design teams implement upstream corrective actions and/or process changes that reduce the time it takes to achieve the desired manufacturing yield in production. However, while conventional yield ramp techniques for a new technology node rely on using designs from previous technology nodes as a starting point to identify patterns for design of experiment (DoE) creation, what do you do in the case of a new design technology, such as multi- patterning, that did not exist in previous nodes? The human designer’s experience isn’t applicable, since there isn’t any knowledge about similar issues from previous designs. Neither is there any prior test data from which designers can draw feedback to create new test structures, or identify process or design style optimizations that can improve yield more quickly.

An innovative new technology, layout schema gener- ation (LSG), enables design teams to generate additional macros to add to test structures without relying on past designs for input. These macros are based on the gener- ation and random placement of unit patterns that can construct more meaningful larger patterns. Specifications governing the relationships between those unit patterns can be adjusted to generate layout clips that look like realistic designs. Those layout clips can then be used in design of experiment (DoE) trials to predict yield, and identify potential design and process optimizations that will help improve yield. By using this new LSG process, designers can significantly reduce the time it takes to achieve the desired yield for designs that include new design techniques.

Issues affecting yield

Wafer yield is typically reduced by three categories of defects. The first category comprises random defects, which occur due to the existence of contamination particles in the different process chambers. A conducting particle can short out two or more neighboring wires, or create a leakage path. A non-conducting particle or a void can open up a wire or a via, or create high resistive paths. FIGURE 1 shows scanning electron microscope (SEM) images of these two types of random defects.

The second category contains systematic defects, which occur due to an imperfect physical layout architecture, or the impact of non-optimized optical process recipes and/ or equipment. Systematic defects are typically the biggest source of yield detraction [1], but a majority of them can be eliminated through design-technology co-optimization (DTCO), in which the design and process sides commu- nicate more freely to achieve faster rates of improvement.

The third category, which we’re not addressing in this article, includes parametric defects (such as a lack of uniformity in the doping process) that may affect the reliability of devices.

Layout schema generation

To demonstrate the use and applicability of the LSG process, let’s look at designs that use the self-aligned multi-patterning (SAMP) process. Multi-patterning (MP) technology with ArF 193i lithography is currently the preferred choice over extreme ultraviolet (EUV) lithography for advanced technology nodes from 20nm on down. At 7 nm and 5 nm nodes, the SAMP process appears to be one of the most effective MP techniques in terms of achieving a small pitch of printed lines on the wafer, but its yield is in question. Of course, before being deployed in production, it must be thoroughly tested on test vehicles. However, without any previous SAMP designs, design of an appropriate test vehicle is challenging. In addition to the lack of historical test data, the unidirectional nature of the SAMP design complicates the design of the conven- tional serpentine and comb test shapes, which contain bidirectional components.

Self-aligned multi-patterning process

In the SAMP process [3], the first mask is known as the mandrel mask. Sacrificial mandrel shapes are printed with a relaxed pitch, and then used to develop sidewalls. The sidewalls are at half the mandrel’s pitch. Depending on the tone, target shapes may exist in the spaces between the sidewalls. The target shapes can be reused as sacrificial mandrel shapes to form another generation of sidewalls. Wafer shapes that don’t have corresponding mask shapes are called non-mandrel shapes. This process can be repeated to achieve SAMP layouts with a reduced pitch. The SAMP process (FIGURE 2) restricts the designs to be almost unidirectional. Generated parallel lines will be cut later by a cut mask at the desired line ends to form the correct connectivity.

Test vehicles

A test vehicle is typically a subset of the masks for a design, designed specifically to induce potential systematic failures or lithographic hotspots on the layer under test. It may also contain some test structures specially designed for the detection of random defects. The main compo- nents in a test vehicle for any new node are serpentine and comb shapes (to capture random defects), and preliminary standard cell designs (with many variations, to assess their quality). Other structures are typically added based on experience derived from production chips of previous nodes.

In a new node, all test structures on the test vehicle are vital for process training and characterization. Feedback from the test process is used for design style optimization. For example, when “bad” layout geometries are discovered after manufacturing, they can be captured as patterns, assigned low scores, and stored in a design for manufacturing (DFM) pattern library [2]. The designer can then use DFM analysis to find the worst patterns in a given layout, and modify or eliminate them. Such early DTCO provides a faster yield ramp for new nodes. Even in mature nodes, test structures are used on production wafers to identify additional opportunities for process refinement and optimization, which will have a positive impact on future yield.

One of the obstacles in test vehicle design is that it depends mainly on human designer’s experience and memory. Although experienced designers have seen multiple design styles in older nodes, the design shapes they are familiar with are limited to those styles. It typically takes a long time to design new test structures that cover new shapes, especially for a new process. The LSG solution adds more macros (generated in a random fashion) to the standard test structures strategy to speed up new shape yield analysis.

Random test pattern generation

The key component of the LSG solution is a method for the random generation of realistic design-like layouts, without design rule violations. The LSG process uses a Monte Carlo method to apply randomness in the generation of layout clips by inserting basic unit patterns in a grid. These unit patterns represent simple rectangular and square polygons, as well as a unit pattern for inserting spaces in the design. Unit pattern sizes depend on the technology pitch value. During the generation of the layouts, known design rules are applied as constraints for unit pattern insertion. Once the rules are configured, an arbitrary size of layout clips can be generated (FIGURE 3).

To begin, the SAMP design rules are converted to a format readable by an automated LSG tool like the Calibre® LSG tool from Mentor, a Siemens Business. Once the rules are configured, the Calibre LSG process can automatically generate an arbitrarily wide area of realistic DRC-clean SAMP patterns. The area is only limited by the floorplan of the designated macro of SAMP test structures. Test patterns can be also generated with power rails to mimic the layouts of standard cells. FIGURE 4 shows a sample clip of the generated output layout. To be ready for the experiments, the SAMP design is decomposed into the appropriate mandrel and cut masks, according to the decomposition rules. This operation also distinguishes between mandrel and non-mandrel shapes.

Design of Experiment

In the design phase of the test vehicle, the generated SAMP patterns are added to the typical contents of regular test patterns. The random SAMP patterns are electrically meaningless, unless they are connected to other layers to set up the required experiment. The DoE determines the way the connections are made from the patterns up to the testing pads, to detect different fail modes. Fail modes include short circuits due to lithographic bridging or conducting particles, and open circuits due to lithographic pinching, non-conducting particles, voids, or open vias.

A via chain can be constructed to connect the random DoE of SAMP structures through a routing layer to external pads for electrical measurement. These clips are decomposed according to the decomposition rules of the technology into the appropriate mandrel and cut masks. The decomposed clips can be tested through simulations, or electrically on silicon to discover hotspots. The discovered hotspots can be analyzed to determine root cause, which can be used to modify design layouts and/or optimize the fabrication process and models to eliminate these hotspots in future production. They can also be used as learning patterns for DFM rule deck devel- opment. By expanding the size of the randomly generated test structures, more hotspots can be detected, which can provide an even faster way to enhance the yield of a new technology node.

To demonstrate the effectiveness of the LSG process, we performed two experiments on a set of SAMP patterns similar to those shown in FIGURE 4.

Detecting random conducting particles

The first experiment collected data about random defects caused by conducting particles. In this experiment, all mandrel shapes are connected through the upper (or lower) via and metal layers, up to a testing pad. All non-mandrel shapes are connected in the same way to another testing pad. The upper routing layer forms two interdigitated comb shapes. FIGURE 5 shows a layout snippet of the connections. All via placements and upper metal routings were made with a custom script, without the intervention of a human designer. Ideally the two testing pads should be disconnected, as no mandrel shape can touch a non-mandrel shape. If the testing probes are found to be connected, this likely indicates a random conducting particle defect, or a lithographic bridge. The localization and analysis of such defects [4] can help with yield estimation and enhancement.

Detecting systematic cut mask resolution problems

One example of a systematic lithographic defect found in SAMP designs is when the cut mask is not resolved correctly. This causes two shapes on the same track to be shorted out through the unresolved cut shape. The testing of such a case requires connecting every other polygon on the same track. This was done with a generating script, without the intervention of human designers. FIGURE 6 shows a snippet of the generated layout with the connections. If the test probes are found to be connected while the two pads (ideally) are disconnected, this may indicate an unresolved cut shape. The analysis of the defect location and data from multiple wafers can prove the root cause of the defect.

Results

The two experiments described above were placed on a test vehicle of an advanced node. The test macro containing the first experiment setup successfully detected several conducting particle defects. A sample SEM image of the discovered defect is shown in FIGURE 7. Statistical data from multiple wafers were used to model the defect density and estimate the yield target.

Repetitive fail data from the test macro of the second experiment indicated systematic failures at particular locations. The analysis showed that the root cause of the failure was a poorly resolving cut shape in some process corners, as was predicted in the DoE. FIGURE 8 shows a snippet of the generated layout and its contour simulation.

To test the effectiveness of the random approach in capturing defects, 20 SAMP design clips were generated with linearly increasing sizes, such that the 20th clip was 20X bigger than the first clip. Lithography simulations were executed on the cut mask to inspect potential failures. The contours were checked, and potential failures were identified and categorized. FIGURE 9 shows the number of the unique hotspots found in each clip. The graph shows that the number of identified hotspots tends to saturate with the chip size. The second clip has 2X the number of unique hotspots found in the first clip, while the 20th clip only sees around a 6X increase. This result is expected, as many hotspots in the larger clips are just replicas of those found in the small clips. Assuming that the LSG tool is configured correctly, this result means most of the potential hotspots can be covered in a reasonable size test vehicle.

Conclusion

Test vehicles are vital for yield ramp up in new technologies and yield enhancement in mature nodes, but it can be difficult to design accurate test structures for new design styles and technologies that have no relevant history. Innovative techniques are needed to achieve comprehensive coverage of potential manufacturing failures created by new design styles, while ensuring full compliance with known design rule checks. A new solution using layout schema generation generates random, realistic, DRC-clean layout patterns of the new design technology for use in test vehicles. Experiments with this technology show it can provide high coverage of new design styles for an arbitrarily-wide design area. Circuitry can be added to the generated clips to make them electrically measurable for the detection of potential failures. The ability to discover lithographic hotspots and systematic failures early in the technology development process is significantly improved, at the expense of additional testing area. This design/technology co-optimization speeds up the yield optimization for new technology nodes, improving a critical success factor for market success.

References

1. Lee, J.H., Lee, J.W., Lee, N.I., Shen, X., Matsuhashi, H., Nehrer, W., “Proactive BEOL yield improvement methodology for a successful mobile product,” Proc. IEEE ISCDG, 93-95 (2012).
2. Park, J., Kim, N., Kang, J.-H., Paek, S.W., Kwon, S., Shafee, M., Madkour, K., Elmanhawy, W., Kwan, J., et al., “High coverage of litho hotspot detection by weak pattern scoring,” Proc. SPIE 9427, 942703 (2015)
3. Bencher, C., Chen, Y., Dai, H., Montgomery, W., Huli, L., “22nm half-pitch patterning by CVD spacer self alignment double patterning (SADP),” Proc. SPIE 6924, 69244E (2008)
4. Schmidt, M., Kang, H., Dworkin, L., Harris, K., Lee, S., “New methodology for ultra-fast detection and reduction of non-visual defects at the 90nm node and below using comprehensive e-test structure infrastructure and in-line DualBeamTM FIB,” IEEE/ SEMI ASMC, 12-16 (2006).

Source Photonics, a global provider of optical transceivers, today announced it recently closed more than $100M in equity to support its growing data center and 5G business.

The funding will be used to further increase the scale of Source Photonics’ operations, as LightCounting reported that the sales of optical components and modules to Cloud Companies grew by 63% in 2016 and 64% in 2017. The growth rate will average roughly 20% per year through 2023. Higher growth rates in 2020-2022 will be driven by first volume deployments of 400GbE. This is a result of the rise of 5G and the cloud.

Planned developments include the creation of a new laser fab, upgrades to existing production facilities and increased investment in the research and development of next-generation technologies, ensuring Source Photonics continues its position as a leading innovator.

“Exciting new applications such as the Internet of Things (IoT), Virtual Reality, and cloud services are growing in popularity every day,” said Doug Wright, CEO at Source Photonics. “These applications all depend on the next standard of connectivity, and 5G depends on the backing of a world-class optical network. We are extremely proud that our investors have shown this confidence in us and are confident that the investment will support our ongoing work to enable the next era of connectivity.”

Upgrades to Source Photonics’ fab in Taiwan have already been completed and production operations have begun for a new fab in Jintan, China, using the latest funding. The funding will also be used towards technology investments for advanced coating technologies to enable next-generation lasers and transceivers for the fast-growing 5G and data center markets.

Source Photonics’ latest range of cutting-edge technology will be exhibited at OFC 2019 at booth 4021. Products on display will include its new 400G-LR8 and DR4 QSFP-DD solutions, which are the latest addition to its PAM4-based optical transceivers portfolio. Other products which will be showcased at OFC, in San Diego, on March 4-7, 2019, include several QSFP28 solutions such as the 100G-DR/FR, 100G-SR4, 100G CWDM4, and 100G-LR4. The company will also demonstrate some of its solutions for the 5G market such as the 50G-ER QSFP28 and 25G LAN DWM SFP28.

Lumileds today announced the appointment of Dr. Jonathan Rich as Chief Executive Officer. Dr. Rich most recently served as Chairman and CEO of Berry Global, Inc., a Fortune 500 specialty materials and consumer packaging company, from 2010 to 2018. Dr. Rich succeeds Mark Adams, who is stepping down as CEO and from the board of directors but will remain in an advisory role to the company.

“I am very pleased to be joining Lumileds and am looking forward to building on the company’s differentiated lighting technology foundation to increase the value we can deliver to customers across a broad set of industries,” said Dr. Rich. “The opportunity for lighting innovation to make a positive impact on safety and sustainability is tremendous.”

Before Dr. Rich held the position of Chairman and CEO of Berry Global, he was president and CEO at Momentive, a specialty chemical company headquartered in Albany, New York. Prior to that, he held positions with Goodyear Tire & Rubber Company, first as President of the Global Chemicals business and subsequently as President of Goodyear’s North American Tire Division. Dr. Rich spent his formative years at General Electric, first as a research scientist at GE Global Research and then in a series of management positions with GE Plastics. He received a Bachelor of Science degree in chemistry from Iowa State University and a Ph.D. in chemistry from the University of Wisconsin-Madison. He has been a visiting lecturer at Cornell University Johnson School of Business since 2017.

“Mark Adams has made significant contributions to Lumileds during his tenure, leading the transition to an independent company and cultivating a culture of innovation and customer focus,” said Rob Seminara, a senior partner at Apollo and chairman of the board of Lumileds. “On behalf of the Board of Directors of Lumileds, we would like to thank him for his service to the company and wish him the very best in his future endeavors. We are very excited Jon will be joining Lumileds to drive the next phase of innovation and growth and we look forward to working with him again.”

Added Adams: “It has been a great experience leading Lumileds’ transition to an independent company that is focused on delivering lighting solutions that truly make a positive impact in the world. I would like to thank the employees of Lumileds and the Apollo team for their support and wish the company much success in the future.”

Materials that are hybrid constructions (combining organic and inorganic precursors) and quasi-two-dimensional (with malleable and highly compactable molecular structures) are on the rise in several technological applications, such as the fabrication of ever-smaller optoelectronic devices.

An article published in the journal Physical Review B describes a study in this field resulting from the doctoral research of Diana Meneses Gustin and Luís Cabral, both supervised by Victor Lopez Richard, a professor at the Federal University of São Carlos (UFSCar) in Brazil. Cabral was co-supervised by Juarez Lopes Ferreira da Silva, a professor at the University of São Paulo’s São Carlos Chemistry Institute (IQSC-USP). Gustin was supported by São Paulo Research Foundation – FAPESP via a doctoral scholarship and a scholarship for a research internship abroad.

“Gustin and Cabral explain theoretically the unique optical and transport properties resulting from interaction between a molybdenum disulfide monolayer [inorganic substance MoS2] and a substrate of azobenzene [organic substance C12H10N2],” Lopez Richard told.

Illumination makes the azobenzene molecule switch isomerization and transition from a stable trans spatial configuration to a metastable cis form, producing effects on the electron cloud in the molybdenum disulfide monolayer. These effects, which are reversible, had previously been investigated experimentally by Emanuela Margapoti in postdoctoral research conducted at UFSCar and supported by FAPESP.

Gustin and Cabral developed a model to emulate the process theoretically. “They performed ab initio simulations [computational simulations using only established science] and calculations based on density functional theory [a quantum mechanical method used to investigate the dynamics of many-body systems]. They also modeled the transport properties of the molybdenum disulfide monolayer when disturbed by variations in the azobenzene substrate,” Richard explained.

While the published paper does not address technological applications, the deployment of the effect to build a light-activated two-dimensional transistor is on the researchers’ horizon.

“The quasi two-dimensional structure makes molybdenum disulfide as attractive as graphene in terms of space reduction and malleability, but it has virtues that potentially make it even better. It’s a semiconductor with similar electrical conductivity properties to graphene’s and it’s more versatile optically because it emits light in the wavelength range from infrared to the visible region,” Richard said.

The hybrid molybdenum-disulfide-azobenzene structure is considered a highly promising material, but a great deal of research and development will be required if it is to be effectively deployed in useful devices.

Researchers at Tokyo Institute of Technology (Tokyo Tech) report a unipolar n-type transistor with a world-leading electron mobility performance of up to 7.16 cm2 V-1 s-1. This achievement heralds an exciting future for organic electronics, including the development of innovative flexible displays and wearable technologies.

Researchers worldwide are on the hunt for novel materials that can improve the performance of basic components required to develop organic electronics.

Now, a research team at Tokyo Tech’s Department of Materials Science and Engineering including Tsuyoshi Michinobu and Yang Wang report a way of increasing the electron mobility of semiconducting polymers, which have previously proven difficult to optimize. Their high-performance material achieves an electron mobility of 7.16 cm2 V-1 s-1, representing more than a 40 percent increase over previous comparable results.

In their study published in the Journal of the American Chemical Society, they focused on enhancing the performance of materials known as n-type semiconducting polymers. These n-type (negative) materials are electron dominant, in contrast to p-type (positive) materials that are hole dominant. “As negatively-charged radicals are intrinsically unstable compared to those that are positively charged, producing stable n-type semiconducting polymers has been a major challenge in organic electronics,” Michinobu explains.

The research therefore addresses both a fundamental challenge and a practical need. Wang notes that many organic solar cells, for example, are made from p-type semiconducting polymers and n-type fullerene derivatives. The drawback is that the latter are costly, difficult to synthesize and incompatible with flexible devices. “To overcome these disadvantages,” he says, “high-performance n-type semiconducting polymers are highly desired to advance research on all-polymer solar cells.”

The team’s method involved using a series of new poly(benzothiadiazole-naphthalenediimide) derivatives and fine-tuning the material’s backbone conformation. This was made possible by the introduction of vinylene bridges[1] capable of forming hydrogen bonds with neighboring fluorine and oxygen atoms. Introducing these vinylene bridges required a technical feat so as to optimize the reaction conditions.

Overall, the resultant material had an improved molecular packaging order and greater strength, which contributed to the increased electron mobility.

Using techniques such as grazing-incidence wide-angle X-ray scattering (GIWAXS), the researchers confirmed that they achieved an extremely short π-π stacking distance[2] of only 3.40 angstrom. “This value is among the shortest for high mobility organic semiconducting polymers,” says Michinobu.

There are several remaining challenges. “We need to further optimize the backbone structure,” he continues. “At the same time, side chain groups also play a significant role in determining the crystallinity and packing orientation of semiconducting polymers. We still have room for improvement.”

Wang points out that the lowest unoccupied molecular orbital (LUMO) levels were located at -3.8 to -3.9 eV for the reported polymers. “As deeper LUMO levels lead to faster and more stable electron transport, further designs that introduce sp2-N, fluorine and chlorine atoms, for example, could help achieve even deeper LUMO levels,” he says.

In future, the researchers will also aim to improve the air stability of n-channel transistors — a crucial issue for realizing practical applications that would include complementary metal-oxide-semiconductor (CMOS)-like logic circuits, all-polymer solar cells, organic photodetectors and organic thermoelectrics.

Zips on the nanoscale


February 28, 2019

Nanostructures based on carbon are promising materials for nanoelectronics. However, to be suitable, they would often need to be formed on non-metallic surfaces, which has been a challenge – up to now. Researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have found a method of forming nanographenes on metal oxide surfaces. Their research, conducted within the framework of collaborative research centre 953 – Synthetic Carbon Allotropes funded by the German Research Foundation (DFG), has now been published in the journal Science.

Two-dimensional, flexible, tear-resistant, lightweight, and versatile are all properties that apply to graphene, which is often described as a miracle material. In addition, this carbon-based nanostructure has unique electrical properties that make it attractive for nanoelectronic applications. Depending on its size and shape, nanographene can be conductive or semi-conductive – properties that are essential for use in nanotransistors. Thanks to its good electrical and thermal conductivity, it could also replace copper (which is conductive) and silicon (which is semi-conductive) in future nanoprocessors.

New: Nanographene on metal oxides

The problem: In order to create an electronic circuit, the molecules of nanographene must be synthesised and assembled directly on an insulating or semi-conductive surface. Although metal oxides are the best materials for this purpose, in contrast to metal surfaces, direct synthesis of nanographenes on metal oxide surfaces is not possible as they are considerably less chemically reactive. The researchers would have to carry out the process at high temperatures, which would lead to several uncontrollable secondary reactions. A team of scientists led by Dr. Konstantin Amsharov from the Chair of Organic Chemistry II have now developed a method of synthesising nanographenes on non-metallic surfaces, that is insulating surfaces or semi-conductors.

It’s all about the bond

The researchers’ method involves using a carbon fluorine bond, which is the strongest carbon bond. It is used to trigger a multilevel process. The desired nanographenes form like dominoes via cyclodehydrofluorination on the titanium oxide surface. All ‘missing’ carbon-carbon bonds are thus formed after each other in a formation that resembles a zip being closed. This enables the researchers to create nanographenes on titanium oxide, a semi-conductor. This method also allows them to define the shape of the nanographene by modifying the arrangement of the preliminary molecules. New carbon-carbon bonds and, ultimately, nanographenes form where the researchers place the fluourine atoms. For the first time, these research results demonstrate how carbon-based nanostructures can be manufactured by direct synthesis on the surfaces of technically-relevant semi-conducting or insulating surfaces. ‘This groundbreaking innovation offers effective and simple access to electronic nanocircuits that really work, which could scale down existing microelectronics to the nanometre scale,’ explains Dr. Amsharov.

Materials that are hybrid constructions (combining organic and inorganic precursors) and quasi-two-dimensional (with malleable and highly compactable molecular structures) are on the rise in several technological applications, such as the fabrication of ever-smaller optoelectronic devices.

An article published in the journal Physical Review B describes a study in this field resulting from the doctoral research of Diana Meneses Gustin and Luís Cabral, both supervised by Victor Lopez Richard, a professor at the Federal University of São Carlos (UFSCar) in Brazil. Cabral was co-supervised by Juarez Lopes Ferreira da Silva, a professor at the University of São Paulo’s São Carlos Chemistry Institute (IQSC-USP). Gustin was supported by São Paulo Research Foundation – FAPESP via a doctoral scholarship and a scholarship for a research internship abroad.

“Gustin and Cabral explain theoretically the unique optical and transport properties resulting from interaction between a molybdenum disulfide monolayer [inorganic substance MoS2] and a substrate of azobenzene [organic substance C12H10N2],” Lopez Richard told.

Illumination makes the azobenzene molecule switch isomerization and transition from a stable trans spatial configuration to a metastable cis form, producing effects on the electron cloud in the molybdenum disulfide monolayer. These effects, which are reversible, had previously been investigated experimentally by Emanuela Margapoti in postdoctoral research conducted at UFSCar and supported by FAPESP.

Gustin and Cabral developed a model to emulate the process theoretically. “They performed ab initio simulations [computational simulations using only established science] and calculations based on density functional theory [a quantum mechanical method used to investigate the dynamics of many-body systems]. They also modeled the transport properties of the molybdenum disulfide monolayer when disturbed by variations in the azobenzene substrate,” Richard explained.

While the published paper does not address technological applications, the deployment of the effect to build a light-activated two-dimensional transistor is on the researchers’ horizon.

“The quasi two-dimensional structure makes molybdenum disulfide as attractive as graphene in terms of space reduction and malleability, but it has virtues that potentially make it even better. It’s a semiconductor with similar electrical conductivity properties to graphene’s and it’s more versatile optically because it emits light in the wavelength range from infrared to the visible region,” Richard said.

The hybrid molybdenum-disulfide-azobenzene structure is considered a highly promising material, but a great deal of research and development will be required if it is to be effectively deployed in useful devices.

By Emmy Yi

Technologies promising huge growth such as Artificial intelligence (AI), 5G, machine learning, high-performance computing, and telematics are ratcheting up pressure on semiconductor manufacturers in the race among product makers to accelerate time to market and capture share. To support rapidly evolving end markets for these and other technologies that are key drivers of industry growth, chipmakers are boosting semiconductor performance, producing more wafer sizes and improving manufacturing efficiency.

At the same time, chip manufacturers must enable unprecedented end-product reliability for exploding markets such as automotive and healthcare markets where, with lives at stake, products can’t afford even the slightest lapse in reliability. In response, chip suppliers are retooling their manufacturing processes to support 3D stacking, package-level integration and miniaturization. But they must do more. Bringing high efficiency to all phases of manufacturing including design and materials is the new imperative.

The key to quality management is not in the traditional post-production testing and damage control but in prevention. Delivering the highest quality and reliability must start in the earliest stages of production with manufacturing and testing design – an approach that reduces not only the cost of downstream testing but minimizes product defects that can damage a supplier’s credibility and lead to lost business.

To that end, SEMI has launched its Quality Assurance Task Force consisting of representatives from industry leaders such as Infineon, NXP, TSMC, UMC, ASE, Unimicron, and GCE. The task force’s goal is to establish quality requirements spanning the supply chain to meet new, higher reliability standards and help safeguard Taiwan’s competitive edge in the global microelectronics industry. Meeting for the first time earlier this month, the companies exchanged ideas for improving quality management in semiconductor manufacturing and ultimately deliver the reliability the market needs.

The company representatives unanimously agreed that the first step is to ensure a QA-friendly environment with quality requirements for various stages of chipmaking ranging from design, manufacturing, packaging and testing to even PCB and CCL production. The SEMI Quality Assurance Task Force this year plans to build on its current membership by enlisting companies from various fields to address critical areas of reliability including statistical process control, surface-mount-technology-based board level reliability control, and 0 dppm quality control for automotive chips.

SEMI Quality Assurance Task Force consists of leading companies in the industry, including Infineon, NXP, TSMC, UMC, ASE, Unimicron, and GCE.

“SEMI’s comprehensive platform of exhibitions, programs, forums, trade meetings and matchmaking events is instrumental in bringing together key industry players to enhance quality management practices and meet the growing reliability requirements of the end markets we serve,” said Terry Tsao, chief marketing officer at SEMI and president of SEMI Taiwan. “The Quality Assurance Task Force is a shining example of how SEMI continues to support the crucial role of Taiwan’s semiconductor industry in the international community.”

For more information about the SEMI Quality Assurance Task Force or to become a member, please contact Emmy Yi at [email protected].

Emmy Yi is a marketing specialist at SEMI Taiwan.