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.
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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.
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7. Slabbekoorn, J. et. al, “Bosch Process Characterization For Donut TSV’s” Eleventh International Wafer-Level Packaging Conference, Santa Clara, CA, November 2014.