Process and Thin-Film
03/01/2003
Overview
One part of successful wafer-level packaging is the need for integrating passive components with IC fabrication, rather than fabricating them separately for integration during conventional assembly and packaging. Wafer processing for such fabrication draws significantly on the use of thin films deposited by sputtering. Successfully fabricating integrated passive devices requires a full understanding of sputter deposition capabilities and, in particular, the needed process conditions related to specific thin films used to fabricate these resistors, inductors, and capacitors.
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Above: Technicians learn under-bump metallization through hands-on training on Unaxis' 300mm Clusterline tool. Clusterline trademarked by & photo courtesy of Unaxis Semiconductors.
Market demand for higher performance and more compact electronic products (mobile phones, PDAs, digital cameras, etc.) is behind the integration of passive components such as resistors, inductors, and capacitors. The requirement for high-frequency signal handling and the integration of optical signals into electronic packages is a major driver for the integration of passives into various devices.
Because of their advantages in performance, size, and assembly cost, integrated passive devices (IPDs) are yet another booming area of advanced packaging. Many of today's electronic products feature 500 or more passive devices, making conventional packages and board assemblies a problem in terms of performance, yield, and cost.
IPDs typically combine a number of passive components (resistors, capacitors, and inductors) in a single package. These devices are increasingly built on a glass thin-film substrate, rather than on silicon-like semiconductor devices or costly ceramic. The IPD is ideally combined with wafer-level packaging, which yields minimal size and low costs, since there is no longer a need for a conventional plastic package.
Another way to integrate passives is to build them right into high-density substrates as integrated passive modules (IPMs). While the process technology is identical to that used for IPDs, the integration of passives into a substrate has the advantage that no extra space is needed and the assembly step of the devices is eliminated. This technology is ideal for products such as mobile phones.
Yet another method in early stages for passive device integration is to combine them at the wafer level together with wafer-level packaging steps. By establishing multiple metal and passivation layers (up to 5), passive components are built directly onto the die between the chip and the first-level interconnect, which is typically a solder bump. The methods for producing precise resistors, capacitors, and inductors on wafers or substrates usually involve thin-film technology for most applications.
Thin-film resistors are preferably made by sputtering TaN, NiCr (including its ternary alloys), and SiCr through a lift-off-type photoresist process. Inductors are typically sputtered to form a plating base and then "enforced" (i.e., relatively thin sputtered layers are increased to 10µm) by an electroplating step to increase the cross section of the trace, or sputtered over the full thickness and etched by subtractive etch technology, the latter being only suitable for ratios of thickness to linewidth below 0.25µm.
 Figure 1. Resistance change by a nonzero TCR — NiCr 150W/sq. (TCR ±25ppm/°K) and Cermet 1500 W/sq. (TCR ±50ppm/°K). |
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Multiple choices are available to create capacitors. Depending on requirements, sputtering can be efficiently done for Al2O3, AlN, SiO2, Si3N4, Ta2O5, TiO2, and other dielectrics. Because of the relatively low sputter rates of dielectric films, PECVD techniques are preferred for the thickness range above 2.0–3.0µm. As a low-cost method, spin-on of high-k dielectrics is also used.
Sputtering considerations
Sputtering is the most appropriate deposition method for thin-film applications used in electronic circuits. In addition, sputtering in general enables changes in the chemical composition of deposited films by introducing gases such as oxygen or nitrogen (i.e., reactive sputtering) to the otherwise inert gas atmosphere inside a sputtering system. Reactive sputtering is used frequently to produce resistor films like Ta2N; control the temperature coefficient of resistance (TCR) of NiCr films (TCR gives information about how much nominal resistance changes with temperature); produce barriers such as TiN, TiW(N), or TaN; and form insulating layers like Si3N4 or Ta2O5.
During sputtering, the reactive gas also reacts with the target material, forming a compound layer on its surface. This compound layer is also sputter-etched during the sputtering process. When the compound sputter-etch rate is higher than the compound-formation rate, most of the target's surface remains uncovered with this compound layer. (This target state is commonly called "metallic" in literature.)
If the compound-formation rate exceeds its sputter-etch rate, the surface of the target is always covered with a compound during the sputtering process. (This target state is called "compound.") The sputter yield of compounds is generally lower than that of the corresponding metals and, consequently, the deposition rate is strongly reduced. Reactive sputtering in the metallic state is the key process used to form most resistor films (i.e., slightly doped NiCr, SiCr, and Ta2N). Reactive sputtering in the compound state is the most common operation mode for insulating films (i.e., Al2O3, Si3N4, and Ta2O5)
Heating or cooling substrates before or during deposition influences the mobility of incoming molecules and atoms and provides a means to control film parameters, such as stress, adhesion, density, and resistance. Similar effects can be achieved by applying DC- or RF-substrate bias.
Thin films for IP applications
Important applications of integrated passives are clock terminators and filter networks used in CPUs such as Intel's Pentium; EMI filters and ESD protectors; zero ohm jumper arrays; bus terminators, such as serial/parallel termination array networks and AC termination networks; and precision resistor arrays, such as isolated and bussed resistor networks, voltage divider networks, and audio resistor arrays.
All these applications can contain a high number of resistors, capacitors, and inductors. High quality requirements, especially for resistor films, lead to sputter-deposited, thin-film solutions.
Resistors
Most important for the practical use of resistors in IPD circuits is the capability to sustain a certain nominal resistance value over the entire time of use. A nonzero TCR leads to a resistance change as a function of variable operating temperatures (Fig. 1), and a long-term drift of the film material leads to a resistance change as a function of time. Figure 2 shows the superposition of both effects for a sheet resistance range from 2–2000W/sq. For this example a temperature change of 100°K during operation and storage for 1000 hrs at 150°C was assumed. The most stable region is between 50–200W/sq.
Sophisticated sputter techniques like reactive sputtering and sputtering of ternary alloys are key for NiCr- and Ta-based films with high stability and low TCR over a wide range of sheet resistivity values (e.g., from 2–2000W/sq.). Also, sputtered thin films exhibit superior performance with respect to noise level, even in the GHz range.
NiCr films cover an especially wide range of applications. Standard NiCr films, reactive sputtered from a compound target in an oxygen atmosphere, are available with a sheet resistance up to 300W/sq., a TCR below ±25ppm/°K, and a long-term stability of about 0.1% deviation from the nominal value after 1000-hrs storage at 150°C.
Resistor films for high-frequency applications adhere well to highly polished glass, quartz, sapphire, or AlN substrates. Dissipation by magnetic materials should be avoided. NiCr films with very low Ni content, reactively sputtered in a nitrogen atmosphere, will match those requirements. Sputtering of ternary alloys such as NiCrAl provides a TCR below ±10ppm/°K and a long-term stability better than 0.03% after 1000 hrs at 150°C.
When higher sheet resistivity values are required, cermets are suitable materials. Compound targets, cosputtering, and reactive-sputtering are techniques to realize a semicontinuous conductive phase with an appropriate composition of dielectric and conductive components. TCR values below 200ppm/°K and a sheet resistivity up to 2000W/sq. have been achieved with SiCrOxNy, as well as for combinations of NiCr with SiO2 or Al2O3. (If TCR requirements are severe, SiCr is a proven cermet material.) Long-term stability is comparable to standard NiCr films. Ta-based thin films are an attractive alternative to the NiCr films above. In addition to its refractory nature, which implies that any imperfection frozen in during deposition will not anneal out for life, Ta belongs to a class known as valve metals, which form tough, self-protective oxides, either through anodic oxidation or through heat treatment in an oxygen atmosphere. Since Ta is such a reactive material, the sputtered films have a tendency to be contaminated during deposition.
 Figure 2. Absolute resistance change of NiCr film as a function of time and temperature (DT TCR = 100°K, Dt = 1000 hrs at 150°C). |
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 Figure 3. TaN resistance versus nitrogen concentration (Ta = 1 Mol). Data obtained from a Clusterline system. |
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Controlled contamination is desirable to achieve useful properties (Fig. 3). The most common process is reactive sputtering in a nitrogen atmosphere to form TaxN films. By increasing the N2 flow, the resistance rises and levels out at ~200–250µW∑cm, whereas the TCR drops down from positive values and stays at ~ -100ppm/°K. The composition of these "plateau" films is very close to Ta2N and displays the greatest stability during load-life tests.
Like NiCr films, Ta2N films need a thermal post-treatment to reach a stable final value. A good indicator for plateau films is that TCR is only negligibly changed during this heat treatment, whereas the resistance value rises, depending on time and temperature. The increase in the sheet resistance appears because, during heat treatment, the surface of the Ta2N film is changed to a self-protective oxide and the electrical effective thickness of the Ta2N layer decreases. This effect may be used for thermal trimming of the resistors without laser cutting, which is important for very high frequency applications.
The common sheet resistance range for Ta2N films is 30–300W/sq. with a TCR between -70 to -130ppm°/K. Long-term stability is better than 0.05% after 1000 hrs at 150°C.
In high-density and high-frequency applications combined with integrated passives, aluminum or copper wiring is used instead of costly gold, which was typically used in former hybrid applications. The thickness range of these layers is between 0.5µm and 5.0µm; deposition of such layers can be kept thin enough to be sputtered in a reasonable amount of time with modern planar magnetron technology. Adhesion promoters such as chrome or titanium and barrier layers such as Ni, TiW, or TiN will help to improve temperature stability of the circuit and reduce diffusion, migration, and segregation of contact material into the resistance layers, and vice versa.
Capacitors
High-value bypass capacitors to be integrated via thin-film techniques are a challenge. For frequencies exceeding 100MHz, thick-film solutions show an increasingly dielectric dispersion; therefore, thin-film capacitors are superior. An integrated thin-film capacitor consists of a sputtered conductive bottom electrode, a sputtered dielectric layer such as AlN, Al2O3, Si3N4, SiO2, or Ta2O5, and a conductive top electrode. Generally, a great variety of metal-insulator combinations are possible, depending on the technical requirements and the compatibility with the materials below and above the capacitor.
Unified solutions with only one base material are Ta-Ta2O5-Ta or Al-Al2O3-Al, which could theoretically be performed with only one sputter source, using the technique of reactive DC- or RF-sputtering. More sophisticated solutions have electrode stacks, including adhesion layers as well as barrier layers. Typical electrode materials are either good conductors, such as Al, Au, Cu, and Pt, or provide good adhesion such as Cr, MnO2, Nb, Ta, Ti, and TiW. Additional barrier layers are used to avoid diffusion and migration, first between the capacitor and the surrounding area, and second between the conductor and insulator of the capacitor itself. The layers are materials such as Ni, NiCr, TaxN, TiN, TiOx, ZrO2, and many more.
Suitable dielectrics for capacitors have high insulation resistance >10¥1012W∑cm, high dielectric strength, and high dielectric constant. Ta2O5, Al203, and Si3N4. will meet all the above requirements when sputtered under correct conditions.
Ta2O5 has a very high dielectric constant of 25 at 100kHz, followed by Al203 with 9, and Si3N4 with 8. With a thickness of only ~200nm, breakdown voltage values of 50V are reported with Ta2O5 and Si3N4, achieving a dielectric strength of 3–5MV/cm. The achievable capacitance/square is comparable to thick-film capacitors. However, the thin-film version appears much more stable and provides better high-frequency performance.
For more information, contact Christian Linder at Unaxis Semiconductors, Unaxis Balzers Ltd., P.O. Box 1000, FL-9496 Balzers, Liechtenstein; ph 423/388-4364, fax 423/388-5415, e-mail [email protected].
Christian Linder, Andreas Huegli, Hanspeter Friedli, Unaxis Semiconductors, Balzers, Liechtenstein