Increasing Endura PVD Capabilities For Thick AI and AIN
Mike Rosa, PhD
Exponential growth in demand for wireless products, combined with their increasing functionality and power management requirements, is driving greater use and ongoing development of physical vapor deposition (PVD) processes in their fabrication.
While CMOS is evolving toward ever smaller devices and thinner film layers—now measured at the atomic scale in leading-edge devices—features and thicknesses typical of the growing class of emerging technologies in the power, MEMS, and analog market segments are comparatively large. Depending on the application, thick films (≥5μm) or relatively thinner ones (50nm–2.5μm) can be required.
Production of this class of devices is readily accommodated on 200mm equipment, spurring Applied Materials’ renewed focus and increased investment in 200mm technology development. The goal is to improve upon existing capabilities and address the unique fabrication challenges of MEMS, power devices, wafer-level packaging, through-silicon vias, analog devices, and CMOS image sensors. This includes a broadened concentration on advanced films for a range of applications that include PVD of metals, compound metals and semiconductor materials.
In this article, we address recent developments in thick aluminum (Al) deposition for power device applications and thinner aluminum nitride (AlN) deposition for RF filter applications commonly used in today’s multifrequency cell phone technologies—both delivered on the Applied Materials Endura 200mm platform. Leveraging the production-worthiness and proven reliability of Applied’s Endura technology, these advances provide state-of-the-art, single-chamber and cluster tool-based solutions for thick Al and AlN that exceed current industry capabilities for volume-production throughput and film quality.
Increasingly, power device designs centered on reduced device footprint, high-power applications, or both, employ thick Al layers to switch high-current loads or act as paths for thermal dissipation. Power devices are used as switches or rectifiers in power electronics and include diodes, power MOSFETs, thyristors, and insulated gate bipolar transistors (IGBTs). Expected to be the largest contributor to the growing power device market in the next five years, the IGBT is present in many everyday products: variable speed motors, air conditioners, stereo systems with switching amplifiers, trains, and electric cars. Their speed is beginning to rival that of MOSFETs, and they exhibit excellent ruggedness at higher power loads.
By comparison, thinner layers of materials such as AlN are used as acoustic modulators in RF filter applications and used for their piezoelectric properties in a variety of MEMS-based sensors and actuators. Over the years AlN has enabled smartphones and tablets to remain compact and lightweight, while becoming more multifunctional and accommodating different service frequencies around the world.
A key driver of this trend is the 4G and long-term evolution (LTE) cellular network technology that makes possible faster data download and improved user interaction with the mobile Internet. AlN is a vital performance enabler in the fabrication of the bulk acoustic resonator (BAW) mechanism on which these technologies rely. Consequently, AlN has attracted particular interest in recent years because it demonstrates the best balance of performance, manufacturability, and reliability for such use.
BAW resonator devices enable precise filtering of the RF signals received from cell phone base stations by mobile phones. With a growing number of signals being received by smartphones (e.g., cell, Wi-Fi, GPS) and the crowding of available frequencies worldwide by multitudes of services, these filters must be exceptionally frequency-selective to avoid slowdowns or interruptions in operation. BAW devices also offer the advantages of high power-load handling, compatibility with small form factors, and relatively easy fabrication using existing CMOS-compatible technologies.
Thick Aluminum Process Development
Figure 1 illustrates examples of the power device structures that incorporate thick layers of Al. Their use ranges from plug-fill applications, thick layers for high current-carrying capability or high thermal-load tolerance, to back-side metallization of the wafer for thermal and electrical conductivity. At present, these layers are typically 3–7μm thick, although work continues on developing the means of depositing layers 11μm or more thick.
Figure 1. Thick layers of Al are used in (a) high-voltage transistors and (b) IGBT power devices.
Table 1 lists target requirements for the thick Al application. Chamber temperature control is the crux of optimizing thick Al deposition to meet these requirements. Productivity demands high throughput; high deposition rates (ideally >2μm/min) are therefore needed to grow the thick layers in as short a time as possible. However, the sputtering energy generated in achieving a high deposition rate builds up heat in the chamber. Elevated temperatures degrade film quality by creating thermal grooving (elastic deformation at grain boundaries), “whisker” growth, and hillock formation (see figure 2). Grooving adversely affects the film’s reflectivity and sheet resistance, which are important for subsequent lithography steps and overall film conductivity, respectively (and also an indicative measure of overall grain size and surface roughness). Overheating causes other issues, such as flaking and higher particle counts.
Table 1. Thick Al requirements vs. process performance.
Grain size control—another challenge—is important in limiting hillock formation. Generally, the higher the deposition temperature the larger the grain size (and the deeper the groove) formed during the deposition of the Al film. While most planar Al films are deposited at or slightly below 200°C, some plug-fill applications require temperatures in the range of 380–450°C. The main consideration is the possible occurrence of electromigration (EM) in the presence of larger grain sizes under conditions of higher current densities. This is predominately due to the concentration of a small percentage of Cu or Si in the Al film along the grain boundaries, within the grooves between the grains. High current conditions may favor EM of ions between the Al grains due to the potential nonuniformity of dopant ions. At lower temperatures, grain sizes will be smaller, with significantly shallower or nonexistent grooving creating more consistent dopant ion concentrations and hence a more uniform and consistent Rs. Because grain sizes are smaller, more boundaries exist where dopant ions can reside. This more even distribution results in a more uniform resistance, which in turn alleviates conditions favoring EM under higher current loads.
Figure 2. Overheating and film stress can produce defects such as (a) grooving, (b) “whisker” growth, and (c) hillock formation.
Consequently, the fundamental hardware challenge is to keep the chamber and wafer cool enough to allow for productivity-enhancing single-step deposition of a layer at least 5μm thick—preferably thicker. Currently, OEM competitor systems rely on multiple chambers to carry out film depositions as a means of managing the heat buildup from the deposition process. In such systems, wafers are transferred through a series of multiple chambers, each depositing some portion of the required thickness.
A number of carefully planned hardware modifications to the standard Al deposition chamber compatible with the 200mm Applied Endura PVD platform have produced industry-leading improvements in film quality and process productivity. Key among these is resolution of chamber heat loading and hot spots on the wafer. The former has been realized by upgrading the DC power supply to 40 kW and improving source-target- and process kit cooling. Edge-feed source cooling has been replaced by a moreeffective center-fed, forced-cooling approach. The process, which was previously not actively cooled, now benefits from a water-cooled adapter that helps stabilize the temperature of the process kit while dissipating the heat load from the higher source power. Hot spots have been eliminated by equipping the electrostatic chuck with wafer lift pins instead of clamps and improving chuck temperature control.
The process kit has also been redesigned specifically for thick deposition processes. A higher deposition ring accommodates a greater depth of Al, while a wider cover ring offers better protection if metal flakes off chamber surfaces onto the wafer.
The above modifications have created a substantially new Al deposition chamber designed to address the needs of the power device market: the high deposition rate (HDR) chamber. The HDR chamber is an Endura-based production-worthy process chamber for thick Al deposition, as shown by the current performance results summarized in table 1 and illustrated in figures 3 and 4. The circled area in the top graph of figure 4 reflects a “spacing matrix qualification” performed at 600 wafers, approximately mid-target life for the process, which also affected the thickness for a given sputter time. In the future, appropriate compensation will apply throughout the target life to deliver a consistent thickness.
Figure 3. (a) Top-down SEMs and (b) high-magnification close-ups illustrate grain-size modification made possible by tuning process temperature.
Already proven in the field for layers up to 6μm, the high-productivity process ( >2.3μm/min) doubles the output of a standard Al deposition chamber, thus reducing the cost of operation, and produces exceptionally uniform films that exhibit no whiskers or hillocks. Enhancing production flexibility, the chamber’s operating temperature range accommodates processes suitable to a variety of Al alloys.
Figure 4. Results from a 1000-wafer marathon using an AlCu (0.5%) target at 40 kW and 400°C show good results for 5.1μm thick film.
BAW filter structures are typically fabricated as either a solidly mounted resonator (SMR) or thin film bulk acoustic resonator (FBAR) devices, both of which are compatible with existing CMOS fabrication technologies (see figure 5). At present, most manufacturers construct them as SMRs, which are easier to implement. In an SMR, the BAW is fabricated on a Bragg reflector stack consisting of alternating pairs of high and low acoustic-impedance materials that limit acoustic losses. The air-gap approach, in which the cavity acts as the reflector, is currently implemented only in leading-edge applications.
Figure 5. (a) The SMR sits on top of reflecting layers that limit loss of acoustic energy into the substrate. (b) In the FBAR, a cavity is etched beneath the active area, creating a suspended membrane.
Advantages of AlN
AlN exhibits several properties desirable in a BAW device. First, AlN has the requisite high degree of c-axis (002) crystal orientation ideal for achieving the highest piezoelectric constant and electroacoustic coupling.[5,6] Depositing the film by reactive, pulsed DC sputtering of pure Al targets in a nitrogen atmosphere predisposes the material to orient in this manner. Recent studies have shown that this orientation can be further promoted by creating as smooth a substrate surface as possible on which to deposit the seed AlN between the substrate and the electrode underlying the AlN. Orientation within the seed layer influences that in the electrode, which in turn affects orientation in the AlN.
Second, AlN possesses a high quality factor (Q), i.e., it can store the maximum acoustic energy within the filter structure. In other words, the oscillations of the acoustic wave decay very slowly. This attribute enhances the signal clarity and strength of a given BAW device. The resonance frequency of a BAW device is inversely proportional to the thickness of the AlN layer and the electrodes above and below it, which makes them ideal for the most demanding 3G and 4G applications. The thinner the electrode/AlN “sandwich,” the higher the frequency. This relationship means that AlN uniformity is extremely important to device performance repeatability.
Lastly, AlN is stable, mechanically very strong, and possesses high thermal conductivity, which also enhances the power-handling capability of the device.
BAW structures are challenging to fabricate for several reasons. Crystal texture and film deposition uniformity are the key requirements. Crystal texture is crucial for optimizing the piezoelectric behavior of the AlN. Here it is significant that the quality of the silicon substrate directly influences the properties of the as-deposited AlN film. Recent studies have demonstrated the importance of both deposition rate and surface roughness of the substrate in achieving the desired AlN crystal texture. Films that grow more slowly (e.g., SiH4 oxide) generally produce better surface quality than thermal oxide. However, experiments have shown that even slow-growing films require subsequent planarization to achieve the desired crystallinity improvement. As noted above, the smoother the underlayer, the better the crystal orientation of the seeding layer between the substrate and the lower electrode, and of the piezoelectric layer itself.
Deposition uniformity must be virtually perfect given the relationship of resonance frequency to thickness of the electrode/AlN “sandwich.” The target for within-wafer uniformity on a 150mm or 200mm wafer is an exceptionally demanding 0.5% or less.
In addition, volume production of such applications requires high deposition rates while maintaining AlN film stress at neutral or slightly compressive (-200-0 MPa),[10,11] particularly in FBAR structures, which can otherwise be fatally distorted. Furthermore, it has been shown that in-film stress produces a shift in the frequency response of the AlN film; the natural frequency of the AlN FBAR devices exhibits a lesser shift when stress values trend toward neutral or tensile values. This effect, in addition to greater film thickness, results in a less sensitive film across the wafer.
Avoiding dopant redistribution and other damage to underlying CMOS layers is another critical consideration that requires post-CMOS processing be performed at temperatures lower than 450°C. Given that AlN deposition quality improves at higher temperatures, this limitation imposes somewhat of a trade-off on process optimization for integrated SMRs, although stand-alone devices could benefit and then be hybrid-integrated into dual IC packages.
Because deposition uniformity is of paramount importance, Applied’s hardware improvement programs focused on redesigning the magnetron to achieve full-face target erosion with a rotating magnet. Four magnet designs were tested with various nitrogen-to-argon ratios in the gas flow. Investigations revealed that some designs delivered less actual power than requested power during sputtering, especially at lower nitrogen-to-argon ratios.
Experiments also showed that higher nitrogen-to-argon ratios produced higher compressive stress levels in the film. These resulted from bombardment by high-energy neutrals formed when lightweight nitrogen ions bounced off the target, were neutralized, and made contact with the AlN with most of their initial energy retained.
These observations influenced magnet design optimization. Considerable process tuning was then required to refine performance and optimize the synergy of all parameters. For example, nitrogen-to-argon ratios had to be tuned so that the greater ionization potential and lighter weight of nitrogen did not unduly reduce sputtering efficiency. Also, high DC power levels—ideal for higher deposition rates—increased film nonuniformity under certain conditions, necessitating compensatory adjustments to other parameters.
Table 2 summarizes the Applied Endura PVD chamber’s performance on 200mm wafers relative to key requirements for AlN deposition in SMRs. Figure 6 illustrates the excellent crystalline alignment of the AlN and molybdenum (Mo) electrode that optimizes the piezoelectric behavior of the AlN, while figure 7 highlights deposition rate and within-wafer (WiW) nonuniformity that both meet requirements.
Table 2. AIN process performance.
It is noteworthy that, unlike available competitive systems, the Endura system’s exceptionally low nonuniformity is achieved without postdeposition trimming. The system further differentiates itself by producing high-quality AlN film at temperatures as low as 200°C, which, as noted earlier, is essential in fabricating CMOS-integrated or FBAR devices suspended above sacrificial materials that can be damaged at higher temperatures.
Figure 6. SEM of AlN deposited on a Mo bottom electrode illustrates the well-matched crystallinity of the two films.
Figure 7. Recent development results for (a) 1μm thick Al deposited at 250°C show (b) nonuniformity of 0.457% and (c) a deposition rate of 74.8 nm/minute.
PVD for 200mm power devices, MEMS and RF applications is a key emerging technology segment for Applied Materials. To serve customers in these markets, the industry-standard Applied Endura 200mm PVD chamber has been enhanced to deliver a competitive, state-of-the-art, single-chamber and cluster solution for thick Al and AlN that meets volume-production throughput targets. Its temperature tunability (250°C–450°C) readily accommodates both applications. Deposition rates exceed 2μm/minute for Al up to 5μm thick; and consistently meet the 75nm/minute requirement for AlN thicknesses ranging from 50nm to 2.5μm. Thick Al layers exhibit superior surface roughness and grain boundary grooving with stable Rs, reflectivity of 70–80 @ 633nm, and neutral stress (~50MPa). AlN layers are exceptionally uniform (<0.5% on 6” and 8” wafers; <0.2 on 4” wafers) and AlN’s inherent high degree of c-axis (002) crystal orientation is enhanced when the oxide underlayer is planarized (FWHM rocking curve <1.3).
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