HYDROGEN PEROXIDE ETCHING AND STABILITY OF P-TYPE POLY-SIGE FILMS

Transcription

1 HYDROGEN PEROXIDE ETCHING AND STABILITY OF P-TYPE POLY-SIGE FILMS 1 B.L. Bircumshaw, 2 M.L. Wasilik, 2 E.B. Kim, 2 Y.R. Su, 2 H. Takeuchi, 2 C.W. Low, 2 G. Liu, 1,2 A.P. Pisano, 2 T.-J. King, and 2,1 R.T. Howe 1 Departments of Mechanical Engineering and 2 Electrical Engineering and Computer Sciences Berkeley Sensor & Actuator Center, University of California, Berkeley, CA , USA ABSTRACT In this paper, a model is developed for the etching of asdeposited, in-situ, boron-doped, LPCVD poly-sige films in hydrogen peroxide. The model is corroborated by etching results for poly-sige alloys with Ge content between 55 and 7%. The results indicate that Ge content in the 55 to 65% range is desirable for maintaining high selectivity with respect to poly-ge sacrificial layers in a peroxide etch while attaining a polycrystalline film structure (at about 425 C, 5% Ge content poly-sige films are amorphous). The drift in residual stress of poly-sige films at room temperature in dry and wet ambients is also reported. Finally, the etching of in-situ, boron-doped, LPCVD poly-ge films in hydrogen peroxide is studied. 1. INTRODUCTION Polycrystalline silicon-germanium (poly-sige) is a promising material for the integration of surface micromachined MEMS (microelectromechanical systems) with electronics. The process flow for poly-sige MEMS fabrication is similar to that for conventional poly-si MEMS fabrication. In this case, however, poly-sige replaces poly- Si as the structural material, while poly-ge can serve as the sacrificial material instead of silicon dioxide. This enables the use of hydrogen peroxide (H 2 O 2 ) as the release etchant instead of hydrofluoric acid (HF). Peroxide selectively etches poly-ge over poly-sige. The selectivity, though, is dependent on doping conditions and the Ge content of the SiGe alloy. Conventional LPCVD (low pressure chemical vapor deposition) techniques can be used to deposit conformal poly-sige and poly-ge films at temperatures below 425 C [1,2]. Consequently, poly-sige MEMS can be micromachined on top of modern foundry CMOS circuitry. Furthermore, research by Sedky et al. has shown that Almetallized CMOS can be annealed at temperatures up to 525 C for 9 utes without significant harm to the underlying electronics [3]. Micromachined resonators can exhibit very high mechanical quality factors. Indeed, unannealed poly-sige MEMS resonators deposited at 425 C have been reported to have Q s in excess of 3, [4]. Due to their high Q s, MEMS resonators are promising as replacements for discrete filters and oscillators in wireless communications systems [5,6]. Integrating RF MEMS directly with CMOS promises to drop parasitic capacitances and inductances, as well as reduce fabrication/integration costs and the form factor of telecommunications devices. After reviewing the problem with etch selectivity that motivated this research, we discuss how Ge content was detered in our processes. A model for the peroxide etching of in-situ, boron-doped (p-type), LPCVD poly-sige is then presented, and the etch rate of in-situ, boron-doped, LPCVD poly-ge is tabulated. Finally, the stability of in-situ, boron-doped poly-sige films at room temperature is studied by measuring their average stress in dry and wet ambients. 2. SIDEWALL SPACER RF RESONATOR PROCESS A five-mask, sidewall-spacer process was employed in an attempt to fabricate poly-sige RF electrostatic resonators with ultra-narrow gaps (Fig. 1). To remain compatible with post-cmos fabrication, all process temperatures were kept at or below 425 C, with the exception of two LTO (low temperature oxide) depositions at 45 C. Gaps between 8 and 1 nm were achieved on 2.4 µm poly-sige films (Fig. 2a). After a one-hour release in 9 C hydrogen peroxide, however, the gaps widened precipitously (Fig. 2b). Figure 1: Cross-sections of the low-temperature, sidewall-spacer process employed to fabricate highfrequency resonators. is used as the structural material, while poly-ge is used as the sacrificial material.

2 By visual exaation of Fig. 2, and assug a linear etch rate, we found that in-situ, boron-doped poly-si.31 Ge.69 etches at a rate of approximately.24 µm/hr (4 Å/). This etch rate is at least 4 times higher than that reported for undoped poly-si.3 Ge.7 films [2,7]. As will be discussed in Section 4, the etch rate of poly-sige is, in fact, non-linear. From the SEMs, in-situ, boron-doped poly-ge was found to etch at a rate of at least.5 µm/; at least 6% faster than undoped poly-ge [8]. It is clear from these experimental results that boron doping greatly enhances the etch rate of both poly-ge and poly-sige films in peroxide. This enhancement is greater for SiGe compared to Ge, reducing the selectivity of peroxide as a release for boron-doped SiGe structures with Ge as the sacrificial. Moreover, the Ge content of the deposited films was several atomic percent higher than expected. These findings indicated the need for us to more fully characterize our poly-sige and poly-ge films. 3. CHARACTERIZING GE CONTENT OF THE LPCVD POLY-SIGE Precise control of Ge content is critical for poly-si 1-x Ge x MEMS technology. Peroxide etch rate, internal stress, resistivity, strain gradient, and deposition rate are all dependent to some degree on Ge content. Unless otherwise stated, all poly-sige films described in this paper were deposited at 425 o C and 4 mtorr in a conventional, horizontal LPCVD furnace. The total gas flow rate was fixed at 22 sccm, and the flow rates of SiH 4 and GeH 4 were varied. For in-situ, boron-doped processes, the B 2 H 6 (1% in H 2 ) flow rate was set at 6 sccm, while for undoped processes, the N 2 flow rate was set at 6 sccm. (a) Poly-Ge 2µm (b) 2µm 1 µm 1 µm Figure 2: Cross-sectional SEMs of completed structures fabricated with the sidewall-spacer process after release in 9 C peroxide for: 13 seconds (a) and one hour (b). To study the dependence of Ge content on process conditions, multiple poly-sige films of varying Ge content were deposited, one on top of another. After depositions, SIMS (secondary ion mass spectrometry) was used to quantify the atomic composition of these multilayer stacks. For each set of process conditions, wafer position dependence was also exaed. It was found that the Ge content of the films varied nearly linearly with the ratio of GeH 4 to (GeH 4 + SiH 4 ) flow rates over the 62 to 75% Ge content region (Fig. 3). The difference in Ge content between wafer positions is attributed to the loading effect inside the furnace. It is important to note that the Ge contents of the films used in this paper were detered by curve-fitting the results in Fig. 3, not direct SIMS analysis of each film. The direct accuracy of SIMS analysis for Ge content is about ± 2% (Ge content). The SIMS data indicated that Ge and/or Si interdiffusion took place at the interface between different Ge content poly-sige alloys at 425 o C. This phenomenon has been reported upon in the literature [9]. Despite the interdiffusion, the multilayer stacks exhibited a stepped profile. To ensure that the multilayer Ge contents were correct, SIMS analysis was used to measure the Ge content in single layer poly-sige films deposited under the same conditions as specific layers of the multilayer stacks. The single film and multilayer Ge contents were compared and found to be in agreement to within ± 1.8% (typically, the agreement was better than ±.8%). The discrepancy between the two measurements is attributed to fluctuations in LPCVD conditions (flow rates, doping concentrations, temperature, etc.) and Ge interdiffusion in the multilayer films. 4. PEROXIDE ETCHING OF POLY-SIGE Qualitative Observations of Peroxide Etching Peroxide etching of high Si content (i.e., < 6% Ge content) poly-sige alloys results in little alteration of the original film for etches under one hour long. Peroxide etching of high Ge content poly-sige alloys results in films that steadily darken and become more glassy (exhibiting reflective properties and coloration common to thin oxides) the longer the films are etched (Fig. 4). An HF dip makes the films duller, indicating that the glassy substance has been removed. Cross-sectional SEMs of a poly-sige alloy with Ge Content of Film 76% 74% 72% 7% 68% 66% 64% Gas Inlet Position Gas Outlet Position 62% 6% Flow Ratio [ GeH 4 / (GeH 4 + SiH 4 ) ] Figure 3: Ge content vs. GeH 4 to (GeH 4 + SiH 4 ) flow ratio for in-situ, boron-doped processes.

3 approximately 66% Ge content that has been etched under a variety of conditions is presented in Fig. 5a-d. A postperoxide etch HF dip removes the silicon dioxide created during the H 2 O 2 etch (compare Figs. 5c and 5d). An HF dip before the H 2 O 2 etch will enhance peroxide etching (compare Figs. 5b and 5d). Fig. 5d appears to indicate that the H 2 O 2 is attacking along grain boundaries (spaced several 1 nm apart at this deposition temperature). The exact mechanism for enhanced etching along grain boundaries and along interfacial surfaces is unknown. From SIMS analysis, however, it is known that ~ 66% Ge (a) 1.6 µm 1.57 µm (c) ~ 65% Ge ~ 63% Ge 3 ~ 62% Ge 1 sec HF (1:1) 3 H 2 O 2, 1 sec HF (1:1) 1.51 µm 1.4 µm 1 sec HF (1:1), 3 H 2 O 2 1 sec HF (1:1), 3 H 2 O 2, 1 sec HF (1:1) Figure 5: Cross-sectional SEMs of a poly-sige alloy with ~ 66% Ge content before and after a 3 ute immersion in 9ºC H 2 O 2, with and without a preceding HF dip. The poly-sige film was deposited at 45ºC and 6 mtorr. (b) (d) Figure 4: Qualitative assessment of the characteristics of 9ºC peroxide etching of films of varying Ge content. The left-hand figure shows the effect of varying Ge content for different etch times. The right-hand figure provides a detailed view of how a single film darkens as the etch proceeds. The schematics below the figures indicate the approximate Ge content of the films and the amount of time they were etched in H 2 O 2. No pre- or post-etch HF dip was performed on these samples. The poly-sige alloys were deposited at 45ºC and 6 mtorr. the films are degenerately boron-doped (~ to boron atoms/cm 3, with this variation exhibited across the load). Thus, the grain boundary enhanced etching is likely due to boron segregation. It should be noted that a separate injector is used for introducing B 2 H 6 into our LPCVD furnace. The high degree of cross-load variation in the boron-doping appears to be due to non-uniformities in the B 2 H 6 flow profile resulting from the condition of the current injector. Proposed Etch Mechanism Previous work had assumed that the etch rate of poly-sige in peroxide is linear with respect to time [7,8]. Heck observed SiO 2 and GeO 2 growth on the surface of poly-sige during peroxide etching [8]. He proposed that once the concentration of Si atoms in the surface oxide reached a critical value (~ 5%), the etch would be arrested. For Si contents below this critical value, he predicted a linear etch rate with time. In this paper, we propose an etch mechanism analogous to the model developed by Deal and Grove in 1965 to explain the thermal oxidation of silicon [1,11]. Chemically, the model can be described as follows: 2H 2 O 2 + SiGe GeO + SiO + 2H 2 O H 2 O 2 + SiO SiO 2 + H 2 O H 2 O 2 + GeO GeO 2 + H 2 O H 2 O + GeO 2 (s) H 2 O + GeO 2 (aq) As a strong oxidizer, hydrogen peroxide is presumed to oxidize Si and Ge atoms exposed on the surface of the poly- SiGe film. The Si is converted into SiO 2 and the Ge into GeO 2. While SiO 2 is stable in H 2 O, GeO 2 is water-soluble. Because peroxide is typically available as a saturated solution of 3% H 2 O 2 and 7% H 2 O, the germanium dioxide is dissolved, leaving behind a porous silicon dioxide film. Overall, for films with high Ge content, material volume is removed as the GeO 2 is dissolved, resulting in the etching of poly-sige micromachined structures. At first, the etching is reaction-rate limited. However, as the etch continues, the porous SiO 2 layer thickens, restricting the flux of reactants to, and the flux of products from, the SiGe SiO 2 interface. Hence, the etch rate is diffusionlimited for long etch times. Assug first order kinetics and steady state diffusion, the etch mechanism described by Eqn. 1 can be modeled by a simple quadratic equation: 2 (1) t = X / B + X /( B / A) τ, (2) where t is time and X = X(t) is the thickness of removed material (drop in step height), while A, B, and τ are constants. A thin layer of SiO 2 and GeO 2 is present on poly-sige films exposed to oxygen. The time it would take peroxide to create and etch this oxide is represented by τ. 2 For short etches, where ( t +τ ) << A / 4B, X ( t) ( B / A)( t + τ ). (3) In this regime, the etch rate is linear with time, and B/A is termed the linear etch rate constant. For long etches, where 2 ( t +τ ) >> A / 4B, X ( t) Bt. (4)

4 In this regime, the etch rate is proportional to the square root of time, and B is the parabolic etch rate constant. Etch Rate Measurement Method Single layer, in-situ, boron-doped poly-sige thin films of varying Ge content were deposited on single crystal silicon (SCS) wafers, as well as SCS wafers covered by a 1 to 3 µm LTO film. The SiGe films were then patterned lithographically using reactive ion etching (RIE) to create test structures with twenty 15 µm wide steps, each spaced 15 µm apart. The initial step height was recorded using a stylus-based profilometer with sub-angstrom resolution and 8 Å or.1% step height repeatability. Due to the high index of refraction of high Ge content films and the high IR (infrared) absorption of boron, the optical techniques available to us could not be employed for step height measurements. Each step height was an average of the twenty step heights of the test structure. Standard deviations over the twenty measurements ranged from 1 to 1 nm, and were typically on the order of 2 to 3 nm. Each wafer was measured in two to ten different locations. After the initial step height measurements, the patterned wafers were dipped in a bath of heated peroxide. The temperature of the bath was fixed at 9 C using a hotplate, temperature sensor (placed roughly two inches above the hot plate), and a feedback controller. Additionally, a thermometer was used to verify the temperature of the bath. Due to the wafer carrier, in nearly all cases, the poly-sige etch structures were exposed to H 2 O 2 at a temperature slightly above 9 C (between 9 and 96 C, accounting for fluctuations in temperature). After a specific timed etch, the wafers were removed and washed thoroughly with deionized water. The average step heights of the test structures were recorded, and then the same wafers were placed back in the H 2 O 2 bath for a specified time. This process was repeated over a total etch period of between 3 utes and one hour. It is known that peroxide decomposes over time [12]. It is therefore critical that we rule out the possibility of our measurements being affected by this decomposition. The oxidation of Ge and dissolution of GeO 2 does not vary with peroxide concentrations above 5% [13]. Moreover, it is Reduction in Step Height (nm) st Test 1 1st Test Best-Fit 2nd Test 5 2nd Test Best-Fit Overall Best-Fit Time (sec) Figure 6: Etched material vs. time for a SiGe alloy of 7.3% Ge content. known that a 3% peroxide bath heated at 9 C for 2 hours will maintain an H 2 O 2 concentration above 5% [8]. All of our measurements were taken within 2 hours of heating a fresh peroxide bath. However, to ensure that the peroxide did not significantly decompose, we topped off the bath with fresh H 2 O 2 periodically. Significant etch rate fluctuations were not observed after refreshing the bath, indicating that the bath remained effective over the period of the tests. Etch Results Fig. 6 is an example of one set of experimental results, along with best-fit quadratic models (Eqn. 2). In this case, a wafer was broken in half and tested on two different days. As can be seen, our model fits the observed etch phenomena very well. However, the model does deviate noticeably in the short term. This is due to the fact that variations in testing conditions (temperature, precise etch time, placement of the profilometer stylus, etc.) are integrated over the etch time. For short etches, or fluctuations have a strong effect on the measurements. For long etches, variations are averaged together and tend to cancel out. The parabolic and linear etch rate constants were extracted using a least squares quadratic best fit. In all cases, the time constant τ was set to zero, which is reasonable since the initial oxide on the steps was on the order of 5 to 3 Å thick. The averaged constants are presented in Figs. 7 and 8. Parabolic Etch Rate Constant, B (nm/sec 2 ) Linear Etch Rate Constant, B/A (nm/sec) Figure 8: Doped Undoped.1 55% 6% 65% 7% 75% Ge Content Figure 7: Parabolic etch rate constant, B vs. Ge content. The undoped data was extracted from [2] % 6% 65% 7% 75% Ge Content Linear etch rate constant, B/A vs. Ge content.

5 For poly-sige films, the correlation factor (R 2 ) is typically greater than 8%; above 64% Ge content, the correlation factor is 99% or better. The correlation factor does drop off as the Ge content falls. This is due to the fact that very little material is etched (~ 1 nm), resulting in significant errors in measurement stemg from inconsistent placement of the profilometer stylus. Overall, the most reliable data is the parabolic etch rate constants for the high Ge content films. It is important to realize that the data in Figs. 6 8 simply records the drop in step height. It does not necessarily relate to the lateral etch rate. Moreover, the conductive part of the films is actually etched more severely than at first glance: the step heights recorded here are for poly-sige covered with porous SiO ETCH RATE OF POLY-GE The etch mechanism for Poly-Ge is analogous to that of poly-sige [8]: H 2 O 2 + Ge GeO + H 2 O H 2 O 2 + GeO GeO 2 + H 2 O (5) H 2 O + GeO 2 (s) H 2 O + GeO 2 (aq) In the case of poly-ge, however, porous silicon dioxide is not formed. Hence, the etch rate is, to first order, reaction rate limited and, therefore, linear with time. The lateral etch rate of in-situ, boron-doped (p-type), LPCVD poly-ge was found by depositing a layer of poly-ge on SCS and LTO wafers. The poly-ge film was then covered with a 1 µm LTO film. Next, the LTO film was patterned into a series of ever-widening oxide bridges, each bridge 1 µm wider than the last. Dice were then etched for various periods of time, and the etch length detered by the widest oxide bridge undercut. The results were then plotted and fit via a least squares method to detere the etch rate. The results are tabulated in Table 1. One of the wafers tested was accidentally dipped for three seconds in piranha (a 12ºC solution of roughly 56:1 H 2 SO 4 :H 2 O 2 ). The poly-ge appeared to be slightly etched. Despite this, we continued processing the wafer without incident. When the etch rate of the piranha-dipped poly-ge was measured, however, we found it to be enhanced by about 8% over those wafers not dipped in piranha. 6. STABILITY OF POLY-SIGE The stability of poly-sige films when stored at room temperature was also studied. Roughly 1 µm of boron-doped poly-sige was deposited on oxidized Si wafers at 45ºC and 6 mtorr (approximately 65 to 66% Ge content). As a control, roughly 1 µm of in-situ, phosphorous-doped (ntype), LPCVD poly-si was deposited on oxidized Si wafers at 65ºC. Some of the wafers were then annealed for one hour in a nitrogen ambient at 825ºC (15ºC for the poly-si control wafers); the other wafers remained unannealed. The backside film of all wafers was then stripped. The wafers were then placed in a desiccator or exposed to laboratory ambient conditions for different lengths of time. During the study, some wafers were oxidized or coated with a SAM (self-assembled monolayer). Using wafer curvature, average Table 1: Etch rate of doped and undoped poly-ge, and the correlation coefficient of the least squares best-fit line. The undoped data is from [8]. Etch Rate Confidence (R 2 ) Undoped Poly-Ge.31 µm/ N/A Doped Poly-Ge.52 µm/ 97.6% Doped Poly-Ge After 3 Sec Dip in Piranha.94 µm/ 84.% film stress was measured regularly over a 7½-month period (Fig. 9). Unannealed SiGe wafers in ambient conditions exhibited an increasingly tensile average film stress, while those in the desiccator maintained a constant stress. We observed no stress drift in the annealed poly-sige wafers, even in ambient conditions. The unannealed and annealed poly-si control wafers exhibited no stress drift, regardless of ambient conditions (wet or dry). Low temperature (36ºC) wet oxidation does not appear to affect the stress drift in poly-sige. Stresses in those wafers oxidized jumped after the oxidation, but then continued their original trend, either drifting upward (i.e., unannealed wafers in the ambient) or maintaining a constant stress (i.e., unannealed wafers in the desiccator or annealed wafers in the ambient or desiccator). Hexamethyldisilazane (HMDS) and the water-based SAM coating Siliclad (from Gelest Inc., [14]) do not appear to halt the stress drift of the unannealed SiGe films in ambient conditions. SIMS analysis of the atomic composition of an annealed and unannealed, 63% Ge content SiGe alloy exposed to Stress (MPa) SiGe3 SiGe5 SiGe SiGe4 SiGe6 # Event # Time (days) Figure 9: At Day, SiGe3, 4, 5, and 6 were unannealed, while SiGe9 was annealed at 825ºC in N 2 for one hour. All wafers were left out in ambient conditions on Day. The events labeled in the figure above are as follows: 1. Day 4: placed SiGe3 in desiccator 2. Day 47: took SiGe3 out of desiccator 3. Day 55: placed SiGe6 in desiccator 4. Day 7: placed SiGe4 and 5 in desiccator 5. Day 147: took SiGe6 out of desiccator, oxidized SiGe3, 6, and 9 in steam at 36ºC for 1 hr 6. Day 165: took SiGe5 out of desiccator 7. Day 174: coated SiGe3 with HMDS 8. Day 217: coated SiGe5 and 6 with Siliclad

6 ambient conditions is presented in Fig. 1. Two days after deposition of the film, the wafer was broken in half; one half of the wafer was annealed for five hours in N 2 at 45ºC, while the other half remained unannealed. The SIMS analysis was performed 29 days after the film deposition. In the interim, between deposition and SIMS analysis, the wafer halves were stored in laboratory ambient conditions. The B, Ge, and Si profiles (not shown) were not noticeably affected by the anneal. The atomic concentrations of H and O, on the other hand, increased more substantially at the surface of the unannealed film compared to the annealed film. Integrating the curves, it was found that the unannealed sample contained 1.9 and 1.4 times more H and O, respectively, than the annealed sample. Accounting for residual O and H from the deposition process, we found that the unannealed film absorbed about 1.8 times more O than H. The higher O incorporation into the unannealed film suggests that the stress drift of SiGe films is due to a water-mediated introduction of oxygen and hydrogen into the SiGe film, not simply water absorption. Exposing the front side of the unannealed SiGe wafers in the stress drift experiment to HF results in virtually no change in the average stress. This indicates that oxidation is not the mechanism causing the stress drift (Fig. 9) and higher O incorporation (Fig. 1). 7. CONCLUSIONS In order to use poly-ge sacrificial layers, release steps for SiGe films with 65% Ge content must be time-limited due to the high etch rate of SiGe in peroxide. To maintain their stress stability, unannealed poly-sige MEMS should be encapsulated in a relatively dry ambient. For RF MEMS resonators, the restrictions are more severe. Most RF electrostatic resonators require gaps of ~ 1 nm or smaller [15]. Unfortunately, a 15-ute release etch will more than double the gap width of a poly- Si.31 Ge.69 resonator, increasing the motional resistance of the device more than 16-fold. However, this problem can be resolved without significant alteration of the process flow by increasing the Si content of the poly-sige films, thereby Atomic Concentration (Atom/cm 3 ) 1E+21 1E+2 1E+19 1E+18 H Annealed H Unannealed O Annealed O Unannealed Depth (µm) Figure 1: SIMS analysis of the atomic composition of an unannealed and annealed (five hours in N 2 at 45ºC), 63% Ge content SiGe alloy 29 days after deposition (27 days after the anneal). The Si, Ge, and B profiles (not shown) were not noticeably affected by the anneal. The poly-sige film was deposited at 425ºC and 6 mtorr. H O lowering the etch rate of the films in peroxide and increasing the etch selectivity between poly-ge and poly-sige. One should note, however, that, as the Ge content approaches 5%, the SiGe films become amorphous at a deposition temperature of 425ºC [2]. Therefore, the optimal Ge content which achieves both sufficient etch selectivity and polycrystalline film structure is on the order of 55 to 65%. Finally, because the gas flow rates have been shown to strongly affect Ge content in the poly-si 1-x Ge x LPCVD process, precise control of gas flow rates is critical. 8. ACKNOWLEDGEMENTS The authors would like to thank Marie Eyoum for contributions to Fig. 1, Dimitry Kouzov for the SIMS measurements, and Marilyn Kushner for making mask sets. Finally, we wish to thank the DARPA NMASP program for their financial support of this research. 9. REFERENCES [1] A.E. Franke, Y. Jiao, M.T. Wu, T.-J. King and R.T. Howe, Post- CMOS Modular Integration of Microstructures using Poly-Ge Sacrificial Layers, Technical Digest, Solid-State Sensor and Actuator Workshop, Hilton Head Is., June 4-8, 2, pp [2] A.E. 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