Leak Rates and Residual Gas Pressure in Cavities Sealed by Metal Thermo- Compression Bonding and Silicon Direct Bonding

Transcription

1 / ecst The Electrochemical Society Leak Rates and Residual Gas Pressure in Cavities Sealed by Metal Thermo- Compression Bonding and Silicon Direct Bonding K. Schjølberg-Henriksen a, N. Malik a,b, Å. Sandvand c,d, G. Kittilsland e, and S. T. Moe a a SINTEF ICT, Department of Microsystems and Nanotechnology, Oslo, Norway b Department of Physics, University of Oslo, Oslo, Norway c Memscap AS, Langmyra 11, N-3185 Skoppum, Norway d IMST, Buskerud and Vestfold University College, 3184 Borre, Norway e SensoNor, Knudsrødveien 7, N-3194 Horten, Norway The residual gas pressure (RGP) in sealed cavities was measured by residual gas analysis (RGA). The cavity sealed by fusion bonding had the lowest RGP of 16 µbar. Cavities sealed by Au-Au thermocompression bonding and plasma bonding had RGPs of 0.18 and 0.22 mbar, respectively. The cavity sealed by Al-Al thermocompression bonding had RGP of 1.3 mbar. Cavities sealed by thermocompression bonding contained mbar Ar. The leak rates of the four seal types were estimated by three methods. RGA measurements revealed that the maximum leakage rates were between and mbar l s -1. Introduction Hermetic packaging, giving a controlled and stable cavity environment, is a key issue for microsensors with high performance and long lifetime. Due to the small cavity volumes of micro electromechanical systems (MEMS), leak rates in the range of mbar l s -1 are often required for device lifetimes of 5 15 years (1 3). Several reviews of vacuum levels required for different MEMS devices (3) and MEMS leak detection methods (1, 4) are provided. Table 1 lists selected leak detection methods and their minimum sensitivities, compiled from references (1 4). The methods have different strength and weaknesses and are suitable for screening or accurate measurements within a range of leak rates. TABLE I. Overview of leak detection methods and minimum sensitivities. Detection method Minimum sensitivity [mbar l s -1 ] He leak test 10-9 Kr Membrane deflection Ne leak test µpirani Q-factor Residual Gas Analysis

2 Both the cavity absolute pressure and the cavity gas composition may be of importance to device performance and lifetime. The cavity gas pressure and composition depend on outgassing from the cavity inner surfaces, leakage rate through fine leaks, permeation through cavity walls, and gas produced by the sealing process. As an example of the latter, the process of silicon direct bonding has been found to produce H 2 and/or H 2 O (5-7). When selecting a wafer bonding technology to seal a specific device, the technology must be integrated into the process sequence, and it must fulfil the requirements of the device in question. Choosing well-proven and qualified technologies are often more costeffective than qualifying technologies, or even developing new technologies. To our knowledge, high temperature annealed Si-Si direct (fusion) bonding is extensively used in industrial sensor applications requiring hermetic sealing (8). However, to our knowledge, low-temperature direct silicon (plasma) bonding has not yet been widely applied in sensor sealing, and the leak rate of seals realized by this technology has not been thoroughly investigated. The leak rate of plasma bonded seals may be assessed by comparing their performance to that of fusion bonded seals, which are known to fulfil the requirements of a number of industrial applications. Similarly, to our knowledge, Au-Au thermocompression bonding is being used in industrial applications (9), while Al-Al thermocompression bonding is an emerging technology receiving increased interest for device sealing. The leak rate of seals realized by Al-Al thermocompression bonding can be assessed by comparison with seals realized by Au-Au thermocompression bonding. However, the seal width may be of importance for the hermeticity of metal thermocompression bonding (10). Increasing the seal width from 10 to 100 µm is expected to increase the device lifetime by a factor of 100 (10). At the same time, cost reduction requires that a minimum area is used for sealing. Investigation of hermeticity performance for a variety of metal seal widths will allow for optimization of the seal width with respect to cost and required performance. In this paper, we present an investigation of leak rates, residual gas pressure and residual gas composition. Fusion bonding, presently used in device sealing, is used as benchmark for plasma bonding, and Au-Au thermocompression bonding is used as benchmark for Al-Al thermocompression bonding. For the metal thermocompression bonded seals, a seal width spanning from µm was applied to reveal possibilities for simultaneous optimization of hermeticity performance and use of wafer area. Since the leak rates of the emerging technologies were not well known, a range of measurement methods was applied. Initial leak rate screening was done by He leak test and optical measurement of membrane deflection. High accuracy leak rate estimates, residual gas pressures, and cavity gas composition were measured by residual gas analysis (RGA). In order to measure all gases present, no getter was present in the sealed cavities. Experimental Test devices were realized by lamination of cavity wafers and bottom wafers, applying four different bonding technologies. Cavity wafers were 280 µm thick, had a diameter of 150 mm and a patterned SiO 2 mask forming 481 quadratic openings. Cavities were realized by tetramethyl ammonium hydroxide (TMAH) etching. The finished cavities had a volume of 1.6 mm 3 and formed thin membranes of different thicknesses. A membrane 306

3 thickness 36 µm corresponded to a side edge of 2.5 mm. The SiO 2 mask was removed in BuHF on all wafers except one, and the membrane thickness was measured on each wafer. The further processing depended on the bonding technology. Cross-sectional sketches of the four die types with dimensions are shown in Figure 1. An overview of the four laminate types and the applied bond parameters is found in Table 2. Plasma: 41 µm 2.5 mm 1.6 mm mm 1.23 mm 750 nm SiO2 Fusion: 46 µm Au-Au: Al-Al: 41 µm 36 µm 750 nm SiO2 400 nm TiW 1.2 µm Au 1 µm Al Figure 1. Cross-sectional sketches of dies from the four laminate types: Plasma, Fusion, Au-Au and Al-Al. TABLE II. Overview of the four laminate types, laminate IDs, bond parameters and conducted tests. Laminate Type Laminate ID Bond Parameters and Anneal He leakage Measurement Residual Gas Analysis Fusion Fusion 50 o C, 350 mbar, 2 min. Yes Yes Anneal 2h 1050 o C, N 2 Plasma Plasma 50 o C, 350 mbar, 2 min. Yes Yes Au-Au Au o C, 8.5 MPa, 15 min Yes Au-Au Au o C, 8.5 MPa, 15 min Yes Al-Al Al o C, 115 MPa, 15 min Yes Al-Al Al o C, 115 MPa, 60 min Yes Al-Al Al o C, 115 MPa, 60 min Yes High-temperature Si Direct (Fusion) Bonding A blanket Si wafer without thermal SiO 2 was cleaned and used as bottom wafer. A cavity wafer with the 750 nm SiO 2 mask and a bottom wafer were cleaned in NH 3 and HCl solutions. They were then cleaned in piranha H 2 SO 4 :H 2 O 2 (2:1) for 15 minutes, rinsed in DIW for 15 minutes, rendered hydrophilic in H 2 O:NH 3 :H 2 O 2 (5:1:1) for 10 minutes, rinsed in DIW for 10 minutes, and spin-rinser dried. The wafers were prebonded in an EVG510 (EVG) in vacuum ambient with pressure below mbar at 50 o C, applying a tool force of 1 kn for 2 minutes. The tool force translated to a bond pressure of Pa. The laminates were kept overnight and then bond annealed for 2 hours at 1050 o C in N 2 ambient. 307

4 Low-temperature Si Direct (Plasma Activated) Bonding A blanket Si wafer without thermal SiO 2 was cleaned and used as bottom wafer. A cavity wafer without SiO 2 and a bottom wafer were cleaned in NH 3 and HCl solutions. They were then cleaned in H 2 O:NH 3 :H 2 O 2 (5:1:1) for 10 minutes, rinsed in DIW for 10 minutes, and spin-rinser dried. Subsequently, both wafers were activated for 1 minute in O 2 plasma created by a source power of 2500W and a bias power of 180W in an AMS- 200 (Alcatel). The cavity wafer was dipped in DI-water for 1 minute and spin-rinser dried. The two wafers were then bonded in an SB6e (Suss) wafer bonder in a vacuum ambient with pressure below mbar at 50 o C, applying a tool pressure of 350 mbar for 2 minutes. The tool pressure translated to a bond pressure of Pa. No further annealing was done. Au-Au Metal Thermocompression Bonding Cavity wafers and blanket bottom silicon wafers, both with SiO 2 layers of 750 nm thickness were sputter deposited with 400 nm TiW and 1.2 µm Au. The metal layers were patterned so that the Au layer formed bond frames of widths µm surrounding the etched cavities. The Au metal pattern on the bottom wafers was 40 µm wider than the bond frames on the cavity wafers. The TiW layer extended at least 50 µm outside the Au pattern on both wafers. The resist was removed by acetone and a plasma cleaning procedure, but no further cleaning was done prior to wafer bonding. The wafers were aligned and bonded in an SB6e (Suss) wafer with an ambient pressure below mbar, applying a tool pressure of 2266 mbar for 15 minutes at 200 o C or 300 o C. Assuming rigid silicon wafers, the tool pressure translated to a bond pressure of 8.5 MPa. Al-Al Metal Thermocompression Bonding Bottom wafers were fabricated by etching silicon frames of widths µm protruding 3 µm above the wafer surface by deep reactive ion etching applying a SiO 2 mask. A layer of 1 µm Al was sputter deposited on the bottom wafers and on cavity wafers which had a SiO 2 layer of 750 nm thickness. The metal was patterned on the cavity wafers, forming metal frames which were 40 µm wider than the protruding metal frames on the bottom wafers. The resist was removed by acetone and a plasma cleaning procedure, but no further cleaning was done prior to wafer bonding. The wafers were aligned and bonded in an EVG 510 wafer bonder (EVG) in an ambient pressure below mbar at a temperature of 350 or 400 o C, applying a bonding force of 60 kn for 60 or 15 minutes. Assuming rigid silicon wafers, the tool force translated to a bond pressure of 102 MPa. Measurements TABLE III. Type and time of measurements performed. Measured item Time denomination Time interval Action t 0 Wafer bonding Laminate t 1 t 1 = t months WLI deflection measurement Laminate t 2 t 2 = t months WLI deflection measurement Die t 3 t 3 = t months He leak test Die t 4 t 4 = t months RGA Analysis 308

5 The dies from the laminates were measured before and after dicing into individual dies. Table 3 lists the measurements and indicates at what time the measurements were performed. The membrane deflection was measured by white-light interferometry (WLI) (Zygo New View 6200). The deflections were used to calculate the pressure difference between the ambient and the sealed cavity by Equation 1 (11). In Equation 1, w is the deflection, a is the membrane side edge length, d is the membrane thickness, ν is the Poisson's ratio, E is the Young's modulus, and P is the pressure difference. In the calculations, the actual, measured membrane thickness, and the corresponding membrane side edge were used. w= (a 4 * (1-ν 2 )* P)/(66*d 3 *E) [1] A maximum leakage rate was calculated using two deflection measurements, performed at times t 1 and t 2. A maximum estimated leakage rate, L deflection, was calculated by Equation 2. In Equation 2, V is the cavity volume of 1.6 mm 3, and t 1 and t 2 are the times of the two deflection measurements. P max is the maximum estimated pressure change in the cavity between times t 1 and t 2. P max was set to 100 mbar, based on overestimating the uncertainty in the deflection measurement to 0.7 µm. Hence, L deflection represented a maximum estimate for the leak rate, while the actual leak rate could be significantly lower. L deflection = P max * V / (t 2 t 1 ) [2] As seen from Table 2, dies from five laminates were subjected to He leak test. The dies were exposed to 3 bar absolute He pressure for 4 hours. Then the He leak rate was measured on individual dies within 30 minutes using an industry standard He leak detector. The number and type of dies subjected to He leak test are listed in Table 4. TABLE IV. Overview of dies subjected to He leak test. Laminate Type Seal width [µm] Number of dies Fusion Plasma Au Al Al One die from each of the four laminates Fusion, Plasma, Au300, and Al was subjected to RGA at SAES Getters (12). The authors refer to (12) for details regarding the experimental procedure. During RGA, the partial pressure of each gas present in the 309

6 sealed cavity was measured. The RGA results were also used to calculate a maximum estimate for the leak rate, L RGA. Assuming that the pressure in the cavity was zero after bonding, a maximum leak rate estimate can be obtained by Equation 3. In Equation 3, V is the cavity volume of 1.6 mm 3, P RGA is the total cavity pressure measured by RGA, and t 4 t 0 is the elapsed time [s] between the bonding and RGA analysis. L RGA = P RGA * V /(t 4 t 0 ) [3] Results The measured actual membrane thicknesses, the measured deflections and the values of P calculated by Equation [1] are listed in Table 5. The WLI deflection measurements indicated that the pressure in the fusion bonded cavity was lowest. No significant difference in cavity pressure between the samples sealed by plasma bonding and thermocompression bonding using Au and Al could be seen from the WLI deflection measurements. Table 5 also lists the cavity pressures as measured by RGA. There was a large discrepancy between the pressures found by the WLI deflection measurement and the RGA. The RGA results showed that the pressure in the fusion bonded cavity was indeed lowest, and a factor 10 lower than the cavity pressures obtained by plasma and Au thermocompression bonding. However, the RGA result revealed that the pressure in the cavity sealed by Al thermocompression bonding was a factor 100 higher than that in the fusion bonded cavity. The results from the He leak test are plotted in Figure 2. Only four dies had a higher measured leak rate than the measured leak rate of the empty test chamber. Two fusion bonded dies, one die with seal width 40 µm from Au200, and one die with seal width 80 µm from Al had He leak rates above mbar l s -1. No systematic differences in leak rate between different seal frame widths could be observed. Maximum estimated leak rates found from repeated deflection measurements, He leak tests, and RGA pressure measurements, are found in Table 6. Table 6 also lists the sensitivities of the respective methods. The gas compositions measured by RGA in cavities from laminates Au300, Al350-60, Fusion and Plasma, are plotted in Figure 3. TABLE V. Measured membrane deflections and cavity pressures as calculated from membrane deflections and measured by RGA. There was a large discrepancy in the pressures found by the two methods of WLI deflection measurement and RGA analysis. Laminate ID Membrane Thickness [µm] Measured Deflection [µm] P Calculated from Deflection [mbar] Cavity Pressure Measured by RGA [mbar] Fusion Plasma Au Au Al Al Al

7 TABLE VI. Maximum estimated leak rates as found from repeated deflection measurements, He leak tests, and RGA pressure measurements. Laminate L deflection [mbar l s -1 ] Leak Rate Maximum from L RGA [mbar l s -1 ] ID He leak test [mbar l s -1 ] Sensitivity Fusion Plasma Au Au Al Al Al Figure 2. Results from He leak tests listed in Table 3. The dashed line at mbar l s -1 shows the maximum measured leak rate of the empty test chamber. Figure 3. Gas composition in sealed cavities, found by RGA. HC means hydrocarbon gases. 311

8 Discussion As seen in Table 5, the lowest residual gas pressure was obtained by fusion bonding, and was 16 µbar. The residual gas pressures occurring by Au-Au thermocompression bonding and plasma bonding were relatively similar at 0.18 and 0.22 mbar. The highest residual gas pressure occurred in the chip sealed by Al-Al thermocompression bonding, and was 1.3 mbar. The 16 µbar obtained in the cavity sealed by fusion bonding is in good agreement with the 22 µbar residual gas pressure obtained in a fusion bonded resonator cavity without the use of getter (13). With the use of getter material, the residual gas pressure was reduced to mbar (13). Figure 3 shows that the highest partial pressures in the fusion bonded chip were those of N 2 and CO/CO 2. In the plasma bonded chip, the three main gases were H 2, N 2, and H 2 O. O 2 was also present in the plasma bonded cavity. The differences in gas composition between the cavities sealed by fusion and plasma bonding are likely to be affected by three main differences between the samples and processes. Firstly, only the plasma bonded sample was exposed to an O 2 plasma. The presence of O 2 in the plasma bonded cavity could be related to this fact. Secondly, the plasma bonded sample was not annealed at elevated temperature, while the fusion bonded sample was annealed at 1050 o C for 2 h. The high temperature anneal is likely to have affected the residual gas composition. Even if H 2 O was present in the fusion bonded cavity directly after bonding, the H 2 O is likely to have reacted with Si during the exposure to high temperature for a prolonged time. H 2 O has been proposed to oxidize Si and produce H 2 at temperatures above 400 o C (5). Further, Ventosa et al. (14) found indications that H 2 O did react with Si at temperatures between room temperature and 250 o C. It is possible that the total residual gas pressure, the presence of O 2, and the ratio of H 2 :H 2 O could be influenced by subsequent high-temperature treatment of the plasma bonded laminate. Thirdly, a thermal SiO 2 layer of thickness 750 nm was present at the fusion bonded interface only. Mack et al. (5) found that H 2 and H 2 O were produced during fusion bonding. Voids, proposed to be gas bubbles containing H 2 O (6) or H 2 (7), have also been reported to arise at the interface of low-temperature silicon direct (plasma) bonding. The presence of SiO 2 at the bonding surface, or bonding wafers with cavities have been shown to suppress the void formation (6, 7). Our results indicate that the H 2 and H 2 O produced during fusion bonding was incorporated in the SiO 2 matrix at the bonded interface, while the H 2 and H 2 O produced ended up in the cavities of the plasma bonded chips. Ar and H 2 were the two main gas constituents in the two chips sealed by metal thermocompression bonding (Figure 3). We think that the partial pressures of Ar of 0.21 (Al) and 0.16 (Au) mbar in these chips is related to the use of Ar as sputter gas in the metal deposition process. It is possible that Ar is implanted in the silicon during backsputtering, or that Ar is incorporated in the metal film during deposition, and that the Ar out-diffuses into the cavity after sealing. It is important to be aware of the presence of Ar in sealed cavities, as the gas does not getter easily. However, a partial pressure of mbar Ar was measured in the fusion bonded chip, showing that Ar could be present in cavities that were not metal sputtered. The reason for the presence of CO/CO 2 and hydrocarbons in the cavities is not currently known. The metal bonded laminates were not cleaned prior to bonding, which could account for the hydrocarbons in the cavities. However, a residual hydrocarbon pressure of mbar was also found in 312

9 the fusion bonded cavity. Further studies are needed to fully understand the reason for the observed gas compositions, including the presence of He in all cavities. Measurement of membrane deflection by WLI did identify the sealing process that resulted in the lowest cavity pressure. However, the difference between 1.3 mbar and 0.18 and 0.22 mbar, obtained in cavities sealed by Al thermocompression bonding on one hand, and plasma and Au thermocompression bonding on the other hand, could not be seen by WLI measurements of membrane deflection. The calculated values of P ranged from 1291 to 1052 mbar. Applying an atmospheric pressure of 1013 mbar, the calculated P values yielded negative cavity pressures. Uncertainties in the geometrical dimensions of the membrane and possible effects from membrane stress on membrane bending are thought to account for the calculated P values exceeding the atmospheric pressure. Accurate measurements of membrane thicknesses were necessary for the calculated residual pressures to be of sufficient quality. The leakage rates listed in Table 6 shows that the leakage rate was low for all bonded chips. The initial screening by He leak test showed that the leakage rate was below 10-9 mbar l s -1, and no differences between the investigated sealing methods or seal widths could be observed. Repeated WLI measurements of membrane deflection over a period of 1 3 months showed that the leakage rate was below mbar l s -1 for all investigated sealing methods. RGA measurements revealed that the leakage rates were even lower, estimating leakage rates between and mbar l s -1. The differences in leak rate maxima estimates arise from differences in residual gas pressure, and do not represent actual differences in leak rates. All the measured leak rate maxima estimates represent maxima, and no difference in leak rate between the four sealing methods has been observed. It is possible that the actual leak rate for all sealing methods is equal, and below mbar l s -1. Conclusions The residual gas pressure in and leak rates of cavities sealed by four different bonding technologies has been investigated. The lowest cavity residual gas pressure of 16 µbar was obtained by fusion bonding. Cavities sealed by Au-Au thermocompression bonding and plasma bonding had similar residual gas pressures of 0.18 and 0.22 mbar, respectively. The cavity sealed by Al-Al thermocompression bonding had a residual gas pressure of 1.3 mbar. The residual gas composition was measured by residual gas analysis. A partial pressure of 0.21 and 0.16 mbar Ar was measured in the chips sealed by metal thermocompression bonding. The leak rates of the four seal types were estimated by three methods. He leak rate measurements showed a maximum leak rate of 10-9 mbar l s -1. Membrane deflection measurements showed a maximum leak rate of mbar l s -1. RGA measurements revealed that the leakage rates were even lower, estimating maximum leakage rates between and mbar l s -1. Acknowledgments This work was supported by the Research Council of Norway through the MSENS project, contract No /O

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