Suss MicroTec. Wafer Bonding Process Manual. Suss MicroTec Applications Group

Transcription

1 Suss MicroTec Wafer Bonding Process Manual Suss MicroTec Applications Group

2 CONTENTS CONTENTS 1 Introduction 2 Overview 3 Anodic Bonding 3.1 Typical process sequence Pre-bond cleaning/preparation Anodic bonding process window Recommended bond parameters 3.2 Results of anodic bonding Bond strength Hermetic bonding 3.3 Machine configuration Graphite pressure plates Patented Star-shaped electrode 3.4 Advantages of anodic bonding 3.5 Drawbacks of anodic bonding 3.6 Anodic bonding application notes: Investigation of the lowest anodic temperature and best abort criteria to end the bond process Lowest anodic bonding temperature Determining end of process 4 Plasma-activated fusion bonding 4.1 Hydrophilic/hydrophobic plasma-activated fusion bonding 4.2 NanoPrep plasma system Applications Plasma gases 4.3 NanoPrep Bonding Typical process sequence Pre-bond cleaning/preparation NanoPrep bond recipes 4.4 Results of NP bonding Bond strength Hermetic bonding Oxide growth Hydrophilic/hydrophobic surfaces Surface roughening 4.5 Machine configuration SiC pressure plates 4.6 Advantages of fusion bonding 4.7 Drawbacks of fusion bonding 5 Glass-glass bonding 5.1 Typical process sequence Borosilicate glass-glass bond recipe Soda lime glass-glass bond recipe

3 CONTENTS CONTENTS 5.2 Results of glass-glass bonding Bond strength Hermetic bonding Surface roughening 5.3 Machine configuration SiC pressure plates 5.4 Advantages of glass-glass bonding 5.5 Drawbacks of glass-glass bonding 6 Eutectic bonding 6.1 Typical process sequence Pre-bond cleaning/preparation Eutectic process window Recommended bond parameters 6.2 Results of eutectic bonding Bond strength Hermetic bonding Bond quality 6.3 Machine configuration SiC heater/pressure plates Inert gas lines 6.4 Advantages of eutectic bonding 6.5 Drawbacks of eutectic bonding 7 Glass frit bonding 7.1 Typical process sequence Pre-bond cleaning/preparation Glass frit process window Recommended bond parameters 7.2 Results of glass frit bonding Bond strength Hermetic bonding Bond quality 7.3 Machine configuration SiC pressure plates Process gas lines 7.4 Advantages of glass frit bonding 7.5 Drawbacks of glass frit bonding 8 Adhesive bonding 8.1 Typical process sequence Pre Pre-bond cleaning/preparation Adhesive process window Recommended bond parameters 8.2 Results of adhesive bonding Bond strength Hermetic bonding

4 8.2.3 Bond quality 8.3 Machine configuration 8.4 Advantages of adhesive bonding 8.5 Drawbacks of adhesive bonding 9 Gold-gold thermo-compression bonding 9.1 Typical process sequence Pre-bond cleaning/preparation Au-Au process window Recommended bond parameters 9.2 Results of Au-Au bonding Bond strength Hermetic bonding 9.3 Machine configuration Inert gas lines 9.4 Advantages of Au-Au bonding 9.5 Drawbacks of Au-Au bonding

5 1 Introduction The purpose of the process recipe manual is to outline the different substrate bonding techniques, applications and process parameters. The objective is to give the owner of this manual the ability to determine the process window required for successful substrate bonding. 2 Overview This manual contains information on the different types of substrate bonding applications. Bonding application described in the manual includes: anodic, plasmaactivated fusion, glass-glass, eutectic, glass frit, and adhesive and thermocompression bonding. The pre-bond cleaning, bond parameters, characteristics, effects, advantages and disadvantages are also described in this manual. 3 Anodic bonding Anodic bonding is a proven and robust bond process that typically refers to the bonding of silicon and Pyrex substrates with the aid of electric current. Anodic bonds are high strength bonds that are irreversible and hermetic. Bond strength data using the pull test method revealed bond strength up to 30 MPa. Some applications of Anodic bonding include pressure sensors and microfluidic devices. SenoNor manufactures pressure sensors from triple stack anodic bonded wafers. As Si and glass are bio-compatible materials, anodic bonded microfluidic devices are used in biomems applications such as Debiotech s piezo-actuated silicon implantable micro pumps used for drug delivery. Bond type: Permanent and hermetic with no intermediate adhesive layer. Applications: MEMS, bio MEMS, Microfluidics. 3.1 Typical process sequence Figure 1: Illustrates the typical anodic bonding process sequence. Step (1) Wafer cleaning with standard clean 1 (SC1) and or standard clean 2 (SC2). Step (2) Optional HF dip step to remove oxide layer on Si for hydrophobic bonding. Step (3) DI H 2 O megasonic clean (CL). Step (4) Align wafers on bond aligner (BA). Step (5) Bond wafers in substrate bonder (SB) Pre-bond cleaning/preparation

6 Typical pre-bond cleaning involves a SC1 and SC2 solution cleaning. Wafers can be cleaned individually or as a batch process in an agitated bath. The recommended cleaning conditions are shown in Table 1. Steps Cleaning agent Composition Temperature Time 1 SC1 NH 4 OH:H 2 O2:H 2 O=1:4:20 60 o C 10 min 2 SC2 HCl:H 2 O 2 :H 2 O=1:4: o C 10 min 3 DI water rinse H 2 O RT 1 min 4 Megasonic DI H 2 O H 2 O RT 1 min Table 1: Typical wafer clean sequence for anodic bonding. A final megasonic DI Water clean and drying is recommended. The megasonic clean removes particulate contaminants and ensures a thorough rinsing of the SC1 and SC2 chemicals Anodic bonding process window Bond parameters Typical values Comments Temperature o C Influenced by strain point and softening point of the glass Force 1100 N Negligible Voltage Current 800 V ma Influenced by substrates resistivity and bond temperature Process time minute When 10% of initial current is reached Table 2: Typical anodic bond process window. Internal stresses that develop during bonding above the strain point temperature will remain permanent after cooling and will have negative consequences on the device. Bond pressure is negligible, however 500 mbar is recommended for holding the wafers in uniform contact to counteract any wafer bow and ensure temperature homogeneity of the wafer stack. Voltage is dependent on wafer stack resistivity and bond temperature as the resistivity of the non-conducting glass substrates decreases with increasing temperature. Note that high voltages can cause electrical break down of the glass Recommended bond parameters Bond parameters Typical values Comments Temperature 400 o C Force 1100 N Voltage 800 V Current 10 ma Process time 10 minute When 10% of initial current is reached Table 3: Typical anodic bond process parameter.

7 3.2 Results of Anodic bonding The quality of anodic bonds can be characterized by the bond strength and hermeticity Bond strength Anodic bonds are generally very strong. Bond strength up to 30 MPa has been measured using the wafer pull test method. Note that during pull testing, Si wafer fractures above 30 MPa Hermetic bonding Anodic bonding are very hermetic bonds and suited for fabricating MEMS pressure sensors and biomems microfluidic devices. 3.3 Machine Configuration Graphite pressure plates and Star-shaped electrode are specifically designed for anodic bonding Graphite pressure plates Graphite pressure plates are recommended for anodic bonding as it has good thermal and electrical conductivity. Importantly, graphite absorbs the sodium (Na) that is liberated at the cathode keeping the wafer free from Na particles. The machine should be fitted with a center pin and field electrode. The graphite pressure plate acts a field electrode and the center pin acts as a central electrode from which anodic bonding can initiate to assist even spreading of the propagating bond front Patented Star-shaped electrode Star shaped electrode is recommended for machines that are dedicated to anodic bonding. The star shape electrode has three different circuits at the center, middle and outer edge of the pressure plate that can be operated in sequence. This enables a more uniform spreading of the bond front that results in void and stress free bonding. Figure 2: Time elapse images of anodic bonding using the Star-shaped electrode on a 100 mm Si-glass wafer stack. 3.4 Advantages of anodic bonding

8 Anodic bonding is the method of choice for biomems devices as Si, glass and SiO 2 are bio-compatible materials. The bonds are more tolerant to surface roughness and particles than fusion bonding. Precise bond temperature and pressure control is not needed compare to eutectic and thermo-compression bonding. It is also possible to do Si-glass-Si or glass-si-glass multi-stack bonding. 3.5 Drawbacks of anodic bonding CMOS compatibility issues due to the high electrostatic charge and alkali (sodium) build up. Oxygen desorption from the from the O 2 rich surfaces inside the cavities affect the pressure cavity pressure. Getters are often used to compensate for O 2 desorption. 3.6 Applications notes on anodic bonding An investigation of the lowest anodic bonding temperature required to bond wafer and best abort criteria to end the bond process is outlined in this section Lowest anodic bonding temperature The question of the lowest bonding temperature is very process specific as bond temperature is influenced by the electrical and mechanical properties of both Si and glass substrates. At specific temperatures, resistivity varies for different types of glass. Cavities, deposited layers and substrate thickness will influence the voltage drop characteristic across the wafer stack. Figure 3 shows the anodic bond temperature-current profile for Semi-standard 100 mm borofloat glass and p-type Si wafer stack. The bond voltage and current was 800 V and 15 ma respectively. Figure 3: Maximum (initial) current vs. bond temperature The Figure 3 illustrates that the wafer stack conductivity increases with increasing temperature (resistivity of glass decreases with increasing temperature). In this

9 setup, no measurable current flowed at temperatures lower that 200 o C and the maximum bond current was reached only at and above 280 o C. The physical properties of substrates confine bonding within a certain temperature range. Too low temperature may result in electrical break down of the glass, while high temperature will soften the glass and deform etched structures. Hence, aside from glass resistivity, a good understanding of the substrate s properties is also necessary to determine the lowest temperature anodic bonding can be carried out Determining end of process Suss bonders users are given the options of (1) time, (2) percentage maximum current, (3) changes in current and (4) moving charge (integral) to end the bond process. For time criteria, the bond process ends after a set bond time is reached. For percentage maximum current, the process ends when the declining current reaches a selected percentage of the maximum (initial) bond current. Changes in current gives the user the option of ending the process at a selected rate of change (derivative) of the continuously declining current and the moved charge (integral) terminates the bond process when a preset value of the integral Q= I dt is reached. Figure 4: Current-temperature profiles & changes in current (derivative) profiles Figure 4 shows the anodic bond current-temperature profile for three different temperatures. The set voltage and current was 800 V and 15 ma respectively. Figure 4 shows that, if time is selected, then the ending currents are different. Likewise, if maximum current is selected, then the bond process times are different. If moving charge was selected, and then there would be more charge (area under graph) applied to wafers bonded at higher temperature. The changes in current criterion had smaller variations between temperatures. Unlike the other three criteria that involved a fixed parameter, the changes in current is a relative parameter and theoretically will result in equal bonding of wafers regardless of bond temperature or fluctuations in bond temperature. Note that after 600 seconds all wafers were well bonded for the temperatures. However on a micro scale, as an example, it is unclear what differences there are with bond hermeticity. Though the difference may seem subtle, keep in mind that at larger wafer sizes, fixed quantities such as the total charge and percentage current applied to the wafer will be exaggerated.

10 4 Plasma-activated fusion bonding Plasma-activated fusion bonding is used for directly bonding of Si-Si, Si-GaAs, and other combination of compound semiconductors without an adhesive layer. One of the main driving forces behind the development of the technology was the need for CMOS compatible fusion bonding and produce electronic quality SOI wafers. Plasma-activated surface treatment has long been used in painting and manufacturing industries for cleaning and to improve adhesion and color coating on metals, glass, polymers and other synthetic materials. Improved adhesion can be achieved through changing the materials surface properties such as roughness, density of dangling bonds and the surface energy by the interaction of the plasma ions or radicals with the surface. Plasma-activated wafer bonding applies the same concept of plasma surface activation used by the painting and manufacturing industries. Plasma activated fusion bonding is an improved method of fusion bonding using surfaces activated by plasma to attain higher room temperature bond strengths. It is hypothesized that plasma treatment activates the surfaces of the wafer either chemically, physically or a combination of both chemical and physical changes on the wafer surface. Bond type: Permanent and hermetic with no intermediate adhesive layer. Applications of fusion bonding are used MEMS sensor manufacturing and material engineering such as SOI and strained Si substrates. 4.1 Hydrophilic/hydrophobic plasma-activated fusion bonding Fusion bonding can be categorized as either hydrophilic or hydrophobic. Hydrophilic bonding refers to wafers with hydrophilic surfaces prior to bonding. The difference between hydrophilic or hydrophobic surfaces lies in the surface chemistry of the wafers. Hydrophilic Si surfaces are normally terminated by hydroxyl (OH) and nitrogen (N) species. Hydrophobic wafers are normally terminated by species such as hydrogen (H), fluorine (F) and CH x groups. 4.2 NanoPrep (NP) plasma system Unlike most plasma system that operates under vacuum, the NP system has the capability to ignite plasma under ambient conditions. This increases the throughput as no time is loss for chamber evacuation and refill Applications The NP system has successfully bonded a variety and combination of substrates. Table 4 illustrates the different types of bond that has been carried out on the NP system. Substrates Si Si SiO SiO GaAs GaAs GaN GaN InP

11 InP ZnSSe ZnSSe Si Epi layers Si Epi layers Table 4: Illustration of the different types of substrate combinations that have been bonded on the NP system. The symbolizes that the substrate combination have been bonded using the NP system Plasma gases Nitrogen (N 2 ), air, argon (Ar) and formine gases are commonly used on the NP system. The different plasmas have different surface effects which results in different bond result. N 2 and air are oxidizing plasma, while H 2 is reducing plasma. Ar plasma is neutral specie that does not change the wafer chemically. Each gas has varying effects on different substrates. Such effects include oxide growth, wettability, surface roughening and bond strength. 4.3 NanoPrep bonding NP plasma bonding is straight forward as there is no chamber evacuation and refill step. The electrode-wafer distance is typically between 2-5 mm Typical process sequence The typical NP plasma bonding process sequence is outlined below in Figure 5. Plasma. There are two distinct paths and an optional HF oxide removal step. Path 1 involves a CL clean after NP treatment (Step 3I) and Path 2 does not involve a CL clean after the NP step (Step 3II). Figure 5: NP plasma activated bonding process scheme. Step 1: The wafers are cleaned with SC1 and or SC2 and with an optional low concentration of HF. The standard cleans are used to remove organic and inorganic contaminants while the HF clean is used to remove any native oxide before bonding. The SC1 clean renders the wafer surface hydrophilic while the HF dips results in a hydrophobic wafer surface.

12 Step 2: DI H 2 O rinse is required to clean the wafers after the wet chemical cleans. ensure that the SC1/SC2 and HF cleaning steps. Step 3I: NP plasma activation is followed by a CL clean prior to bonding. Step 3II: plasma activation is not followed by a CL clean prior to bonding. Step 4: Alignment of the wafers Step 5: Bond/annealing of the wafers pair Pre-bond cleaning/preparation SC1 and SC2 cleaning is commonly applied to wafers before bonding. The chemical components of SC1 and SC2 are (NH 4 OH:H 2 O 2 :H 2 O) and (HCl:H 2 O 2 :H 2 O) respectively. The mixture, bath temperature and cleaning times vary from user to user. For hydrophobic bond surfaces, wafers are soaked in a diluted (2-5%) HF solution is used for approximately 10 seconds. The soak removes any native oxides on the Si wafer and renders the surfaces hydrophobic. Steps Cleaning agent Composition Temperature Time 1 SC1 NH 4 OH:H 2 O 2 :H 2 O=1:4:20 60 o C 10 min 2 SC2 HCl:H 2 O 2 :H 2 O=1:4: o C 10 min 3 2% HF HF:H 2 0=1:49 RT 20 s 4 DI water rinse H 2 O RT 3 min 5 Megasonic DI H 2 O H 2 O RT 60 s Table 5: Typical wafer clean sequence for fusion bonding. CL cleaning parameters are very general and the parameters below can be applied for a variety of substrates and surfaces. Parameter Typical values Comments Number of scans 4 Clean spin speed 500 rpm Dry spin speed 2000 rpm Dry time 60 s Larger wafers requires longer drying times IR power 80% Substrate dependent Table 6: CL 200 cleaning recipe. Drying time is increased for wafers with topography NanoPrep bond recipes The Process window for NP plasma bonding is described below. The table shows the usable parameters and their respective ranges. After NP treatment, the wafers are annealed at o C for 2 hours to form permanent bonds.

13 Parameter Typical values Comments Gas H 2, N 2, Ar, air, H 2 :N 2 Process dependen Gas flow rate Power Scan speed Lpm W mm/s Scans 1-20 Process dependent Electrode distance Pulsation 2-5 mm Off Cycle time NA (pulsation plasma parameter) Duration time NA (pulsation plasma parameter) Table 7: Process Window for NP plasma bonding. More specific NP recipes are shown in Tables 8-14 for several different substrate combinations. Parameter Typical values Comments Gas N 2, Air, Process dependent Gas flow rate Power Scan speed Scans 1 Electrode distance Pulsation 50 Lpm 400 W 5 mm/s 0.25 mm Off Cycle time NA (pulsation plasma parameter) Duration time NA (pulsation plasma parameter) Table 8: Recommended NP plasma treatment parameter Si-Si. Parameter Typical values Comments Gas N 2, Air Process dependent Gas flow rate Power Scan speed Scans 1 Electrode distance Pulsation 50 Lpm 400 W 5 mm/s 0.25 mm Off Cycle time NA (pulsation plasma parameter) Duration time NA (pulsation plasma parameter) Table 9: Recommended NP plasma treatment parameter Si- SiO. Parameter Typical values Comments Gas N 2 Process dependent Gas flow rate 50 Lpm Power 300 W Scan speed 20 mm/s Scans 2 scans Electrode distance 0.5 mm Pulsation Off

14 Cycle time NA (pulsation plasma parameter) Duration time NA (pulsation plasma parameter) Table 10: Recommended NP plasma treatment parameter SiO-Si Epi layer. Parameter Typical values Comments Gas Ar Process dependent Gas flow rate Power Scan speed Scans 1 Electrode distance Pulsation 50 Lpm 300 W 25 mm/s 0.25 mm On Cycle time? (pulsation plasma parameter) Duration time? (pulsation plasma parameter) Table 11: Recommended NP plasma treatment parameter Si-GaAs. Parameter Typical values Comments Gas H 2 :N 2 Process dependent Gas flow rate Power Scan speed Scans 1 Electrode distance Pulsation 50 Lpm W mm/s 0.5 mm Off Cycle time NA (pulsation plasma parameter) Duration time NA (pulsation plasma parameter) Table 12: Recommended NP plasma treatment parameter Si-Ge. Parameter Typical values Comments Gas H 2 :N 2 Process dependent Gas flow rate 50 Lpm Power W Scan speed mm/s Scans 1 Electrode distance 0.5 mm Pulsation Off Cycle time NA (pulsation plasma parameter) Duration time NA (pulsation plasma parameter) Table 13: Recommended NP plasma treatment parameter Si-InP. 4.4 Results of NP bonding Fusion bonds are characterized by bond strength, voids, hermeticity, process repeatability and process window and are disused in more details in this section Bond strength

15 There is a general increase in the pre-bond strength when the wafers are annealed. Figure 6: Illustrates the bond strength after annealing hydrophobic and hydrophilic bonded wafers Hermetic bonding Fusion bond are hermetic and have been used to fabricate devices that require high levels of hermiticity Oxide growth One effect of the NP plasma treatment is the growth of oxides. The growth is influence by the substrate, plasma gas, plasma power, number of pass (treatment). The effect of plasma treatment on oxide growth is illustrated by Figure 7. Figure 7: Illustration of the effect of NP treatment on oxide growth using N 2 plasma. Oxide growth is influenced by influence by the substrate, plasma gas, plasma power, number of passes Hydrophilic/hydrophobic surfaces Influence of different gasses, hydrophobic, hydrophilic surfaces and plasma parameters. Not enough data Surface roughening:

16 Influence of different gasses, hydrophobic, hydrophilic surfaces and plasma parameters. Not enough data 4.5 Machine configuration Fusion bonding has no special machine requirements SiC pressure plates SiC plates are recommended as they provide a stable surface for holding alignment between the wafers as it maintains flatness at process temperatures. SiC also has matching thermal expansion coefficient to Si which is ideal for keeping alignment accuracy. 4.6 Advantages of fusion bonding A big advantage of NP plasma bonding is the ability to form permanent fusion bond with low-temperature annealing. Permanent hermetic bonds requiring no intermediate layers are formed at temperatures as low as 250 o C. Fusion bonding without plasma activation requires a minimum temperature of 1000 o C. When intermediate layers are required for bonding more process steps are involved as in the case of glass frit and adhesives. Drawbacks of fusion bonding Fusion bonding requires expert cleaning to overcome particles and void generation during annealing. Out-gassing (due to cleaning chemistries) will occur if at the bond interface if cleaning is carried out under non optimized condition. This out gassing will change the desired pressures inside the bonded cavities. 5 Glass-glass bonding Suss has developed a new low-temperature ambient plasma-activated process for directly bonding glass-glass substrates. The direct bonding of glass substrates is of great interest for bio MEMS lab-on-chip applications as glass substrates are extensively used to fabricate microfluidic devices. Without plasma treatment, hermetic bonding of glass substrates could only be achieved at high anneal temperatures of ~600 o C, which is in excess of the strain point and or approaching the softening point of most glass. Devices bonded above the strain point of glass resulted in internal stresses and glass softening caused deformation of the micromachined cavities and channels. It is critical that lower anneal temperatures are applied as it is less detrimental to the device and ultimately results in improved yield. Soda lime and borosilicate glass surfaces activated by plasma bonded spontaneously at room temperature and permanent hermetic bonds were formed after annealing at 300 o C. The effect of different plasma gases, temperature, bond pressure and time on bond strength is demonstrated. Pull-test on bonded samples revealed bond strengths up 25 MPa. Scanning electron microscopy (SEM) images of the bond interfaces showed no deformation of etched channel structures for low-temperature bonds. We have successfully fabricated microfluidic electrophoreses chips.

17 5.1 Typical process sequence The process sequence and cleaning are very similar to anodic bonding described in section 3. One difference is there is no need for HF cleaning as there is no native oxide on glass wafers. The investigation of glass-glass bonding suggests that only the use of air plasma will result successful boning. N 2 and Ar plasma failed to bond the substrates Borosilicate glass-glass bond recipe The investigation of glass-glass bonding suggests that air plasma results successful bonding. N 2 and Ar plasma failed to bond the substrates. The borosilicate glass-glass bond process is shown below in Table 15. NP Parameter Typical values Comments Gas Air, Process dependent Gas flow rate 50 Lpm Power 405 W Scan speed 20 mm/s Scans 1 Electrode distance 0.5 mm Pulsation Off Cycle time NA (pulsation plasma parameter) Duration time NA (pulsation plasma parameter) Bond (Anneal) Parameter Typical values Comment Temperature 500 o C Time 2 hr Force N Maximum substrate load temperature >100 o C Susceptible to thermal shock. Table 15: Borosilicate glass substrate bond recipe Soda lime glass-glass bond recipe The soda lime bond recipe is different from borosilicate. Soda lime can be bonded at much lower temperature, as low as 200 o C. Soda lime is an easier material to bond than borosilicate. NP Parameter Typical values Comments Gas Air, Process dependent Gas flow rate 50 Lpm Power 405 W Scan speed 20 mm/s Scans 1 No bond with multiple passes Electrode distance 0.5 mm Pulsation Off Cycle time NA (pulsation plasma parameter)

18 Duration time NA (pulsation plasma parameter) Bond (Anneal) Parameter Typical values Comments Temperature 300 o C As low as 250 o C Time 2 hr Force N No maximum substrate load temperature >100 o C Not susceptible to thermal shock. Table 16: Soda lime glass substrate bond recipe. 5.2 Results of glass-glass bonding Glass-glass bonds are characterized by bond strength, voids, hermeticity, process repeatability and process window and are disused in more details in this section Bond strength The bonds are irreversible and have high bond strength. The estimated bond strength is around 30 mpa Hermetic bonding Glass has a very low permeation rate on the order of cc/s. As a result, the bond very hermetic and have been used for micro fluidic applications Surface roughening Has not been investigated. 5.3 Machine configuration The standard machine configuration is suitable for glass-glass bonding SiC pressure plates Pressure plates: SiC plates are recommended. SiC Provides a stable surface for holding alignment between the bonded wafer as it maintains flatness at process temperatures and has matching thermal expansion coefficient to Si which is ideal for keeping alignment accuracy. 5.4 Advantages of glass-glass bonding Glass-glass bonding has applications in micro-fluidic bio MEMS devices. 5.5 Drawbacks of glass-glass bonding More sensitive to particles in comparison to anodic bonding. 6 Eutectic Bonding

19 Eutectic wafer bonding exploits the property of certain metal combinations that are able to form eutectic systems. The eutectic point is the lowest melting point composition of two or more metals. Only two-component systems are used for wafer bonding applications as it is requires less processing of the wafers. For wafer bonding applications, the first metal of the eutectic component is deposited on the first wafer and the second metal of the eutectic component is deposited on the second wafer. The application of heat and pressure during bonding causes the metals to diffuse at the bond interface forming the eutectic alloy. The bond is permanent and irreversible and is highly hermetic due to the low permeability of metals. Eutectic metals that are commonly used for wafer bonding are gold-si (Au-Si) eutectic point 363 o C, gold-tin (Au-Sn) eutectic point 282 o C, and lead-tin (Pb-Sn) eutectic point 183 o C. The most commonly used eutectic in wafer bonding is Au-Si given that Si is conveniently present as the substrate. Figure 8: Gold-silicon eutectic phase diagram. The eutectic point is 363 ±2 C at a composition is approximately 81 Wt % Au: 19 Wt % Si. 6.1 Typical process sequence Eutectic bond processing typically includes cleaning, aligning followed by bonding. One critical aspect is bonding must be done in an inert environment to prevent oxidation of the metals Pre-bond cleaning/preparation Care should be taken when cleaning the metal coated wafers as standard clean solution can be abrasive to the thin metal layers. For the case of Au-Si eutectic bonding, the Si wafer can be cleaned using anodic or fusion cleaning methods described earlier in Section Eutectic bonding process window

20 Eutectic bonding process window is smaller than other bond processes as bond quality is strongly dependent on temperature, temperature heating and cooling rates, pressure and bond time. The bond temperature is the eutectic point of the metals. Bond parameters Typical values Comments Temperature o C Depending on the eutectic metals Temperature ramping 5 o C/min Important for controlled diffusion of the metals during heating and cooling Force N Atmosphere Inert Prevents oxidation of the eutectic surfaces Process time 15 minutes Process dependent Table 17: General process window for eutectic bonding. Bonding is strongly influenced by heating and cooling rates Recommended bond parameters Bond parameters Typical values Comments Temperature Au-Si 363 o C, Au-Sn 282 o C, Pb-Sn 183 o C. Depending on the eutectic metals Temperature ramping 5 o C/min Important for controlled diffusion of the metals during heating and cooling Force N Atmosphere Inert Prevents oxidation of the eutectic surfaces Process time 15 minutes Process dependent Table 18: Recommended bond parameters for eutectic bonding. 6.2 Results of eutectic bonding Eutectic bonds are very strong and have high hermeticity given the metallic bond interface Bond strength Eutectic bonds are permanent and are of high bond strength. Pull test data shows bond strength in the range 25 MPa Hermeticity The bond is hermetic due to the metallic bond interface. However, no hermitic data is available at this time Bond quality The bond quality is strongly influence by the process parameters and the process window is small. The ability of the equipment maintains temperature uniformity, pressure uniformity and heating and cooling ramp rate is also critical for bonding.

21 The importance of good process and machine control is illustrated below in Figures Figure 9: Bond interface surface image of gold-indium (Au-In) eutectic bond. The bond process was carried out under non-optimized process conditions. The bond pressure was insufficient and the temperature ramp-up time was too short. Figure 10: Bond interface surface image of gold-indium (Au-In) eutectic bond. The bond process was carried out under optimized process conditions.

22 Figure 11: Bond interface surface image of gold-indium (Au-In) eutectic bond. The bond process was carried out under non-optimized process conditions. The bond time was too short. The dark areas show the inhomogeneous bond interface. 6.3 Machine configuration The same hardware configuration used for fusion bonding is recommended for fusion bonding. See section Inert gas lines Eutectic bonds must be carried out in an inert environment. Oxygen rich environment causes oxidation of the metals that result in inferior bonding. It is critical that an inert environment such as N 2, Ar or formine is kept the chamber during bonding. 6.4 Advantages Eutectic bonds form very strong and highly hermetic bonds. Bonding is considered a low-temperature process as the bond temperature is generally below 450 o C and has the ability to bond different type substrates. Another important property of eutectic bonding is there is no out gassing during bonding. 6.5 Disadvantages Deposition of the eutectic metals layers means more process steps are required for bonding. In the case of Au eutectic process, additional adhesion and diffusion barrier metals layers must be deposited on the Si before the actual Au eutectic layer is deposited. Chromium (Cr) and or titanium (Ti) layers are deposited onto Si to improve the adhesion of Au and to prohibit the fast diffusing Au ions from back diffusing in the substrate and reaching active device layers. Similar to fusion bonding, the bonds are affected by surface deviations and particles. Eutectic bond must be carried out in an inert environment as oxidation of eutectic layers can interfere with bond formation that will reduce hermeticity and bond strength. Eutectic layers can induce stress due to thermal mismatch of the eutectic metals. 7 Glass frit bonding Glass frit bonding is a widely used production process. The technology is widely used for fabricating MEMS sensors for automotive applications. 7.1 Typical process sequence No cleaning is required for glass frit bonding. The typical process involves aligning and bonding wafers. One important property of glass frit is the process demands high alignment accuracies. Precaution must also be taken to ensure that the screen coated frit does no outgas during bonding.

23 Ferro FX is a common type of glass frit that is widely used commercially. It is comprised of a low melting point glass (TF= C). The active ingredient is Pbsilicate-glass and the filler material is Ba-silicate-glass. It is also comprised of a binder for paste forming when mixed with solvents. The maximum particle size is approximately 15µm Pre-bond cleaning/preparation There is no cleaning process for glass frit wafers. The pre-bond treatment is confined to the preparation of the glass frit. The frit is screen printed frit, dried then glazed. Each device manufacturing will choose the frit preparation process that best suits their respective products Glass-frit bonding process window Bond parameters Typical values Comments Temperature o C Depending on the frit Temperature ramping (From 300 o C) 5 o C/min Important for controlled flow of the glass frit Force N Pressure ramping???? Atmosphere Process dependent Determined by the device requirement Process time 20 minutes Process dependent Table 19: General process window for glass frit bonding. Bonding is strongly influenced by temperature and pressure uniformity of the bond system Recommended bond parameters Bond parameters Typical values Comments Temperature o C Depending on the frit Temperature ramping (From 300 o C) 5 o C/min Important for controlled flow of the glass frit Force N Atmosphere Process dependent Determined by the device requirement Process time 20 minutes Process dependent Table 20: Recommended bond parameters for glass frit bonding. 7.2 Results of glass frit bonding Glass frit bonding is reliably and produces high percent device yields under optimized conditions. Figure 12 illustrates the difference between a good and poor bond.

24 Figure 12: SEM image of a good and poor glass frit bond. This difference in bond quality can be seen from the side walls Bond strength The bond strength is in excess of 30 MPa, the Si wafers breaks before the wafer can be pulled apart Hermeticity Glass frit material has a very low permeability. The permeability is equivalent to glass, which translate to high levels of hermeticity Bond quality Glass frit bond have very good bond quality. The process is very repeatable and reliable under optimized process conditions, 7.3 Machine configuration The same hardware configuration used for fusion bonding is recommended for eutectic bonding. See section Process gas lines The devices made from glass frit bonding usually dictate the use of special gases. These gases are required for proper functioning of the MEMS devices. It is recommended that that bonder has extra gas lines with flow control capability. 7.4 Advantages Glass frit bonding has the ability to bond a variety of substrates and is not limited by wafer topography. The process is CMOS compatible as the bond temperature is less than 450 o C. Glass frit material has a very low permeability. The permeability of the frit is equivalent to glass, which has a low permeability and high levels of hermeticity. 7.5 Disadvantages The frit footprint consumes wafer real estate and the added process steps associated with screen printing is a drawback. Also screen printing is considered a dirty

25 process. Frit reflow can lead to difficulties maintaining 3D bond line uniformity across the wafer if the process parameters and machine conditions are not optimized. 8 Adhesive bonding Adhesive bonding is the process of choice for temporary and low-hermitic wafer bonding applications. Temporary bonds are wafer bonds that can be de-bonded by dissolving the adhesive in solvents or de-bonding with UV light. Adhesives are categorized as polyamides, BCB, photoresist, UV curable polymer and other types of adhesive polymers. Some adhesive are also capable of forming permanent wafer bonds. Spin-on glass (SOG), Benzocyclobutene (BCB) and other polyamides forms very strong bonds that cannot be undone by soaking in solvents. The big advantages of adhesive bonding are that they are low-temperature bonds with maximum bond temperature of 220 o C and are photo-patternable using stand photolithography technique. Ultraviolet (UV) polymer bonding application is also possible when one or both substrates are transparent. Another advantage is different substrate types and wafer topography can be bonded using adhesives. 8.1 Typical process sequence Adhesive bond processing typically includes curing cleaning, aligning followed by bonding. Adhesives that are spin coated must be cured to prevent out gassing during bonding. Curing is achieved through soft baking on a hot plate or convection oven. Without curing, the trapped solvent in the spin coated adhesive will outgas and create bubbles in the adhesive film. Out gassing will generate bond voids that increase the pressure inside of etched cavities and degrade the bond quality Pre-bond cleaning procedure See section if pre-bond cleaning is applicable. SC1 and SC1 cleaning is not applicable to adhesive coated wafers Adhesive process window Adhesive bond process temperatures are significantly lower than process temperature of other bond processes. Bonding can even be achieved at room temperature for example when using UV curable polymers. In fact, some UV curable applications require active cooling during UV treatment to prevent overheating of the wafers pair Recommended bond parameters Bond parameters Typical values Comments Temperature RT-200 o C Depending on the adhesive Temperature ramping (From RT) 5 o C/min Important for controlled flow and homogeneity of the adhesive Force N Atmosphere Process dependent Determined by the device requirement Process time 10 minutes Process dependent Table 20: Recommended bond parameters for adhesive bonding. 8.2 Results of adhesive bonding

26 Adhesive bond process is very forgiving to wafer topography, particles and surface roughness. The bonds can be as high as 20 MPA, and are suitable for moisture-tight hermetic applications Bond strength Bond strength up to 20 MPa can be achieved with adhesives Hermetic bonding Adhesive bonds are capable of low level hermetic seals that are most commonly uses for moisture-tight sealing applications. In most cases, adhesives are organic polymers, which have a higher permeability than metals, glass and other crystalline material. Hence, hermeticy levels are inferior to those of eutectic, anodic and fusion bonds Bond quality The bond quality is strongly influenced by the spin coat uniformity and the curing/soft baking conditions. A uniform spin coated adhesive film and film curing is absolutely critical. The effects of curing/soft bake time and temperature are illustrated below in Figures 13 and o C for 10 minute 100 o C for 20 minute 100 o C for 30 minute Figure 13: BCB bond interface images for soft bake temperature of 100 o C for 10, 20 and 30 minutes respectively. 10 minute at 125 o C 10 min at 150 o C 10 min at 174 o C Figure 14: BCB bond interface images for soft bake time of 10 minutes and bake temperature of 125, 150, 175 o C respectively. (Pascual) Figures 13 and 14 illustrate the importance of the prebond treatment of the adhesives. In this case, the curing temperature was more influential than cure time.

27 8.3 Machine configuration No special machine configuration is required for adhesive bonding. The general configuration used for fusion and glass frit bonding is also recommended for adhesive bonding. The machine configuration is described in section Advantages of adhesive bonding Adhesive bonding is a low temperature process, which makes it CMOS compatible. Adhesives are easily laid down using spin coating technique. Cavities can be constructed by standard photolithography techniques if the adhesives are photopatternable such as photoresist and BCB. Adhesives have the advantage of being able to bond different types of substrates and different types of topography. 8.5 Drawbacks of adhesive bonding Adhesive bonding biggest drawback is its low hermeticity. This is due to the high permeability nature of polymers. Adhesive bond will limit any post bond processing where the bonded wafer might experience process temperatures up to 300ºC. Adhesives will thermally degrade at high temperature and erode the bond. 9 Gold-gold (Au-Au) thermocompression bonding Thermocompression bonding of Au is a technique that is used for fabricating and packaging of MEMS devices. Similar to eutectic bonds, the Au bonds are highly hermetic and provide an electrical conductive bond due to the low permeability and high electrical conductance respectively of Au. 9.1 Typical process sequence Pre-bond cleaning is very critical for successful Au bonding as the gold surface layers requires very clean surfaces. SC1 and SC2 cleaning are recommended before bonding, following cleaning, the wafers are aligned and bonded. It is recommended that bonding is carried out in an inert atmosphere to prevent oxidation of the Au surfaces Pre-bond cleaning procedure The cleaning process for Au thermocompression bonding is very similar to the eutectic bond cleaning process. Care should be taken when cleaning the metal coated wafers as standard clean solution can be abrasive to the thin metal layers. The cleaning process is the same that is used for cleaning anodic, fusion and eutectic wafers described earlier in Section Au-Au process window We have determined that temperature and pressure ramping during bonding is also a very important in addition to bond temperature and pressure. The quality of the bond is improved when set process temperature and pressures are reached through gradual and control ramping. The bond temperature will range between o C and pressure of mbar.

28 The gold layer will form clusters during bonding if the temperature ramping is not controlled. Gold cluster formation is illustrated in Figure 15 below. Figure 15: Au Surface image after an unsuccessful bond. The gold layers form clusters during the bond process at 350 o C. The formation of clusters will result in poor wafer bonding Recommended bond parameters Bond parameters Typical values Comments Temperature 330 o C Depending on the frit Temperature ramping (From 100 o C) 20 o C/min Important for controlled flow of the glass frit Pressure N Pressure ramping 1000 mbar/min 400 mbar/min Atmosphere Process dependent Determined by the device requirement Process time 20 minutes Process dependent Table 21: Recommended bond parameters for gold-gold (Au-Au) bonding. 9.2 Results of Au-Au bonding Au thermocompression is one of the more difficult bond techniques and as a result, it is more difficult to achieve perfect bonding. Effects such as gold clustering, rifts at the bond interface and weak bonding are common if bonding is carried out under non-optimized conditions. Figure 16 and 17 shows a rift in the bond interface when Au thermocompression bond process is not optimized.

29 Figure 16: 100x magnification of Au-Au bond interface at bond temperatures of 350 o C. The figure shows the very distinguishable bond interface between the wafers identified as A and B. Figure 17: 100x magnification of Au-Au bond interface at bond temperatures of 300 o C. The figure shows the very distinguishable bond interface between the wafers identified as A and B. The bond interface is very evident for wafer bonded at 300 and 350 o C. The rift between the two substrate indicated discontinuity of the Au-Au layers. This bond is not hermetic and was easily depended. Under optimized conditions, it is very difficult to identify the Au-Au bond interface. Wafers bonded at 330 o C undistinguishable (A) 100 x magnification (B) 200 x magnification Figure 18: 100x magnification of Au-Au bond interface at bond temperatures of 330 o C. The figure shows the very distinguishable bond interface between the wafers identified as A and B. When the bond interface not easily recognizable, it is an indication that the bond good as there is homogeneous continuity between the gold layers Bond strength Au-Au thermocompression bonds form very strong and irreversible bonds Hermetic bonding

30 Au-Au thermocompression bonds highly hermetic bonds as the metallic gold has very low permeability. 9.3 Machine configuration The same hardware configuration used for fusion bonding is recommended for Au-Au bonding Inert gas lines As in the case of eutectic bonding, Au-Au thermocompression bonds must be carried out in an inert environment. Oxygen rich environment causes oxidation of the metals that leads to inferior bonding. It is critical that an inert gas such as N 2 or Ar is fed to the chamber during bonding. 9.4 Advantages of Au-Au bonding The advantages are quite similar to those of eutectic bonds discussed in section 6.4. Like eutectic bonds, Au-Au thermocompression bonds form very strong and highly hermetic bonds. Bonding is considered a low-temperature process as bond temperature is generally below 350 o C and has the ability to bond different type substrates. Another important property of eutectic bonding is there is no out-gassing during bonding. 9.5 Drawbacks of Au-Au bonding Deposition of the eutectic metals layers means more process steps are required for bonding. Additional adhesion and diffusion barrier metals layers must be deposited on the substrate especially Si before the actual Au eutectic layer is deposited. Chromium (Cr) and or titanium (Ti) layers are deposited onto Si to improve the adhesion of Au and to prohibit the fast diffusing Au ions from back diffusing in the substrate and reaching active device layers. Similar to fusion bonding, the bonds are affected by surface deviations and particles. Au-Au bond must be carried out in an inert environment as oxidation of Au layers can interfere with bond formation that will reduce hermeticity and bond strength.