Microsyst Technol DOI 10.1007/s00542-013-1834-8

TECHNICAL PAPER

Low cost anodic bonding for MEMS packaging applications Robin Joyce • Kulwant Singh • Himani Sharma Soney Varghese • J. Akhtar

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Received: 24 April 2013 / Accepted: 29 May 2013 Ó Springer-Verlag Berlin Heidelberg 2013

Abstract Anodic bonding of Pyrex 7740 glass to bare silicon and oxidized silicon wafer is presented for micro electro mechanical systems (MEMS) device packaging. Experimentally it has been observed that anodic bonding process parameters are varying with different 3D structures. The effects of bonding temperature and voltage are discussed by keeping the temperature constant and varying the voltage. The bonding interface has been studied by scanning electron microscope observations. Effective parameters for MEMS structure such as bonding temperature, voltage has been discussed.

fusion bonding, anodic bonding, eutectic bonding, solder bonding have been reported (Hanneborg 1991; Bhat et al. 2007; Ko et al. 1985). Anodic Bonding is one of the most used primary level wafer packaging method because of its low level temperature (350–400 °C) requirement. Ever since its inception, the power of knowledge, visualization and experience made people to bring out the best from the processes by simple modification on the machine. Here we discuss a novel low cost anodic bonding machine which utilizes the properties of the sodium doped borosilicate glass (CORNING PYREX) to bond over processed silicon wafer.

1 Introduction 2 Theoretical aspects Since the development of bonding technique by Wallis and Pomerantz in (1969), it has been extensively used in microfabrication industry for device packaging. As time passed, wafer bonding has become a key technology for microelectronics, optoelectronics and micro electro mechanical systems (MEMS) packaging and encapsulation (Kanda et al. 1990). Bonding for MEMS level packaging requires high quality, precision, low cost and man skill for the success in fabrication. Various bonding techniques like R. Joyce (&)  K. Singh  S. Varghese Nanomaterials and Device Research Laboratory, School of Nano Science and Technology, NIT, Calicut, India e-mail: [email protected] R. Joyce  K. Singh  H. Sharma  J. Akhtar Sensors and Nanotechnology Group, CSIR-Central Electronics Engineering Research Institute, Pilani, India H. Sharma DAAD Fellow, Korean Institute of Science and Technology, Saarbruechen, Germany

Figure 1 shows the classification of wafer bonding techniques in microfabrication packaging. Wafer bonding can be divided into direct and mediated bonding. Among all of them, due to the temperature suitability, easiness and cost effectiveness, anodic bonding is widely used for MEMS packaging, hermetic sealing and encapsulation. Anodic bonding is a solid state, irreversible bonding technique by electrostatically bonding two dissimilar materials together (glass, metals, alloys, semiconductors) having a coefficient of thermal expansion (CTE) close to each other (Wei et al. 2003a, b). Variety of glasses such as borosilicate glasses, Corning 7070, Schott 8330, Soda lime 0080, potash soda lead 0120, aluminosilicate 1720 have been bonded to tantalum, titanium, aluminium, Ni-Co alloys, GaAs and silicon. Anodic bonding of silicon against Pyrex 7740 glass is the most established method among all. Corning Pyrex 7740 and Schott 8330 are sodium borosilicate glasses having a CTE close to silicon. Figure 2 shows the basic principle of anodic bonding.

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Fig. 1 Classification of wafer bonding techniques

Fig. 3 Experimental set up for anodic bonding

The system comprises a hot plate made of copper, thermocouple, DC power supply together with a pair of aluminium electrodes. This low cost set up was assembled in our laboratory with the available workshop materials. Precleaned silicon and glass substrates were sandwiched between the pair of electrodes. The copper plate was heated to 450 °C by resistive heating technique followed by applying the required voltage and measured the current flow through the circuit with respect to time. Fig. 2 Schematics of Anodic bonding

3.2 Pre-cleaning and wafer preparation In anodic bonding, mirror grade polished glass and silicon wafers are brought in contact under an external negative voltage (600 V) and at medium temperature (400–450 °C) (Wei et al. 2003a, b). Pyrex glass is connected to cathode and silicon wafer is connected to anode. At these temperatures, the viscosity of the glass decreases and the doped sodium, potassium, calcium atoms break down into ions and become mobile. The applied heat allows glass to act as electrolyte during the process. The applied electric voltage drives the sodium ions towards cathode and neutralizes them, whereas oxygen anions are attracted to anode. The area very near to the silicon glass interface is depleted of sodium ions which gives rise to a large electric field. The strong electrostatic force of attraction at the junction due to the large electric field pulls the wafers, initiating a bond between them. The oxygen from the glass transports to the interface, where it combines with the silicon to form silicon dioxide (SiO2). 3 Experimental 3.1 Experimental setup An experimental setup for anodic bonding designed at the SNTG, CSIR-CEERI laboratory is shown in Fig. 3.

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One of the first considerations that must be made in wafer processing is on how wafers can be cleaned prior to any process step. Attempts are made to keep wafers clean at all times, but more care must be taken prior to high temperature processing steps such as thermal oxidation, diffusion, epitaxial growth, or chemical vapour deposition. There are three effective processes of wet cleaning, (1) Degreasing (2) RCA 1 & 2 (Radio Corporation of America) (3) Piranha. We used RCA cleaning processes as it is the most standard method for Si wafer cleaning before thermal oxidation. We took three quarter pieces of 300 Si ð111Þ wafer and two new 300 Si ð111Þ wafer and proceed for following cleaning process. RCA 1: This solution contains deionized (DI) water: H2O2: NH4OH in the ratio 5:1:1. It is also sometimes called ammonium peroxide mixture (APM). Wafers are immersed in this solution and heated to 65 °C for about 10–15 min, followed by thorough rinse in DI water. This procedure removes organic dirt present on the Si wafer. RCA 2: This solution contains DI water: H2O2: HCl in the ratio 6:1:1. Wafers are immersed in this solution and heated to 65 °C for about 10–15 min. This procedure

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Fig. 4 Graph showing current vs. time for anodic bonding of silicon on Pyrex glass at -400 V. Graph showing current vs. time for anodic bonding of silicon on Pyrex glass at -600 V. Graph showing current vs. time for anodic bonding of silicon on Pyrex glass at -800 V. Graph

showing current vs. time for anodic bonding of silicon dioxide on Pyrex glass at -600 V. Graph showing current vs. time for anodic bonding of silicon dioxide on Pyrex glass at -800 V. Graph showing current vs. time for anodic bonding of silicon dioxide on Pyrex glass at -1,050 V

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removes metal ions. The second RCA cleaning process is necessary before oxidation and diffusion to keep the furnaces free of metal contamination. Both cleaning processes leave a thin oxide on the wafers. For stripping the oxide layer from the wafers, we dip the wafers in dilute HF (DI water: HF = 50:1) for few seconds. Since HF reacts with the glass, Teflon beaker must be used. After cleaning treatment, quarter pieces of bare Si wafers are ready for bonding process and rest two Si wafers loaded for thermal oxidation on a quartz boat and inserted in a furnace after the 1,100 °C temperature is reached. Wafers should be inserted in the furnace slowly to avoid cracks, which may be caused due to the very high temperature (1,100 °C) at the middle of the furnace. The process is carried out as: 10 min by dry, 150 min by wet, 10 min by dry at 1,100 °C. Flow rate of wet O2 was 0.6 liters/min and N2 is used as the carrier gas at a flow rate of 1 liter/min. We used the above standard thermal oxidation process for MEMS to match bonding parameter of MEMS processed wafer. The thickness of the thermally grown SiO2 is measured using the elipsometer and found to ˚ , which is very near to our requirement in be 8,000 A MEMS devices. 3.3 Bonding process The pre-cleaned silicon wafer (quarter piece of 3 inch wafer) was placed on the hot plate which was structurally connected to the anode. Diced and cleaned Pyrex glass (3 cm 9 3 cm) was placed over the silicon wafer and the cathode was placed on the top of glass. Cleanliness of the hot plate and the top electrode were ensured by cleaning the surface by methanol before starting the bonding process. Thermocouple was attached on to the hot plate as shown in the Fig. 3. Power supply was connected to the machine. For a constant temperature and voltage, the current flow was measured with respect to time which is named as run 1. Then for the same temperature, voltage level was increased to next step, the current flow with respect to time was measured which is named as run 2. Three runs were made for both silicon-glass bonding and Si/SiO2- glass bonding and the graphs were plotted. When the current flow nearly approached zero value, the power supply was switched off and allowed the plate to cool. The bonded sample was taken out and inspected.

4 Results 4.1 Bonding current The bonding mechanism starts as soon as the external voltage is applied. The current flow becomes high to a level

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and decreases exponentially with respect to time as shown in Fig. 4a–f (Schmidt 1998; Lee et al. 2000). This initial peak current corresponds to the transport of sodium, potassium ions from the glass to the cathode where these ions are neutralized. As the temperature is raised, more ions decompose from Na2O and K2O and the conductivity of the glass increases. Higher voltage produces higher electric field and increases the drift velocity of the sodium ions. Higher applied voltage also accelerates the sodium ion detachment from the lattice and increases the sodium ion concentration, which contributes to higher electrostatic force and strong bond. The parameters that influence the bonding include temperature, voltage, time and pressure, out of which bonding temperature and voltage play key role in anodic bonding process. Hence more emphasis is made on these two parameters. It has been observed from the graphs (Fig. 4a–f) that as the applied voltage increases, the current flow increases and the bonding time decreases. The maximum current flow and total bonding time for silicon-Pyrex glass and silicon dioxide-Pyrex glass are given in Table 1. The bonding time for silicon diode-Pyrex glass was observed to be more since a thin film (0.8 lm) of oxide layer was already been grown over silicon and it becomes difficult for the oxygen molecules to diffuse into the silicon surface and enhance the growth of oxide layer. Hence the current flow is reduced through the interface layer and more voltage has to be applied to the substrates for a better bonding. Bonding was observed better at -800 V and -1,050 V. 4.2 Interface integrity The interface integrity was characterized by SEM. Figure 5a–e shows the SEM images of bonded samples with constant temperature and varying voltages for both Si and SiO2 with glass. It has been observed that bonding of wafers were uniform and smooth at higher voltages.

Table 1 Parameters of anodic bonding process Bonding layers

Voltage (V)

Max. current (mA)

Total bonding time (s)

Bonding status

Si-Pyrex

-400

0.42

900

Bonded

Si-Pyrex

-600

0.59

650

Bonded

Si-Pyrex

-800

0.72

430

Si ? SiO2-Pyrex

-600

0.05

1,850

Unbonded

Si ? SiO2-Pyrex

-800

0.13

1,000

Bonded

Si ? SiO2-Pyrex

-1,050

0.16

786

Bonded

Bonded

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Fig. 5 SEM image of silicon-Pyrex glass bonding at -400 V. SEM image of silicon-Pyrex glass bonding at -600 V. SEM image of silicon-Pyrex glass bonding at -800 V. SEM image of silicon

dioxide-Pyrex glass bonding at -800 V. SEM image of silicon dioxide-Pyrex glass bonding at -1,050 V

5 Conclusions

to time were analyzed and was found that bonding time decreases on increasing magnitude of the applied voltage. It was observed that total time and applied voltage required for bonding silicon dioxide wafer to Pyrex glass was more when compared to Si-Pyrex bonding. Since most of the fabricated wafers at the packaging level are oxidized and hence anodic bonding for device packaging can be obtained at 1,050 V, 450 °C.

A novel low cost anodic bonding machine has been conceptualized and developed according to the theoretical aspects. Addressed the experimental setup of the new machine, pre cleaning/preparation of the wafers and bonding process in detail. The effect of bonding current on different voltages for a constant temperature, with respect

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Microsyst Technol Acknowledgments The Director, CSIR-CEERI, Pilani, Dr. Chandra Shekhar and Head of SNST, NIT Calicut, Dr. C. B. Sobhan are thanked for their constant support and encouragement.

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Ko WH, Suminto JT, Yeh GJ (1985) Bonding techniques for microsensors, micromachining and micropackaging for transducers. Elsevier, Amsterdam Lee TMH, Lee DHY, Liaw CYN, Lao AIK, Hsing I-M (2000) Detailed characterization of anodic bonding process between glass and thin-film coated silicon substrates. Sens Actuators 86:103–107 Schmidt MA (1998) Wafer-to wafer bonding for microstructure formation. Proc IEEE 86(8):1574–1585 Wallis G, Pomerantz DI (1969) Field assisted glass-metal sealing. J Appl Phys 40:3946–3949 Wei J, Xie H, Nai ML, Wong CK, Lee LC (2003a) Low temperature wafer anodic bonding. J Micromech Microeng 13(2):217–222 Wei J, Nai SML, Wong CKS, Sun Z, Lee LC (2003b) Low temperature glass-to-glass wafer bonding. IEEE Trans Adv Packag 26(3):289–294