Amorphous Si/Au wafer bonding

P. H. Chen1, C. L. Lin1 and C. Y. Liu1,2 1Dept. of Chemical Engineering and Materials Engineering,

2Institute of Materials Science and Engineering, National Central University, Jhong-Li, Taiwan, R. O. C.

Abstract:

Crystal (100)-Si/Au and amorphous Si /Au bonding for wafer bonding

applications were studied. It was found that large craters and air voids could form

at the c-Si/Au bonding interface. However when the (100) Si wafer was coated

with an amorphous Si layer the craters that formed at the bonding interface were

smaller and more uniformly distributed. Remarkably, no voids could be observed

at the amorphous Si/Au bonding interface. We believe that the uniformity of the

Au(Si) liquid alloy was a result of the fast amorphous Si/Au reaction and that this is

the key to achieving a void-free bonding interface. In other words a uniform liquid

Au(Si) eutectic that slowly reacts with the (100) Si prevents air void formation at the

bonding interface.

Introduction:

Au-Si eutectic bonding has been successfully used in IC (Integrated Circuits)

packaging and MEMS applications, such as for sealing, die attachment and electrical

interconnects. [1] Recently, Au-Si eutectic bonding has also been implemented

into wafer bonding techniques. [2, 3] The requirements for wafer bonding are

much stricter than those for conventional die attachment and sealing applications.

A uniform and void-free bonding interface is required for successful wafer bonding.

Jang observed however, that voids and craters often formed at the Au-Si bonding

interface. The presence of voids at the bonding interface has a serious affect on the

reliability of the bonding layer and the performance of the bonded wafer device. [4,

5] So, to make Au-Si bonding practical for wafer bonding applications, the

problem of voiding at the Au/Si bonding interface has to be solved. [6] The length

of the Au-Si wafer bonding process is another issue that must be dealt with. The

formation of the Au-Si bond is highly dependent on the inter-diffusion rate between

Au and Si. It usually takes over 15 minutes to accomplish the bonding process.

The Au-Si bonding process is typically done at a temperature of 400 ℃, 40 ℃ over

its eutectic point (363 ℃). [3] This means that delicate devices on the wafers are

likely to suffer thermal damage during the wafer bonding process. In this paper, we

report a novel Au/amorphous Si bonding process with which it is possible to achieve

void-free Au-Si eutectic wafer bonding.

Experiments:

Figs. 1(a) and (b) show schematics of the two bonding structures studied in this

work. A 1-μm amorphous Si layer was deposited on a (100)-Si wafer by the

PECVD (Plasma Enhanced Chemical Vapor Deposition) process. To ensure good

adhesion between the Au layer and the Si wafer, a Cr/Pt bi-layer was deposited on

the Si wafer prior to the E-Gun deposition of the Au seed layer and subsequent to Au

electroplating. A 10-μm Au layer was electroplated on the E-Gun Au seed layer.

Then using graphite bonding fixtures, the Au/Pt/Cr/Si wafers were respectively

bonded with cleaned crystal (100)-Si (c-Si) wafers and amorphous Si coated Si

wafers (a-Si). A graphite bonding fixture was used to apply the proper bonding

force to the two bonded Si wafers. The entire bonding process was performed in a

vacuum furnace maintained at around 4×10-6 torr throughout. After the specified

bonding times, the bonded Au-Si samples were removed from the bonding fixture

and mounted with epoxy for SEM cross-sectional examination.

Results and Discussion:

Fig. 2 shows cross-sectional SEM images of the a-Si/Au and c-Si/Au bonded

samples. As shown in Fig. 2(a), we can observe relatively big craters at the bonding

interface of the c-Si/Au sample. Facet crater formation at the c-Si(111)/Au bonding

interface has been observed by Jang. [6] Owing to the anisotropic nature of

crystalline Si, the c-Si/Au reaction rate is highly dependent on the crystallography of

the Si wafer. In other words, Si dissolution into the Au layer proceeds preferentially

along particular planes, usually the planes with the highest surface energy. [6] In

crystalline Si, the (111) plane has the lowest surface energy, so is the most resistant to

dissolving in Au. This means that, as Si reacts with and dissolves in Au, facet craters,

i.e., V-shape grooves or inverted pyramids form on the c-Si surface. Fig. 2(a) shows

inverted-pyramid craters that formed on the (100) Si side; they are constructed of four

(111) Si planes. In addition, observe that big air voids formed inside the craters.

Note that the densities of Si and Au(Si) liquid alloy are very different, 2.33 g/cm3 and

18.64 g/cm3, respectively.[7] We believe that the formation of air voids in the craters

is associated with the fact that there is density mismatch between the c-Si and the

Au(Si) alloy. As Si dissolves into Au during the Au-Si bonding process, the

incremental volume of the Au(Si) alloy is insufficient to compensate for the volume of

Si consumed, which means that the amount of liquid Au-Si alloy formed does not

completely fill the craters in the Si side. Thus, air voids will form inside the craters

at the bonding interface, as seen in Fig. 2(a). Those air voids result in a weak

bonding interface and poor thermal and electrical conductivity across the bonding

interface.

As seen in Fig. 2(b), there are much smaller craters at the bonding interface than

for the c-Si/Au case and, remarkably, no air voids can be found in the craters. We

suggest that the primary reason for the void-free quality of the bonding interface is the

fast formation of a uniform layer of Au-Si liquid alloy. Due to the looseness of the

structure of the amorphous Si coating layer, the dissolution of Si into Au can occur at

a very fast rate, without any preferential orientation. After the rapid consumption of

the a-Si coating layer to form a liquid eutectic Au-Si alloy, the Au(Si) liquid alloy

would start to react with the underlying c-Si wafer. Since the liquid Au-Si alloy

already contains a certain amount of Si, the reaction rate of the c-Si/Au-Si liquid alloy

would be much less than that of the c-Si/Au reaction case. The slow c-Si/Au-Si

alloy reaction prevented the formation of large craters on the c-Si side, also making it

unlikely for air voids to form at the bonding interface.

Prior to examining the reactive Si surfaces, we used an aqua-regia solution to

remove the Au(Si) bonding layer. Fig. 3(a) shows the exposed Si surface for the

c-Si/Au bonding case. Relatively big craters can be seen on the c-Si bonding surface.

The distribution of the crater size is quite large; it ranges from 10 μm to 90 μm. On

the other hand, the craters on the a-Si surface are much smaller and uniformly

distributed. Fig. 4 shows SEM cross-sectional images of the edged of the bonded

samples. As seen in Fig. 4(a), the eutectic Au-Si alloy exhibits excellent wetting on

the a-Si surface. The wetting angle is about 17o. In contrast, the c-Si surface of the

eutectic Au-Si alloy does not seem to wet well.

Above we have shown that a fast a-Si/Au reaction is the key to the

prevention the formation of air voids in craters at the Si/Au bonding interface. The

rates of a-Si/Au and c-Si/Au reactions in a prepared c-Si/Au/a-Si sandwich structure

were compared. The sandwich sample was then annealed at 380 ℃ for 2 minutes.

Remarkably, the thin amorphous Si coating reacted substantially with Au, as shown in

Fig. 5, while, no reaction was observed on the opposite c-Si side. The implication is

that fast Au/a-Si inter-diffusion greatly reduces the incubation time necessary for the

formation of eutectic Au-Si bonding. There are two reasons for faster formation of a

Au(Si) liquid alloy at the a-Si/Au interface. First, Au atoms have a much faster

diffusion rate in Au. Second, it takes much less energy to release Si atoms in

amorphous Si than in crystalline Si.

Conclusions:

An amorphous Si coating layer on a (100) Si wafer rapidly reacted with the

Au layer to form a uniform Au(Si) liquid alloy, which prevented the formation of

voids at the bonding interface. This amorphous Si/Au bonding process should

enable the implementation of Si/Au bonding in wafer bonding applications.

References:

[1] A.L. Tiensuu, M. Bexell, et al., Sensors and Actuators A, 1994; 45: 227.

[2] R.F. Wolffenbuttel, K.D. Wise, Sensors and Actuators A, 1994; 43: 223.

[3] R.F. Wolffenbuttel, Sensors and Actuators A, 1997; 62: 680.

[4] D.S Gardner, P.A. Flinn, Electron Devices, 1988; 35: 2160

[5] K.Hinnode, I. Asano, Y. Homma, Electron Devices, 1989; 36: 1050.

[6] Jin-Wook Jang, Scott Hayes, Jong-Kai Lin, and Darrel R. Frear, Appl. Phys.,

2004; 95: 6077.

[7] William D.Callister, Jr., Materials Science and Engineering an Introduction

(Wily), appendix A.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Captions:

Fig. 1 The structure of bonding test. (a) is the c-Si(100) bonded with Au lauer and

(b) is pre-deposit 1μm a-Si on c-Si(100). The Cr and Pt can ensure adhesion and be

a barrier layer

Fig. 2 Cross section image of (a) crystal Si-Au bonding (b) amorphous Si-Au

bonding. The voids that relative big void can be fined in crystal case.

Fig. 3 The bonding interface of (a) crystal Si-Au bonding (b) amorphous Si-Au

bonding. The amorphous Si-Au bonding has more uniform size and distribution of

craters.

Fig. 4 Cross-section image of both bonding case. Case (a) is the c-Si/Au bonding.

Au-Si allow seem not wetting on c-Si surface. Case (b) is the a-Si/Au bonding.

Au-Si alloy spread out on the a-Si surface.

Fig. 5 The SEM image of rate determine test. The degree of reaction that in

amorphous Si side is higher than crystal side.