Room-temperature Hermetic Packaging Using Ultrasonic Cu – Cu Bonding with Compliant Rim

In this paper, hermetic packaging using room-temperature (RT) Cu–Cu bonding is demonstrated for microsystem packaging. The bonding of the compliant rim made of Cu to electroplated Cu was performed with ultrasonic assist at RT. Owing to the cone-shaped cross section of the compliant rim, it easily deforms under pressing load with ultrasonic assist and bonds to the counter Cu film at RT. As a result of a simple vacuum test, the air leak rate of the cavity inside the bonded sample was approximately 2 × 10−11 Pa·m3/s, which may be sufficiently low for applications of vacuum packaging of microdevices.


Introduction
(3)(4)(5)(6) In addition, once microdevices are packaged into a single chip or wafer, it is easy to handle them in a subsequent process.In the packaging of microdevices, anodic bonding, (7,8) glass fritting, (9) and solder bonding (10) are the commonly used bonding technologies.However, anodic bonding requires an elevated temperature above 300 °C and a high voltage.Besides, the materials that can be used for anodic bonding are limited to the combination of glass and a conductive material.Glass fritting and solder bonding also require a high-temperature process because of their high melting points.The high-temperature process may degrade the performance of the sealed devices and constrain not only structural design but also the materials involved.
Recently, metal-to-metal bonding has attracted interest in the three-dimensional (3D) microsystem stacking multilayers of active electronic components because of excellent electrical interconnections and enhanced heat dissipation.(13) Therefore, the development of room-temperature (RT) Cu-Cu direct bonding has been of great importance for not only hermetic packaging but also 3D heterogeneous integrations such as LSI and MEMS integrations.
The direct bonding methods of Cu include thermocompression bonding, surface-activated bonding, and ultrasonic bonding.Thermocompression bonding is a well-known bonding method for metals (e.g., Au-Au (14) and Cu-Cu (15)(16)(17)(18) ).It is usually performed at elevated temperatures of 250-400 °C to obtain sufficient bonding strength.Compared with Au-Au bonding, Cu-Cu bonding in ambient air is difficult since the Cu surface is easily oxidized.A Cu-Cu metallic bond is formed at the bonding interface by atom diffusion across the oxide layer at a high temperature for a long time.In surface-activated bonding, the RT bonding of Cu (19)(20)(21) has been demonstrated for microsystem packaging.A large bonding strength can be achieved by the attractive force between atomically cleaned bonding surfaces using a dry etching process such as argon fast atom beam bombardment.In general, Cu-Cu bonding is performed under ultrahigh vacuum to avoid the reoxidation of the activated surface.Although this bonding method is one of the promising candidates, the bonding surface should be strictly planarized (22) and the bonding environment cannot be chosen freely.Recently, low-temperature Cu-Cu bonding has been achieved using a surface activation process based on the reduction effect with formic acid treatment. (23)(26) The low-temperature Cu-Cu bonding of conventional bumps in ambient air has been investigated using ultrasonic assist with surface treatments. (27)(30)(31) RT bonding in ambient air has been reported using mechanical caulking with Cu compliant bumps. (28)However, the fabrication cost significantly increases because the slit formation in the counter electrode requires additional photolithography.To reduce the fabrication cost, we have proposed the RT bonding of Cu compliant bumps to a Cu planar electrode with ultrasonic assist. (30,31)Recently, RT vacuum packaging has been demonstrated using compliant rims made of Au with ultrasonic assist. (32)A large Au-Au bonding strength could be achieved, and the ability of the compliant rim was experimentally verified.Therefore, we have focused on compliant rims made of Cu. (33) In this study, we demonstrated RT hermetic packaging using a Cu compliant rim for microsystem packaging.The RT bonding of an electroplated Cu film to the Cu compliant rim was performed in ambient air with ultrasonic assist.The bonding quality and sealing performance were evaluated by the die shear and hermeticity tests.

Materials and Methods
Compliant rims made of Cu were fabricated on 4-inch-diameter Si wafers.Figure 1 shows a schematic illustration of the process of fabricating the Cu compliant rim (top size: 2 μm; base size: 10 μm; height: 9-10 μm).First, a TiW film (thickness: 250 nm) and a Au film (thickness: 80 nm) were deposited by sputtering in sequence on a thermally oxidized Si wafer as a seed layer.Next, a photoresist pattern having undercut holes was formed by photolithography. (32)e used a chemically amplified negative-tone photoresist.After that, Cu electroplating was applied to fill these undercut holes.The photoresist was removed with acetone.Then, seed layers were removed by ion etching.The Si wafers with Cu compliant rims were cut into 5 × 5 mm 2 chips using a dicing saw.Figures 2(a) and 2(b) show SEM images of the Cu compliant rim on a Si chip.The Cu compliant rim is a square-shaped frame of 500 μm length on one side.On the other hand, Cu sealing frames on glass wafers (width: about 10 μm; height: about 3 μm) were fabricated by standard photolithography and electroplating.The glass wafers were cut into 7 × 7 mm 2 chips using a dicing saw.
Prior to bonding, the chips were chemically cleaned with H 2 SO 4 , KHSO 4 , and CH 3 COOH to remove contaminants from the Cu surface. (30,31)Subsequently, the RT bonding of glass chips to Si chips was carried out by ultrasonic Cu-Cu bonding in ambient air. Figure 3 shows a schematic illustration of the ultrasonic bonding of Si chips to glass chips.(32) The ultrasonic amplitude was 1.5 μm and the ultrasonic frequency was 48.5 KHz.The bonding time was 1.5 s and the pressing load was 40 N/rim.

Results and Discussion
Glass chips were successfully bonded on Si chips at RT.A die shear test was performed in order to measure the bonding strength of the bonded samples.The bonding strength was about 3.5 N/rim.Figure 4 shows the bonding strengths obtained from Au-Au and Cu-Cu bondings carried out under the same ultrasonic conditions, namely, an ultrasonic amplitude of 1.5 μm, an ultrasonic frequency of 48.5 KHz, and a bonding time of 1.5 s.In the RT bonding of the electroplated Au film to a Au complaint rim, a large bonding strength was obtained under the pressing load of 10 N.However, for Cu-Cu bonding, a similar bonding strength could not be achieved under the same conditions.When the pressing load was increased to 40 N, a large bonding strength was obtained.Compared with Au-Au bonding, Cu-Cu bonding requires a larger pressing load since Cu is harder than Au.After the bonding with the pressing load of     in water.This test is simple and a good indicator of gas leakage. (32)Figure 6(b) shows an optical microscopy image of a sealed area after immersing it in water.The dark and bright areas represent penetrated water and air, respectively.We can clearly see that the water does not penetrate into the sealed Cu package.This result indicates that no leakage can be identified from the sealed Cu package and strong Cu package sealing was achieved.
A simple vacuum test was also performed.A through hole (diameter: 100 μm) inside the sealing frame on a glass chip was formed for the vacuum test.After that, a Si chip was bonded to a glass chip with a through hole.As shown in Fig. 6(a), the cavity inside the package was evacuated using a turbomolecular pump (TMP) through the hole.The pressure in the cavity was approximately 8.7 × 10 −6 Pa, which is close to the value obtained from evacuation without the test piece (pressure: 4.6 × 10 −6 Pa).After the gate valve was closed, the change in pressure for the cavities was measured using the vacuum gauge.The volume of the vacuum chamber and cavity was approximately 2.5 × 10 −3 m 3 .Figure 6(b) shows the time dependence of the chamber with the test piece.We can see that the pressure was maintained at less than 1 × 10 −2 Pa after 400 h.As calculated from this data, the air leak rate was approximately 2 × 10 −11 Pa•m 3 /s.This value might be sufficient for the vacuum sealing of microdevices.Our RT Cu-Cu bonding technology indicates the good bonding quality for hermetic packaging.

Conclusions
We demonstrate RT hermetic packaging using a Cu compliant rim with ultrasonic assist for microsystem packaging.Strong package sealing was achieved by the Cu-Cu bonding and good sealing performance was confirmed.As a result of the simple vacuum test, the air leak rate is approximately 2 × 10 −11 Pa•m 3 /s, which may be sufficiently low for use in the vacuum packaging of microdevices.These results show the potentials for 3D heterogeneous integrations such as CMOS and MEMS integrations that require an RT process.

Fig. 2 .
Fig. 2. (a) SEM image of Cu compliant rim on Si chip and (b) magnified view of (a).

Fig. 5 .
Fig. 6. (Color online) (a) Schematic illustration of vacuum simple test and (b) time dependence of pressure of a vacuum chamber with a test piece.(a)(b)