Temporary wafer bonding – key technologogy for MEMS devices
In near future new solutions are required for miniaturized electronic systems with higher performance, lower cost and reduced thickness down to some 10 µm . Wafer thinning is one of these promising solutions which facilitates 3D integration of sensors or electronic systems. These systems consist of individual components, which are vertically stacked and have electrical connections. 3D integration requires the wafer thinning of both MEMS and CMOS devices. Efficient thermal management and a high through silicon via (TSV) density due to lower aspect ratio are some of the advantages of thinned wafers. Compared to thick wafers, the handling of thin wafers is challenging. Ultrathin silicon wafers down to 50 microns have low stability, low bending stiffness and cannot be handled with standard wafer processing equipment. In order to enable the fabrication of thin Si substrates or to stabilize them during the mechanical processing, it is necessary to temporarily attach these device wafers to carrier wafers [l]. For thinning process, grinding, etching and polishing are used. Grinding in two steps (rough and fine) removes most of Si material and leads to surface defects and imperfections. With a subsequent Si wet etching, these imperfections can be removed and the crystal lattice are restored. Finally the chemical mechanical polishing (CMP) generates a perfect surface with Ra < 1 nm. One method for thin wafer handling is provided by Brewer Science. This method involves spin coating of device wafer with an adhesive. The carrier is coated with a release material to remove the thinned wafer in a debonding process. Figure 1 shows the process flow for the thin wafer handling technology used in this work. First the device and carrier wafers are prepared. The wafers are then temporarily bonded and the subsequent back thinning is performed and finally the mechanical debonding takes place.
Usually, temporary wafer bonding technologies are divided into wet and dry techniques . For each group several specific technologies are available. Adhesive bonding technology falls into the group of wet process with different methods for the final debonding. The different approaches are: thermal slide, chemical, laser and machanical release. Fig.2 shows an overview for different temporary wafer bonding methods separated into wet and dry processes.
In temporary wafer bonding there are some traditional methods such as thermal slide release approach. New technologies now focus on room temperature debonding processes. Here Brewer Science provided the ZoneBOND technology, which is a combination of chemical and mechanical release. The main part of this technology is that the carrier is separated in two zones. During the bonding process, two zones are formed. In the middle of the wafer, there is a fragile adhesive region (zone 1). In contrast to the zone 1, there is a strong adhesive area on the wafer edge (zone 2). Beside the ZoneBOND material Brewer Science provides new materials with different requirements for the adhesive wafer bonding and de bonding processes. The BrewerBOND method employs either laser release or mechanical release. The laser release results in very low stress for the thinned substrate, but is only feasible with glass material which is transparent to laser irradiation between 270 nm and 360 nm wavelengths. The adhesive bonding materials used for ZoneBOND and new BrewerBOND are listed in Table.
The mechanical release needs an additional de bonding force to separate the wafers. The advantages of mechanical release are the low costs as well as the possibility of using different types of adhesive material. The process flow is significantly simplified compared to ZoneBOND in which the anti-sticking layer is coated on the complete carrier wafer. As a result the wet chemical etch or the laser release before de-bonding is not required. In comparison to ZoneBOND method the BrewerBOND technology has simple preparation steps. The comparative overview of these technologies is shown in Fig.3.
Before starting the wafer coating it is necessary to do an edge trim of the device wafer. Previous results shown, that the wafer edge is less susceptible for damage. The edge trim width is 2 mm with a depth of 260 µm. Fig.4 shows the wafer edge after the edge trim of an 8-inch wafer. After the thinning step the wafer diameter is reduce by 4 mm.
Experimental results using ZoneBond material
Spin coating of the device and the carrier wafers is carried on the semi-automatic RCD8 spin coater from SUSS MICROTEC. The device wafer is prepared by the deposition of the thermoplastic adhesive (ZoneBOND 5150-30). The subsequent curing takes place at 230 °C for 2 min. The viscosity of the adhesive is 10000 centipoise high. The adhesive layer thickness depends on chuck rotating speed. A layer thickness of around 20 µm requires a rotating speed of 1 600 revolutions/min with a dispensing quantity of around 5–10 ml (Fig.5).
The first step of the carrier wafer fabrication on the RCD8 is forming of a zone on the wafer edge with the adhesive ZoneBOND EM 2320-15. An exact centering of the carrier wafer on the vacuum chuck is important to get uniform coating on the wafer edge zone. The thickness of the layer is in the range of 0.5–3 µm. After baking at 220 °C, an anti-stick layer (ZoneBOND ZI 3500-02) is dispensed on the center of the carrier wafer. Compared to 400 centipoise of ZoneBOND EM 2320-15 the viscosity of the anti-stick layer is only 50 centipoise. Compared to the device wafer preparation the adhesive layer thickness of the carrier wafer depends on chuck rotating speed. A Layer thickness of around 2 µm requires a rotating speed of 300 revolution/m in with a dispensing quantity of around 1.5 ml. The adjusting adhesive width is around 1.5 mm (Fig.6).
The subsequent adhesive wafer bonding is realized by using a SUSS MicroTec SB8 wafer bonder. The adhesive wafer bonding is carried out under low vacuum (process pressure < 5 mbar) with a 170 kN/m2 bonding pressure using a bonding temperature of 200 °C. Figure 7 shows the wafer stack on a 6inch fixture and the formed intermediate layer with approximately 30 µm thickness after the adhesive wafer bonding in the SB8e.
One way to characterize the quality of the bonding interface is infrared microscopy. Fig.8 shows two examples after the adhesive wafer bonding process.
The left one shows clearly visible voids between the bonding partners. In contrast, in the bonding interface on the right side no voids are detectable. In previous work 5 to 5 mm structures are used to evaluate the bond strength by applying an increasing shear force until the interface cracks. A maximum shear strength around 8 MPa with a dicing yield more than 90% was obtained for the strong adhesive zone (Fig.9). Compare to zone 2, the yield and the bonding strength of zone 1 is very low.
After chemical, mechanical and thermal processes (T < 250 °C), the edge zone of the strong adhesive area is released with mesitylene. Then, the wafer stack with the device wafer face down is mounted on a tape frame and placed in the SUSS de-bonding system DBI 2T.
In the DBI 2T, the mounted wafer stack is fixed on both sides by vacuum. The mechanical separation of device and carrier wafers is done over the C-cut at the wafer edge. The carrier wafer is clamped over the blade and vertically moved on a flex plate over power cylinder at room temperature. A pressure roll is moving backwards and control the speed of debonding procedure. The schematic drawing including a picture of the DB 12T is shown Fig.10 and Fig.17.
In the final step, the device wafer on the mounted tape frame and the carrier wafer are cleaned in a combined puddle dispense- und spray process. Similar to the edge release process, the cleaning is performed in the SUSS MicroTec AR12 module. The used chemistry is limonene or mesitylene with a subsequent isopropanol rinse. With this process flow, a thinned wafer down to 50 µm can be fabricated (Fig.11).
In the cleaning step the wafer is rinsed with mesitylene. In puddle dispense process the arm rotate from right to left side for two seconds supported with ultrasonic. The solvent remove the adhesive from the wafer. The cleaning process needs to be as short as possible, because the UV-sensitive tape decomposed by the solvent mesitylene. The puddle dispense eliminate the whole adhesive layer in 15 min.
The tape loses their adhesive strength after 30 min in mesitylene bath. But the thinned wafer needs the tension of the tape (Fig.12).
Experimental results using BrewerBond material
The fIrst step of the device wafer processing on the RCD8 is the coating with the adhesive BrewerBOND 305 material. BrewerBOND 305 is a polymer material with a viscosity of 6 700 centipoise. Challenging is the dispensing without air inclusion. The thickness of the layer is in the range of 35–50 µm using a HEIDENHAIN measurement system. After baking at 220 °C for 2 min without proximity to the hotplate the device wafer is ready for further processing. The second step of substrate preparation before adhesive wafer bonding is the layer deposition on the carrier wafer with the BrewerBond 510 material. This release material enables the mechanical debonding without wafer cracking at room temperature. The expected datasheet layer thickness is around 5–6 run using speed rotation of 1 250 rpm. The ellipsometry measurement showed a thickness of the release material around 3.5 nm. The numeric model and the wafer map after thickness measurement of the BrewerBond 510 release material are represented in Fig.13.
The subsequent adhesive wafer bonding is carried out under vacuum (process pressure < 5 mbar) with 180 kN/m2 bonding pressure. With the infrared inspection an outgassing of the material was observed (Fig.14).
One is bonded at 200 °C with a heating and cooling ramp of 10 K/min. The next one has a longer hold time at 100 and 150 °C for 20 min. The last bonded interface show the wafer stack after heating the wafer (200 °C, 10 min) before coating. Nevertheless the subsequent wafer thinning of the 200 mm wafer is with the combined three step process (grinding, spin etching and chemical mechanical polishing) possible. With this conventional back grinding process the wafers can be thinned down up to 50 µm thickness. Regarding to silicon surface the roughness after grinding is around 14 run with a wafer thickness of 115 µm (Fig.15).
Regarding to the thickness variation control a subsequent fine grinding is favorable. With the downstream stress release spin etching process defects or micro cracks are reduced. In additional the surface quality of around 4nm is improved. With the final CMP step the perfect surface quality with a roughness smaller than 0.5 nm is realized (Fig.16).
Besides the mechanical wafer thinning there exist many other chemical and physical processes, which are partially tested and compatible to the adhesive wafer stack. These are:
• silicon dry etching;
• wet etching and cleaning process with different acids and bases like diluted hydrofluoric acid.
The most critical parameter is the temperature. Processing temperatures up to 300°C are possible.
After some tested processes the wafer stack with the device wafer face down is mounted on a tape frame and placed in the debonding system DB 12T (Fig.17). In the subsequent debonding process the device is separated from the carrier wafer. The debonding process of the 200 mm wafer is a room temperature peel-off process and identical to the mechanical release ZoneBOND process.
During the process the device wafer is fixed and the carrier wafer is vertically moved on a flex plate over two power cylinder. The debonding force is lower than 500 N and finished after less than 5 min. The cleaning is performed in AR12 module with mesitylene and subsequent isopropanol rinse compared to ZoneBOND process. In the cleaning step the wafer is rinsed by mesitylene. In the puddle dispense process the chuck is rotate right to left side for two seconds with changing the direction. The solvent remove the adhesive from silicon surface. Regarding the sensitive tape decomposed by the mesitylene the cleaning process needs to be as short as possible.
For the manual tape frame removing the wafer is placed on a vacuum chuck after UV exposure. With minimized nitrogen support the 50 µm thin wafer can be taken from the vacuum chuck by hand (Fig.18).
In sununary the increasing of functionalities in one system in parallel with the component size reducing, required the 3D MEMS integration. One key technology is the temporary bonding/debonding method with a high priority in 3D IC.
Compared to the 3D IC in the MEMS integration there exist different challenges.
• different functionalities (like Optic, Mechanic, Fluidic plus Electronic);
• many technologies and materials (like silicon, ceramic, glass, metal or polymers);
• sensible components with the need of a hermetical package;
• TSVs with high aspect ratio and large dimensions.
In the summary there exist no standard solution. It depends on the application. Actually in 3D-MEMS integration the complex ZoneBOND two zone approach is used. But Brewer Science presented new adhesive material called BrewerBOND which allow temporary wafer bonding processes to produce 50 µm thin wafer with subsequent laser or mechanical release process. The process flow is significantly simplified. Compared to ZoneBOND, the anti-sticking layer is coated on the complete carrier wafer. As a result the wet chemical edge release before de-bonding is not required. In additional the material enables backside temperature processing up to 300°C. ■