Pressure indicating film characterization of wafer-to-wafer bonding
08/01/2009
Wafers are bonded by applying precise combinations of physical pressure, temperature, and/or voltage. Pressure is measured as an average, assuming perfectly flat pressure plates. Applied pressure characterization is important for high yielding eutectic/thermocompression bonds.
Kwan-yu Lai, Micralyne, and Jeffrey G. Stark, Sensor Products
Wafer-to-wafer bonding has become an enabling semiconductor technology in industries such as 3D packaging, MEMS, MOEMS, and SOI. In a typical wafer bonding process, two flat substrates are permanently joined (bonded) to one another by applying precise combinations of physical pressure, temperature, and/or voltage (Figure 1). Each of the above factors is set depending on the substrate materials being bonded, and the control of these parameters is crucial to a successful, high-quality, high-uniformity manufacturing process.
 Figure 1. Wafer bonding process. |
Of these major parameters in a bond recipe, voltage and temperature are readily measurable within a wafer bonding chamber using common electronics and thermocouples. Pressure, on the other hand, is measured in the tool as the total amount of force exerted over the pressure column. This measured force is then used to calculate the average pressure, assuming perfectly flat pressure plates. In practice, the pressure plates are often non-ideal, or they may have degraded over time. This leads to potential pressure variations which would not be detected with control software alone. Such poor distribution of pressure can lead to unbonded wafer areas, cracked wafers, or even premature wear of the pressure plates.
The significance of a uniform applied pressure in a bonding process depends largely on the specific materials being bonded. For example, in an anodic bonding process, silicon is bonded to glass (typically Pyrex) by applying a large electric field (e.g. 1000V) at elevated temperatures (e.g., >300??C). At such temperatures, sodium impurities in the bulk of the normally insulating glass becomes mobile, thus making the glass much more conductive. When a high voltage is applied to the anode in this state, the sodium ions move toward the anode, leaving oxygen ions at the bond interface. The reaction between silicon and oxygen forms a strong SiO2 bond.
The applied voltage also creates a large electrostatic force on the bond stack, which assists with the bonding process. Because the magnitude of the electrostatic pressure is generally sufficient for a full bond, physically applied pressure is neither critical nor required for this type of bond process.
 Figure 2. Magnified image of a wafer bonding fixture shown with pressure sensor film in place. |
However, in an eutectic/thermocompression bonding process, two arbitrary substrates are bonded together using thin intermediate films that are often metallic alloys (Fig. 1). A common bond metal for silicon is Au-Si eutectic bond with a eutectic temperature of 363??C. In this bond, the Si surface contacts Au deposited on the other substrate, and the stack is brought to a temperature just beyond the eutectic point for a short time to allow the alloy to form. Given a fixed temperature, if too much pressure is applied, the eutectic alloy can spill out into unwanted regions and cause short circuits. Conversely, too little pressure would typically result in weakly bonded or unbonded regions. And in practice, spill outs and unbonded regions are often found on the same pair of substrates due to pressure and/or temperature non-uniformities. Therefore, the characterization of applied pressure is important for these bond processes to achieve high yields.
Pressurex film (Sensor Products) is a direct and economical way to detect and correct such pressure variations. The thin flexible film measures pressure from 2???43,200 PSI (0.14???3,000 kg/cm2). When placed between contacting surfaces of a wafer bonding fixture it instantly and permanently changes color directly proportional to the amount of pressure applied. The precise pressure magnitude is determined by comparing color variation results to a color correlation chart (much like interpreting Litmus paper).
 Figure 3. Bond tool images show pressure inconsistencies. |
By running a bond recipe with the pressure set to 4 bar on an appropriate grade of pressure film, a direct imprint is formed. Figure 3 shows Pressurex Micro sensor film 2???20 PSI (0.14???1.4 kg/cm2) taken from a 6" diameter bonding tool with poor pressure uniformity. Analyzing the pressure distribution with the Topaq Tactile Force Analysis System, this image is transformed into a color-coded pressure map, revealing a high-pressure ring (>10 bar) with relatively little pressure applied at the center. The line scan further elaborates these pressure inconsistencies.
 Figure 4. Bond tool images show improvement to the pressure uniformity as captured by pressure-indicating film. |
A series of adjustments to the pressure column of the bond tool were made, and the pressure uniformity was checked each time by running the same bond recipe on the same range of pressure film. The resulting series of images are shown in Figure 4, which confirms that the actual pressure is more uniform. After the adjustments, the pressure film analysis shows an offset from the intended recipe pressure of 4. By using properly calibrated pressure film, the offset can be corrected. Similarly, it can also be used to match processes across multiple bond tools.
The same pressure film can be used as a tool performance log in manufacturing practices such as Six-Sigma statistical process monitoring. Cost savings will inure to users of pressure indicating film through decreased scrap rate and increased time efficiency. There are also specific benefits that are distinct to each type of bonding application.
Metal eutectic bonding
Pressure film prevents the eutectic alloy from spilling out into unwanted regions and causing short circuits, which, given a fixed temperature, can occur if too much pressure is applied. It also can minimize weak bonded or un-bonded regions that occur if too little pressure is applied. Pressure film reveals the magnitude and distribution of pressure across the bonding platen and part.
Anodic bonding
Here, the film reveals whether the top and bottom plates are in uniform contact.
Fusion bonding
The pressure film can help minimize trapped air pockets between the bonded substrates, which on certain applications can be caused by non-uniform applied pressure.
Metal diffusion bonding
Using the film can help minimize un-bonded wafer sections, detecting if pressure is too low. Wafers won’t bond if the forces are too low.
Glass frit bonding
Pressure indications can ensure hermetic seal is formed around the device, which will not occur if pressure is too low. Overly high pressure could prevent the glass frit from flowing into the device.
Polymer adhesive bonding
Use pressure measurement to minimize voids caused by polymer thickness non-uniformity. While this is not a direct problem related to the amount of pressure, pressure non-uniformity can exacerbate the problem.
Conclusion
Pressure-indicating film is a quick and direct research tool that provides a snapshot of the pressure distribution of a bond tool at room temperature. Through calibrated post analysis, it also provides a method to compare processes and tools implemented during manufacturing.
References:
- U. S. Patent No. 3,397,278, Wallis and Pomerantz, “Field Assist-ed Glass-Metal Sealing”, Jour. of App. Phys. , Vol. 40, No. 10, September, 1969,
- Bonding in Microsystem Technology, Jan A. Dziuban, Springer 2006
Kwan-yu Lai is R&D engineer, development engineering department, at Micralyne Inc. [email protected], www.micralyne.com.
Jeffrey G. Stark is president of Sensor Products Inc. [email protected], www.sensorprod.com.