PSA CHALLENGES AND APPLICATIONS IN SEMICONDUCTOR MANUFACTURING

Transcription

1 PSA CHALLENGES AND APPLICATIONS IN SEMICONDUCTOR MANUFACTURING Laurent Vésier, Senior Quality Engineer, Rohm and Haas Electronic Materials CMP Technologies, Newark, DE Jason Lawhorn, Product Engineering Technician, Rohm and Haas Electronic Materials CMP Technologies, Newark, DE George Feeley, Senior Materials Engineer, Rohm and Haas Electronic Materials CMP Technologies, Newark, DE Overview: Semiconductors and Chemical Mechanical Planarization Chemical Mechanical Planarization (CMP) is used increasingly in today s semiconductor chips manufacturing. CMP was first used for polishing Tungsten interconnects and Silicon Dioxide (SiO2) inter layers. Currently, CMP is an enabling technology in the manufacture of semiconductor chips, including the more advanced devices using Copper and new dielectric materials. The CMP process requires a polishing pad and slurry combination to planarize wafers used in the construction of semiconductor devices. (Figure 1 represents a wafer planarization step with a polishing pad). Adhesives are used in providing adherence between layers of the polishing pad and to adhere the pad to the rotating table. Economic trends in matured parts of the CMP process also require increased consumables lifetime and, therefore, reduced failures of the adhesive layers with increased pad life. Long term reliability of the adhesives in demanding liquid environments (high temperatures, low ph) is, therefore, a constant expectation. Manufacturing Operations Rohm and Haas Electronic Materials CMP Technologies manufactures polishing pads used to planarize semiconductors (called wafers due to the thin configuration). The polishing pads used in these operations contain pressure sensitive adhesives (PSAs) between pad layers (different pad layers are used to give the pad different polymeric properties that will in turn allow it to respond to different polishing recipes). The PSAs are also used to adhere the pad to the rotating table (platen) while planarizing the wafers. Rolls of PSAs are used during manufacturing to laminate the pads. Lamination is achieved using standard industrial equipment and may include the use of heat as appropriate but is generally done using roller pressure. Customer Requirements and Selection Process Customer requirements as they relate to PSA selection can be categorized in the following subgroups: - Reliable adherend adhesion (no delamination of the layers as well as consistent adhesion to the platen, and PSA integrity)

2 - Ease of application and ability to reach the appropriate bond strength without air entrapment. Some amount of repositioning ability is required. - Ease of removal from the platen after the pad is consumed (worn) due to ergonomic considerations (low pulling force required). - Cleanliness (no visible spots or inconsistent surfaces by color or texture, no visible embedded particles) - Consistent key product characteristics (high manufacturing process capability) Selection of the PSA to be used between the top pad and sub-pad layers is usually based on semiconductor polishing end use conditions. Selection is based on the chemicals used during the polishing process, the pressure used during the polishing operation, the temperature reached on the surface of the pad due to friction between the pad and the wafer and the chemical nature of the polymers used in the pads. Shear stresses induced by pressure on the wafer and chemicals used during polishing can have a deleterious effect on the PSA performance. Cleanliness As the semiconductor industry is continuously faced with feature size reduction, the polishing pad industry is faced with contamination reduction challenges. A particle embedded in polishing pad layers may impact the defectivity of the overall polishing process by impacting the local deformation of the pad. Another potential effect of particle contamination is the potential leaching of contaminants into the slurry stream used in the polishing process. Examples of contaminants found in PSA could include: - fibers - metallic contaminants, - stains from oils used in PSA roller lubricants A picture of a potential contaminant is shown in Figure 2. The manufacturing operations used in polishing pad s manufacturing are Class 100 certified (Figure 3). Globally, nearly all fabs that polish wafers use Class 1000 clean rooms, so paper and paper by-products are strictly forbidden in those areas. For the platen side, this necessitates a PSA that has a non-paper release liner. Rohm and Haas Electronic Materials CMP Technologies uses a 2 mil clear polyester on a double-coated rubber-based adhesive. All our customer surveys report that our clear release liner is a positive, since the individual operators can do a pre-inspection of the pad before applying it to the polishing platen. A $500 pad could be polishing $50, worth of chips at any given time. As a result, dirt or foreign particles cannot be tolerated on the pad and are rejected in the manufacture of the pad. In the early years of the CMP business we used a series of inspection steps to remove all pads with defects. Now we are also actively involved with our vendors to improve the cleanliness of the PSA we

3 apply. Our suppliers improvements and our Shine Program has been effective in reducing contamination issues. Feedback to our adhesive vendors helps institute improvements in the level of cleanliness we receive with each PSA roll we purchase. Eliminating all sources of contamination and variation is a necessity for each manufacturer to maintain high device yields. Leading PSA suppliers to the CMP market are faced with similar demands. PSA Failures in CMP Use Failure issues encountered with the use of PSAs in polishing pads can be classified in two categories: - Loss of adhesion of the pad to the platen or intermediate layers of the pad during polishing (referred to as delamination). Failures can have various degrees of severity (delamination on the whole surface or localized, for example at pad edges). Those failures result in productivity losses, potential breakage of wafers and unacceptable production downtime. - Pad s being too hard to remove from the platen after the polishing process is completed. The consequence of this kind of failure is that more strength is required from the operator to pull the pad off the platen for changeover of the pad, potentially creating safety issues from ergonomic factors. Time dependencies and rheological properties of the adhesives are a prime consideration for this kind of long term behavior. Pad Application Initial surface contact between the adhesive and the platen is critical to proper adhesion. Clear, plates were used to understand the effect of the application technique on PSA surface contact. As shown in Figure 4, adhesive to platen contact results in a gray color while the absence of contact appears as the adhesive s natural tan color. Four methods of application were evaluated (industry standard is application by hand with a light weight puck): Application with hand smoothing over the top of the pad limited pressure application Application using the industry standard method with moderate force Application using the industry standard method with high force Application using a 4.5 lb roller with no operator added force potential method The four pictures in Figure 5 show the results of the applications described above. The limited pressure application shows a distinctly lower surface contact area when compared to the moderate and high force application methods. The contact difference between excessive and moderate force is limited, but visible. A significant increase in contact is realized with a roller method evaluated in this study. The ability of the roller to concentrate force in a small area increases the uniformity of the applied pressure.

4 Test Methods and Customer Application Standard adhesive test methods such as peels to glass often use high pressure for the sample preparation when compared to the forces described above for CMP pad application to a platen. This could explain the lack of correlation between end user failure and adhesive test values. Understanding the effect of reduced application pressure on adhesive bond strength may help relate adhesive characteristics and physical properties to the performance of CMP pad platen adhesives. T-Peel versus Applied Pressure Pressure Sensitive Adhesives properties are such that they are generally set by light forces applied at PSA / adherend interfaces. While initial bonds may require very slight pressures, the time dependency of that bond during use may be more critical and dependent upon the initial pressure given at the PSA/ adherend interface. It is also difficult to judge the level of adhesion of a PSA during use unless some calibration is used with relevant parameters (for example with the use of pressure measuring instruments). The stresses applied during use may also impact bond reliability and change the required amount of initial applied pressure. The graph below (Figure 6) shows changes in peel to glass (similar to PSTC-1 (2) except that these were performed on ½ wide specimens) versus applied pressure. Time factors are not considered in this graph as testing was performed immediately following application. As one can see, varying force can lead to varying initial adhesion as measured by the peel test. The increasing amount of initial peel is dependent on the actual contact pressure exerted by the roller. The contact pressure exerted by the roller also depends on the roller diameter (1). While the 4.5 lb. roller used in PSTC tests is appropriate for machine laminations, manual application of PSA could be better represented by test methods using roller weights lower than 4.5 lbs. An opportunity for improvement in the end use application of the PSA could also be to understand the pressure necessary to achieve a reliable bond over a prolonged period of time in customer use. New fundamental application tests to characterize this time and pressure dependency could be developed by PSA manufacturers so that end users can make deliberate decisions about the utilization of consumables for their equipment. This would help reinforce the importance of application procedures. As an example, if time is required to build bond strength for pads adhering to a platen, this time could be specified by PSA suppliers in guidelines for use of the product. Application pressure, temperature of the substrate at application and dwell time are significant variables that impact adhesion. Generic dwell times for adhesion build up have been communicated by the PSA industry but are rarely specified in PSA products. Test Methods and Environmental Conditions The ability to predict PSA behavior during use has been mainly discussed in the literature as being dependent on the following characteristics: shear and peels (2,3). The same tests have been helpful in understanding PSA behaviors immersed in slurry. Samples of polymeric pad/psa/subpad were

5 immersed in liquids of interest to understand performance during customer use of the pads. T-Peel test results were recorded prior to immersion in the liquid and recorded after immersion in the liquid. The specimens were immersed for 48 hours in an acidic and oxidizing formulation representing slurries used in the CMP industry. The specimens were 1 wide by 10 long, machine laminated and completely immersed in the chemical substance for 48 hours at 23 C. The test containers were covered to prevent excess evaporation. At the end of the testing period, the test specimens were conditioned for 1 hour at room temperature. For each condition, a total of six specimens were used to calculate the T-peel prior to immersion and after immersion. The calculated % loss in T-peels can be represented in Figure 7. The Y-axis of the graph in figure 7 represents the % loss in peel after and before immersion in the liquid of interest, i.e., the PSA part called Rubber-based 1 had a 12% lower peel after immersion than prior to immersion. Generally, the Rubber-based PSAs appear to have performed better overall than the Acrylic-based PSA in the acidic and oxidizing environment. The results in Figure 8 showed also that the % loss T-peel after immersion were related to after immersion shear results. The X-axis on the chart in Figure 8 shows the failure time of different adhesives on a polymeric substrate. The tests were conducted at room temperature and measurements of peel changes were conducted before and after immersion. Shear times to failures were recorded after immersion in the same slurry composition. The Y-axis depicts the % loss in T-peel results on the same substrate as those tested in the modified shear tests. The % loss is calculated from T-peel results for a particular adhesive/ substrate combination before immersion and after immersion in an acidic, oxidizing environment. In this case, high T-peel changes after immersion, an undesired result, were related to short failure times as measured by the shear test. Summary: It has been the authors experience that reliability of adhesives can be largely affected by immersion in liquids. The Rubber-based adhesives have generally shown better reliability as measured by a combination of T-peel and shear tests than Acrylic-based adhesives in an acidic, oxidizing environment. End use adhesion in products requiring manual application is more dependent on consistent product characteristics than machine laminated interfaces. Fundamental tests to characterize optimal application techniques could be developed to further the understanding of end use of PSAs in these applications with pressure, temperature and dwell time as specified variables. PSTC tests with roller weight may need to include comments indicating that lower weights applied at end use may not be representative of typical end-use performance. References: (1) K. L. Johnson, Contact Mechanics, (Cambridge University Press, Cambridge, 1987)

6 (2) Test Methods for Pressure Sensitive Adhesive Tapes 12 th Edition, Pressure Sensitive Tape Council, 1996 (3) FTM-17- Chemical Resistance-Immersion method- FINAT Technical Handbook 6 th Edition, 2001 Illustrations: Slurry dispensing ω wafer ω platen wafer Polishing pad PSA layers Rotating table Beneath pad Figure 1. Example of Polishing with a Pad Adhering with PSA to a Rotating Table 0 Figure 2. Type of Common Contaminant Found on a PSA Surface (0.04 in)

7 Figure 3. Rohm and Haas Electronic Materials CMP Technologies CMP pads are transferred using a pass-through to isolate the inspection area from the manufacturing area. Each pad goes through a final inspection before being packaged in a Class 100 environment. No adhesive to platen contact. Adhesive to platen contact. Figure 4. Clear Platen for Evaluation of Surface Contact

8 Limited Pressure Application Moderate Pressure Application Excessive Pressure Application Potential Method (4.5 lbs roller) Figure 5. Applications with Different Pressures

9 Peel to Glass Roller force on PSA Figure 6. One-way Analysis of Peel to Glass (oz) by Roller Fforce on PSA (lbs), on ½ Wide PSA %loss in Tpeel PSA / Substrate (before and after 48 hrs ph2) by PSA type Acrylic based Rubber based 0.0 Rubber based 1 Rubber based 2 Rubber based 3 Acrylic based 1 Acrylic based 2 Acrylic based 3 Figure 7. Before and After Immersion % Loss in T-peel at the Polymer Pad / PSA/Pad Interface

10 Peel Change after immersion, % Modified acrylics Rubber based Shear test time to failure, min Figure 8. Relationship of T-Peel Change after Immersion versus a Shear Test after Immersion for Acrylic and Rubber-based PSAs (Immersion at Room Temperature).

11 Laurent Vésier Laurent Vésier is a Senior Quality Engineer leading improvement projects with Rohm and Haas Electronic Materials, CMP Technologies, Inc. Prior to that position, he was a Materials Engineer with Rohm and Haas Corporate Asset Management Group Mr. Vésier has been teaching and using Six Sigma process improvement tools in his current position since joining Rohm and Haas Electronic MaterialsMr. Vésier began his career at the Institute of Paper Science and Technology as a Research Scientist. Mr. Vésier is a Master of Science graduate of Georgia Institute of Technology