Dow Corning WL-5150 Photodefinable Spin-On Silicone

Transcription

1 Dow Corning WL-515 Photodefinable Spin-On Silicone Properties and Processing Procedures Introduction Dow Corning WL-515 is a silicone formulation which can be photopatterned and cured using standard microelectronics processing techniques. The silicone material is prepared in mesitylene with a solution viscosity of 45 cp. The standard method for application of Dow Corning WL-515 is by spin coating although other methods of application (such as spray coating and extrusion coating) may also be applicable. Application, patterning and cure of Dow Corning WL-515 is accomplished in a simple six-step process that utilizes commercially available equipment. Updated information on the processing of the Dow Corning WL-515 materials is available from your local Dow Corning representative. Material Properties Dow Corning WL-515 can be solvent cast to form free standing monoliths or spincoated onto substrates to form sub 5 µm thick films. The mechanical properties of Dow Corning WL-515 thin film (2 µm) and bulk samples (~ 2 mm thickness) are listed in Table 1 for samples cured at 25 C for 3 minutes. The room temperature modulus of Dow Corning WL-515 has been measured by a continuous stiffness (CSM) nanoindentation technique on spin-coated thin films. Figure 1 shows a typical modulus vs. displacement curve for a 2 µm thick film of Dow Corning WL-515 cured at 25 C for 3 minutes. The plot shows the change in the modulus as the indenter tip penetrates into the film surface up to a maximum penetration depth of 22 nm. Modulus (GPa) Mechanical Property Value Nanoindentation Modulus (MPa) 31 Nanoindentation Hardness 9.5 (MPa) Film Residual Stress on Si 2.6 Wafer (MPa) Bulk Modulus (MPa at 25 C) 16 Bulk Tensile Strength (MPa) 6. Bulk Elongation (%) 37.6 CTE (ppm/ C) 236 Table 1. Mechanical properties of Dow Corning WL Displacement Into Surface (nm) Figure 1. Modulus vs. displacement curve for Dow Corning WL-515 (frequency = 75 Hz). Dow Corning WL-515 provides a low modulus, low stress film after cure with a film surface modulus of approximately 3 MPa. The increase in modulus with indenter displacement into the film is due an increasing substrate effect with penetration. The dimensional change of Dow Corning WL-515 as a function of temperature is shown in Figure 2. A linear expansion is observed throughout the temperature region of 1 to 3 C. The thermal stability characteristics of Dow Corning WL-515 cured at 25 C for 3 minutes are listed in Table 2. Good thermal stability and modulus retention have been demonstrated to a temperature of 3 C. 1

2 Dimension Change (µm/m) 1e+5 8e+4 6e+4 4e+4 2e+4 Cured films of Dow Corning WL-515 show excellent optical transparency down to a wavelength of 22 nm. The UV-visible spectrum for a 24 µm thick 25 C cured film on a quartz substrate is shown in Figure 4. The electrical properties and moisture absorptivity for Dow Corning WL-515 are listed in Table Figure 2. Dimension change vs. temperature for Dow Corning WL-515. Condition Weight Loss (%) Isothermal at C/6 minutes Isothermal at C/6 minutes Dynamic to 25 C 1. (1 C/minute) Dynamic to 3 C 1.7 (1 C/minute) Table 2. Thermal stability characteristics for Dow Corning WL-515. The weight loss profile for Dow Corning WL-515 is shown in Figure 3 for a sample cured at 25 C for 3 minutes. Weight Percent Remaining Temperature ( C) Percent Transmittance Wavelength (nm) Figure 4. UV-visible spectrum for a cured film of Dow Corning WL-515. Electrical Property Value Dielectric Constant (1 MHz) 3.2 Dissipation Factor (1 MHz).7 Leakage Current (A/cm 2 ) 1 x 1-11 Breakdown Voltage (V/cm) 3.9 x 1 5 PCT Moisture Absorption (%).24 Table 3. Electrical properties and moisture absorptivity for Dow Corning WL-515 measured on monolithic specimens. Material Shipping and Storage 9 3 Dow Corning WL-515 is shipped cold 88 2 with blue ice at a temperature of 86 approximately 5 C. If the solution arrives at its destination warm please contact your 8 local Dow Corning representative. Upon Time (minutes) receipt the solution should be immediately stored at a temperature of 15 C. Prior to Figure 3. Thermal weight loss for a 25 C/3 minute cured Dow Corning WLuse the solution should be allowed to equilibrate to room temperature. 515 sample analyzed under helium. During use Dow Corning WL-515 should not be allowed to remain at room 2

3 temperature for more than 3-4 days to maintain good performance. Material Waste Handling In disposing of Dow Corning WL-515 or rinse and developing solvents containing Dow Corning WL-515 it is important that these do not come into contact with basic materials (including ammonia, alkyl amines, ammonium hydroxides, alkyl ammonium hydroxides, metal hydroxides) as well as oxidizing metals and oxidizing agents. These in combination with water can result in the evolution of hydrogen gas from the composition. Consult the MSDS provided for additional information on materials waste handling. Processing A step-by-step summary of processing for Dow Corning WL-515 is presented schematically in Figure 5 and in Table 4. 1 Spin Coat 2 Soft Bake 3 UV Exposure 4 Post Exposure Bake 5 Solvent Develop 6 Hard Bake (Cure) Figure 5. Application, patterning and cure process steps for Dow Corning WL-515. The process consists of six general process steps to arrive at a cured patterned film. The target thickness is approximately 22 µm. Process Step Standard Conditions 1. Spin Coat a) Spread 5 rpm/1 sec. b) Spin 5-25 rpm/3 sec. c) Edge Bead 15 rpm/3 sec. 2. Soft-bake Hot Plate 11 C/12 seconds 3. UV Exposure i-line or Broadband 1 mj/cm 2 4. Post-exposure Hot Plate 15 C/12 sec. Bake 5. Development Stoddard Solvent Automated Development Process (see Step 5 comments) 6. Hard Bake Oven (Air, N 2 or vacuum) 15 C/12 min. or 18 C/6 min. or 25 C/3 min. Table 4. Processing conditions for Dow Corning WL-515. Step 1. Coating Process Surfaces for coating Dow Corning WL- 515 should be free of surface impurities prior to coating. It is recommended that the spin-coaters used to deposit the solutions be equipped with a backside rinse capability and bowl exhaust. The coater should also be equipped with two dispense heads, one for dispensing Dow Corning WL-515 and a second for dispensing a solvent for edge bead removal. Depending on the size of the substrate approximately 8-1 ml of Dow Corning WL-515 is applied to a wafer in either static or dynamic dispense modes. The wafer speed is then set to 5 rpm for 1 seconds to allow the solution to spread across the wafer. The speed is then increased to achieve a uniform film coating across the wafer. A speed of 15 rpm is recommended to achieve a film thickness of 3

4 approximately 22 µm after full processing (completion of steps 1-6). Expansion of the spin speed range between 5 and 3 rpm allows for final fully processed film thicknesses ranging from 4 to 15 µm respectively. During the spin step it is recommended that a backside rinse be implemented to eliminate polymer string (cotton candy) formation at the wafer edge. Edge bead removal is conducted by spinning the wafer at speeds not faster than the spinon speed and applying mesitylene or a Stoddard type of solvent (such as Air Products Negative Resist Developer ) to the topside of the wafer to remove the bead formed at the wafer edge. The solvent can be applied using a dispenser or manually by syringe. Step 2. Soft-bake After coating, the film produced from Dow Corning WL-515 is subjected to a low temperature bake to remove residual solvent. The standard process is to heat the wafer on a hotplate to a temperature of 11 C for a period of 2 minutes. After the soft-bake the film will remain tacky; however, the tackiness can be reduced by further heating the film on a hotplate at a temperature of 11 C for up to 5 minutes. It may be desirable to perform an oven rather than hotplate soft-bake. Any oven process will need to be optimized to the equipment used. If such an approach is used it is important that the coatings remain in a horizontal rather than vertical orientation to avoid a downward flow of the material on the substrate. Step 3. UV Exposure UV-light exposure of films prepared from Dow Corning WL-515 causes the activation of a photo-sensitive compound in the formulation that results in selective cross-linking (setting) of the UV-light exposed regions during the post-exposure bake (Step 4). As the photoactivation and subsequent cross-linking takes place in the light exposed regions, dissolution and removal of the non-cross-linked regions in Step 5 results in a negative image of the original photomask. Films prepared from Dow Corning WL-515 can be exposed to patterned UV-light using a mask aligner or a stepper. Due to the surface tackiness of the films after soft-bake direct contact of the films with the UV exposure equipment components must be avoided. For UV-light exposure using a mask aligner a typical proximity gap of 2-4 µm is used. It is recommended that the UV-light exposure be conducted using either i-line or broadband radiation. The sensitivity of the films to g- and h-line radiation is low and exposure at these wavelengths is not recommended. The standard dose of broadband UV-light from a medium pressure Hg lamp for films with a thickness of 15 to 4 µm is 1 mj/cm 2. Doses in the range of 6 to 1 mj/cm 2 can be used but may result in slightly reduced film retention. Exposure to doses lower than 6 mj/cm 2 will result in low or no film retention. Step 4. Post-Exposure Bake Following Step 3 it is necessary to subject the film to a post UV-light exposure bake. Omitting this step will result in the complete removal of both the irradiated and nonirradiated portions of the film during the development step. It is during this step that the UV-light irradiated portions of the film undergo a cross-linking process rendering those areas insoluble in the developing solvent. The standard condition for the postexposure bake is placement of the substrate onto a hotplate at 15ºC for a period of at least 9 seconds and not exceeding 18 seconds. The standard hot-plate exposure is 15ºC for 12 seconds. Exposure of the film to a temperature of 15ºC for periods longer than 18 seconds can result in a significant increase of film residue (scumming) in the non-exposed regions. Exposure at 15ºC for periods less than 9 seconds can result in insufficient cross-linking and a significant reduction in the final film thickness after development. 4

5 Step 5. Development The solvent used to develop the negative image in the film is a Stoddard type of solvent such as Negative Resist Developer (NRD) that is available from Air Products. This solvent can be used as both developer and rinse solvent (Step 1). Alternatively, mesitylene has also shown to be a good development solvent for Dow Corning WL-515. After the post UV-light exposure bake (Step 4), the negative image pattern can be developed by the following process using an automated coater with two dispense heads, one for NRD and one for IPA: second static dispense of NRD to coat the wafer second dynamic dispense (2 rpm) of NRD to rinse wafer second NRD puddle second NRD dynamic rinse at 5 rpm second NRD dynamic rinse at 1 rpm second IPA dynamic rinse at 1 rpm second spin dry at 3 rpm. During development a portion of material in the exposed regions is removed in addition to the non-irradiated regions. This results in the post-developed film having a final thickness less than the original thickness achieved after the soft-bake step. The percentage of the final film thickness relative to the thickness after the soft-bake step (Step 2) is referred to as the film retention. The film retention for fully processed (Steps 1-6) films from Dow Corning WL-515 is in the range of 8 to 9 percent with the higher values obtained for the thicker films. Dow Corning WL-515 is an organic solvent developable film. Water or aqueous alkali types of developers cannot be used. Step 6. Hard Bake CSM Modulus (MPa) maximization of the cure level and mechanical properties. Curing of Dow Corning WL-515 films is a result of an addition cure mechanism that does not involve the evolution of cure by-products. This feature results in negligible shrinkage (~ 2%) and low stress after cure. The standard cure process for Dow Corning WL-515 films is 25ºC for 3 minutes in an oven. The cure atmosphere can be air, nitrogen or vacuum. No heating or cooling ramps are necessary to bring the films to the cure temperature. Films can also be cured at a temperature of 18ºC for 6 minutes. The final modulus as a function of cure condition is shown in Figure 6 for 25ºC/3 minute and 18ºC/6 minute cures and well as a third condition of 18ºC/6 minutes followed by 3 excursions to 26ºC for 1 minute each C/ 3 min 18 C/ 6 min 18 C/1 hr + 3 x 26 C/1 min. Figure 6. CSM modulus against cure condition for Dow Corning WL-515 films. Lithographic Process Performance The lithographic performance for fully processed 2 µm thick films of Dow Corning WL-515 is summarized in Table 5. The results were generated for films processed using a mask aligner and standard process conditions for steps 1 through 6. Key performance features include good resolution down to 15 µm sized pads, a sidewall slope of 6º and minimal shrinkage during cure. The objective of the hard bake is to complete the film cure process resulting in 5

6 Process Property Value Film Thickness Range 15 to 4 µm Minimal Feature Size 15 µm Aspect ratio (ht/w) <1.3 Height Uniformity <5% Sidewall Slope ~6 Film Retention 8 to 9 % Film Shrinkage ~2 % (During Hard Bake) Table 5. Lithographic process performance parameters for Dow Corning WL-515. Figure 6 provides the trend in the post lithography/cure film thickness as a function of the coating speed in Step 1. The post processing film thickness range for Dow Corning WL-515 is approximately 15 to 4 µm. Film Thickness (µm) Spin Speed (rpm) Figure 6. Final (post pattern and cure) thickness as a function of Step 1 spin speed for Dow Corning WL-515. Chemical Contamination Certain classes of materials, chemicals, curing agents and plasticizers can alter the cure profiles and patternability of Dow Corning WL-515. These include sulfur, polysulfides, polysulfones or other sulfur containing materials; amines, urethanes or amine-containing materials, and some solder flux residues. Of note is the solvent n- methylpyrolidinone (NMP), which is frequently used as a carrier solvent for dielectric materials and as a stripping solution for some photoresists. Exposure of Dow Corning WL-515 to NMP in either liquid or vapor form can result in undercure and loss of photopatternability. Care must be taken to isolate usage of NMP away from the area used for processing. Undesirable alteration of the cure characteristics of Dow Corning WL-515 is typically observed as either overcure or undercure of the patterned films. Overcure of the films is observed as excessive residue or scumming in the non-irradiated regions following the patterning process. Undercure of the films is observed after the patterning process as either thinner than predicted films with poor image resolution or no film retention at all. While the introduction of contamination that results in overcure and undercure of the films is possible at all points in the patterning process, the post exposure bake step (Step 4) is particularly susceptible to contamination. Frequently, if contamination can be avoided at this step, the patterning process can proceed with normal results. If persistent contamination issues are identified on processing equipment, aggressive cleaning techniques should be employed. Particular attention should be paid to any hot plates or ovens used in the patterning process, as these are frequently the source of contamination. In the event of a persistent contamination problem, please contact your Dow Corning representative for an updated list of known contaminants and recommended cleaning procedures. Rework/Strip Dow Corning WL-515 can be wet-etched (stripped) following the post-exposure bake step by immersing the film in a room temperature solution of Dow Corning WL Silicone Film Remover. Removal of a 2 µm thick film requires <8 minutes immersion. Following immersion the film is rinsed with IPA and water. Fully cured films of Dow Corning WL- 515 can also be reworked. The immersion time to rework a cured film will depend on the cure temperature and time. 2 µm thick 6

7 films cured at 18 C for 1 hour can be removed by immersion for approximately 4 minutes, and films cured at 25 C for 3 minutes can be removed by immersion for approximately 9 minutes. Following film removal, the substrate should be washed with IPA and water. Dow Corning WL-1975 cannot be used as a developing solvent for Step 5 as it will remove the entire film. Rework of Dow Corning WL-515 films can results in the evolution of trace amounts of hydrogen gas requiring good ventilation for processing. Please consult the MSDS provided with Dow Corning WL-1975 for additional details. Plasma and Reactive Ion Etch Processing Films derived from Dow Corning WL- 515 contain silicone and as a result are sensitive to O 2 and air plasmas. Excessive exposure to these plasmas may cause the films to crack. O 2 and air plasmas at low exposure dose may be used to increase the surface energy of the films to improve the adhesion and wettability to other electronics materials. Following processing and cure, films derived from Dow Corning WL-515 may leave a small amount of polymer residue on the surface of the developed regions of the substrate. This residue is not visible to the eye and is typically less than 2 Å thick. To remove this unwanted material a reactive ion etching process may be used. As Dow Corning WL-515 contains silicone it is not desirable to use a pure O 2 etch. An etching gas combination of SF 6 /O 2 is recommended with an optimum SF 6 level of 5 percent. After SF 6 /O 2 etch, a water wash is recommended to avoid contamination of the substrate. Additionally, CF 4 /O 2 gas systems have been demonstrated as an effective etch system. Please ask your local Dow Corning representative for the latest detailed results in etching processes. 7