Purified Kodak Photoresist
Stable, purified kodak photoresist materials with improved performance in resolution, adhesion, etch resistance, and low pinhole density – for use in advanced planar semiconductor technology and microelectronics.
PKP – Type II
Purified KTFR photoresist for superior adhesion
PKP PURIFIED KODAK PHOTORESISTS
PKP photoresist product is a stabilized solution of purified Kodak “KTFR.” This purified photoresist offers improved resolution, adhesion, etch resistance, low pinhole density and best all-around properties available for negative-working resists. The need for membrane filtration of the photoresist in use is not essential. Resolution in the micron range is practical, while photoresist procedures will be found consistently reproducible in production. PKP resist is produced under conditions which assure high quality and comply with the critical and exacting requirements essential for advanced integrated circuit technology.
KTFR product is purified by special processing based on a centrifuging technique under controlled isothermal conditions. The purification process yields photoresist material with excellent quality. The polymeric composition (cis-polyisoprene) shows uniformity in molecular weight and size distribution. In effect the centrifuging technique is a fractionation process with the very high molecular weight polymers removed. Consequently, resist films obtained from the purified products reveal very low pinhole density.
PKP-Type II (Purified KTFR)
PROPERTIES OF PKP PHOTORESIST
|PKP Solutions||Type II|
|% Solids (by wt.)||28 ± 0.8|
|Viscosity (cps – 25 °C)||500 ± 35|
|Sp. Gr. (25 °C/4 °C)||0.897|
|Flash Point (°F)||84|
|Polymeric Composition||cyclized cis-polyisoprene|
|Aver. Mol. Wt. Membrane Osmometry||65,000 ± 5,000|
|Sensitizer * 2,-6-Bis-(p-Azido Benzylidene)-4-methylcyclohexanone)|
|Peak Spect. Sens. (nm)||355|
|Spectral Absorb Range (nm)||310-480|
|Thresh. Expos. Intensity(joules/cm2)||0.01|
|PKP Films (exposed)|
|Thermal Elongation (%/°C)||.04|
|Coef. Therm. Expan. (in/in/°F)||4.5 x 10-5|
|Dielectric Constant (1 mc)||2.4|
PROPERTIES OF PKP CONTINUED
|Water absorb. (25 °C)||nil|
|Vol. Resistivity (25 °C) (ohm-cm)||5 x 1014|
|Equatorial Spacing (A) at phase transition temp.158 °C148 °C
Traces represent 40%, 50%, 60%, and 67% dilutions.
NOTE: For optimum adhesion of photoresist the equatorial spacing should correspond to lattice constant of substrate material: (Aluminum -4.0 A: Silicon 5.4 A; SiO2 – 4.9 A)
PKP II INSTRUCTIONS
The use of Purified Kodak Photoresist (PKP) has significantly eliminated many problems inherent in photoresist work. Through the Transene process of thermally controlled centrifugal filtration, a more uniform photoresist material relatively free of conglomerates has emerged. However, application and processing of photoresist films remain to some degree an empirical art. The following pages form a general outline of procedure for photoresist technology used throughout the industry. Yet, because environmental and equipment differences exist, it is usually, necessary to adjust conditions and process parameters until satisfactory results are obtained.
Several precautions are worthy of note:
- Dust and lint cause pinholes. These can be minimized by the use of clear-flow liquids.
- Relative humidity must be controlled between 30 – 50%.
- Appropriate lighting of gold fluorescent, yellow incandescent, or white fluorescent with yellow or orange filters must be used.
- Adequate ventilation is necessary because of solvent fumes.
PKP materials are purified expressly for your order. To remain conglomerate free, precautions must be taken:
- PKP materials should be refrigerated to minimize polymerization. Allow PKP to warm to room temperature before opening.
- PKP materials should be stored away from light sources and not be removed from shipping bottles until used.
- Substrate Preparation
Clean and prebake 20-30 min. @ 120-130°C
- Photoresist Application
Motor-driven rotary spinning
20 min. @ 82°C
- Photoresist Exposure
1-10 sec. minimum light source 10mW/cm2
- Photoresist Develop
10-60 sec. spray (PKP II Developer) followed by butyl acetate rinses followed by compressed air or nitrogen blow-off.
Minimum 10 min. 120°C (max. 148°C)
- Photomask Remover
Transene Negative Resist Remover NRR-001 at 50-60 °C effectively removes PKP II photoresist.
- Substrate Preparation:Only if the substrate is perfectly clean and dry prior to resist application will a well-defined etch pattern free from defects be developed. Removal of surface particles and organic residues is accomplished by cleaning with solvent such as trichloroethylene followed by baking at temperature between 120°C – 200°C for up to 20 minutes.
- Application:The standard technique of spinning a substrate on a motor driven rotary vacuum chuch yields the most uniform and reproducible thickness. Under this program centrifugal forces distribute a uniform coating of resist without significant solvent evaporation or excess resist being thrown off at the substrate edges. Begin by flooding substrate with photoresist. Never apply resist to substrate while spinning. An uneven distribution may result.Square or rectangular substrates are best coated at low rpm (50-1000 rpm). Undiluted resist at 75 rpm yields about 2.5 µ coatings, the substrate edges and corners being thicker.
Circular substrates (the most common) are coated at high speed between 2,000 – 5,000 rpm, resulting resist thickness being determined by resist viscosity, rpm and acceleration. RPM speed > 5,000 has little effect on resist thickness. The lower the viscosity of a resist, the less revolution speed (rpm) affects coating thickness. Time of acceleration to peak rpm is generally regarded to be the determinant of thinner and more uniform thickness across the substrate. Also, the slightly thicker resist coating at the edge of the substrate is reduced. The optimum acceleration time to attain peak rpm is 0.1 sec.
Resist film thickness used for thin film circuits vary from 0.3 to 2 µ. At lower thickness limits, dilution of resist can cause a discontinuous film. Thick films (1-2 µ) give added protection against etch penetration and pinhole formation at the expense of less resolution. A better approach of spinning two thin coats results in assured quality and desired thickness of resist films.
- Prebaking:Evaporation of residual solvents results in maximum adhesion of resist to substrate. Resist coatings not thoroughly baked will have variable exposure requirements because residual solvents will inhibit cross linking of functional groups. Excessive baking may also create problems such as fogging and decomposition of resist material. Bake-out time is dependent upon film thickness.
KTFR – 20 min. @ 82°C (above 104°C will affect adhesion)
- Exposure of Photoresists:Photoresists may be exposed with any light source having output in the near UV spectrum. Large-area light sources are used only for coarse (50 µ or 0.002 inch or larger) lines. To resolve fine details a less diffused light source is necessary. Generally, for fine lines patterns point sources (carbon arc, high pressure mercury vapor or Xenon flash lamps) are used at a point remote from substrate.This will insure uniformity of light intensity across substrate. Proper exposure of KTFR requires light energies of about 10mW/cm2. Correct exposure is dependent upon thickness and processing variables. An exposure of 1-10 seconds is sufficient providing the light source is capable of yielding minimum irradiation intensity of 10mW/cm2 at substrate surface.
For good fine-line definition and reproducibility, the exposure energy must be controlled within 10% of the optimum value. Light intensity at the substrate surface should be monitored. Due to diffraction effects of resist material, overexposure produces cross linking of resist under the mask, the effect being line broadening to as much as 2.5 µ.
Underexposure resulting in cross-linking only at the surface of the resist film can cause the pattern to be washed off when the image is developed. Line broadening can also be the fault of substrate irregularity and/ or insufficient contact between mask and resist surface. The use of a vacuum to hold the mask against a perfectly smooth substrate will solve this problem.
The KTFR sensitizer decomposes without cross-linking the polymer if exposure takes place in the presence of oxygen. This reaction is limited only to the surface of the film, which remains soluble in the developer. This oxygen effect will not be noted in working with resist coatings of > 1 µ. Film coatings of 0.5 µ or less may be affected greatly, not offering enough protection in the subsequent etch operation.
- Photoresist Developement:In general, the best procedure involves a 10 to 60 second spraying of the developer onto the coated substrate. Follow this with several non-aqueous rinses (butyl acetate). Using pure compressed air or nitrogen residual solvent is blown off the surface. Transene PKP II Developer is recommended.
- Postbaking:Post baking of the developed resist will evaporate residual solvents, enhance chemical stability of the polymer coat, and further enhance adhesion. Post baking temperature should never exceed 148°C. Recommended postbake is 10-20 min. at 120°C.
- Photoresist Removal:Transene Negative Resist Remover NRR-001 is recommended for removal of PKP II photoresist.