Process of Using Absorptive Layer in Optical Lithography With Overlying Photoresist Layer to Form Relief Pattern on Substrate

The following Examples further illustrate this invention, it being understood that the invention is in no way intended to be limited to the details described therein. In the Examples, all parts and percentages are on a weight basis and all temperatures are in degrees Celsius, unless otherwise stated.

Thirty grams of pentaerythritol triacrylate, 3.0 grams of Macrolex Yellow 6G, Mobay Corp., and 1.5 grams of the benzophenone-type photoinitiator Irgacure 651, Ciba-Geigy Corp. were stirred into 70.0 grams of Dowanol PMA, propylene glycol methyl ether acetate(Dow Corp.). The mixture was stirred for one hour under ambient conditions and filtered through a 10 micrometer PTFE filter.

The solution was spin-coated onto a silicon wafer at 4000 rpm for 30 seconds. The coating was irradiated for 60 seconds under a nitrogen atmosphere with a pulsed xenon lamp having a current density of 5000A/cm.sup.2 and an output of 200 to 800 nm. After irradiation curing, the wafer was baked at 200.degree. for 10 minutes to remove any residual solvent and complete the polymerization of the monomer. The thus-formed absorptive layer was 0.7 micrometer thick. The absorptive layer was washed with acetone to remove any dye which might be exposed on the surface and which might dissolve into the subsequently applied photoresist layer. The absorptive layer was found to be totally insoluble in the conventional photoresist solvents propylene glycol methyl ether acetate, ethylene glycol ethyl ether acetate, and cyclopentanone.

The positive resist Microposit 1400-30, Shipley Corp., was applied to a control silicon wafer and to the coated silicon wafer prepared in Example 1 under identical conditions, i.e. spin-coating at 6000 rpm for 30 seconds and baking at 90.degree. for 15 minutes. The combined thickness of the resist plus the absorptive layer was 2.0 micrometers, thus the thickness of the photoresist layer was 1.3 micrometers. This was the same as that of the photoresist layer deposited directly on a silicon wafer, measured with a profilometer. That the measured thickness of the photoresist layer plus the absorptive layer was the sum of their individual thicknesses indicates that there was substantially no mutual dissolution or interreaction between the layers.

The wafer coated with the absorptive layer and the photoresist layer prepared in Example 2 was flood irradiated for 60 seconds with UV light from a medium pressure mercury lamp having an output exceeding 100mj/cm.sup.2. The wafer was immersed in Waycoat HPRD LSI Developer, Phillip A. Hunt Chemical Corp., diluted 1:1 with deionized water, for 60 seconds to completely remove the photoresist layer. The thickness of the absorptive layer remained unchanged at 0.7 micrometers.

The wafer and the control wafer of Example 2 were placed in a conventional oxygen plasma reactor and etched for 20 minutes. The 0.7 micrometer absorptive layer was completely removed and the surface of the wafer was clean. The thickness of the photoresist coating on the control wafer was found to be 1.1 micrometers. These results indicate that it is feasible to pattern etch the subject absorptive layers in an oxygen plasma without significant loss of thickness in an overlying photoresist layer.

A wafer containing absorptive and photoresist layers prepared according to Examples 1 and 2 was pattern irradiated with light at 436 nm by a TRE 800 Stepper, TRE Semiconductor Equipment Corporation. The irradiating dosage was about 20 percent higher than conventional levels to compensate for loss of reflected light from the substrate. The pattern, a series of uniform lines and spaces, was developed as in Example 3. Patterns of equal lines and spaces as fine as 1.0 micrometer were clearly discernable. No separation of the photoresist pattern from the absorptive layer was observed. The pattern was uniform over the entire wafer.

The wafer was placed in a conventional plasma etcher and etched in an oxygen plasma for 8 minutes at 28.degree., 400 W of power and 10 mtorr of pressure with an oxygen flow of 30 sccm. The exposed portion of the absorptive coating was completely removed to the silicon substrate. The photoresist pattern was transferred through the absorptive layer with no loss in resolution. Erosion of the photoresist layer was only 0.4 micrometer.

The substrate for this example was a disk-shaped sapphire wafer three inches in diameter having on its surface epitaxial silicon islands 120 micrometers square and 0.5 micrometer high. The wafer was coated with a 0.3 micrometer thick coating of highly reflective tantalum disilicide by conventional cosputtering of tantalum and polycrystalline silicon. The tantalum disilicide surface was primed with hexamethylene disilazane vapor and then coated with 0.7 micrometer of the absorptive composition and 1.3 micrometers of photoresist as described in Example 2. The structure was irradiated, developed and oxygen plasma etched as described in Example 4. The oxygen plasma etch was carried out for 12 minutes. A pattern of 1.25 micrometer lines was cleanly etched to the tantalum disilicide surface without loss of resolution due to surface reflection and without the constrictions, conventionally termed "notching" that often occur utilizing conventional resist systems where a line is etched over the edge of substrate topography.

The advantageous properties of the subject resist system are clearly demonstrated by this and the foregoing examples.