For those who are interested in generating DOEs in glass using direct write process with a following etch step, CMI developed a laser beam direct write LDW-glass which has a much superior selective etch ratio than the e-beam direct write HEBS-glass. Due to a very large etch ratio of the laser exposed area, versus unexposed area within a LDW-glass plate, DOE as well as refractive micro lens arrays can be fabricated in LDW-glass with the laser direct write approach.

The laser beam direct write LDW-glass is marketed under the name "LDW-glass Photomask Blanks." For information on LDW-glass photomask blanks, please request CMI product information number 95-08 from info@canyonmaterials.com.

Absorption-Phase Shift Mask

A binary absorption HEBS-glass mask becomes a binary phase shift mask upon selective wet chemical etching or a dry etching process. Starting with a HEBS-glass blank having an e-beam sensitized glass layer which is sufficiently thick so that the binary phase-shift mask is still sensitive to e-beam, a second e-beam exposure produces an absorption-phase-shift mask.

Advantages of HEBS-glass Masks

Direct write on HEBS-glass creates instant photomasks, and eliminates chrome and photoresist, and their associated processing chemicals. This is a zero-waste, inexpensive solution for mask making. By employing HEBS-glass blanks, photomasks meeting specifications containing the most stringent defect levels can be prepared consistently. Advantages gained in using HEBS-glass photomask blanks include the following:

• Photomasks can be patterned from HEBS-glass blanks without a number of processing steps

• The advantages that can be expected from eliminating the post exposure processing steps include faster turnaround, better line width control and much lower defect densities.

• Defects such as intrusion, extrusion, lack of adhesion, excess material/chrome spots and scratches in chrome are eliminated due to the elimination of chrome and resist as well as the associated processing steps.

• No post exposure process-induced CD variation. No process induced image quality problems (e.g., line distortion) due to resist swelling during baking.

• HEBS-glass photomasks are non-reflective, and have near zero difference in reflectivity between darkened and undarkened areas. Reflectivity is 4% which is much less than that of the anti-reflective chrome at all wavelengths.

• White light is a safe light for the HEBS-glass photomask blanks enabling inspection of the mask-blanks with intense white light before, during, and after the pattern is generated.

• The HEBS-glass mask is more durable than a chrome mask since the HEBS-glass mask is less sensitive to surface scratching due to its volume effect, i.e., the masking pattern is within the glass surface rather than on the surface.

• For contact printing, the HEBS-glass masks have long life times and more wafers produced per mask

• The e-beam sensitivity of a HEBS-glass is very uniform throughout the whole mask surface. Therefore, good CD control is not limited to the center of the mask. In contrast, chrome blanks often have the non-uniform photoresist coating thickness near the edges of the chrome plate.

• Easy re-inspection of accepted masks; since there exists no scattered light from clean HEBS-glass photomask (without the chrome features which scatter light in the image plane), defects such as dust particles, fingerprints, and scratches are readily observable/detected in the passage of an intense light beam in a dark room. No expensive inspection equipment is needed to reinspect used masks or could-be contaminated masks.

• Unlike chrome blanks, there is no chemical waste problems.

Applications

One of the applications in high volume consumer products is a diffractive lens element which replaces up to more than 3 molded glass lens in the laser head for various optical disc drives. The diffractive optical head enables a smaller read-write-erase spot, higher efficiency and has a much less weight which benefits servo tracking at a higher rotational speed.

Another of the applications in high volume consumer products is a micro-mirror array or a micro-prism array which provides a micro-mirror or a micro-prism behind each pixel of LCD (liquid crystal display) so that LCD has a high contrast nearly independent of the angle of view.