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Defence Scientific Information & Documentation Centre

 Chrome Mask Blanks

Chrome mask blanks find extensive use in the fabrication of masks for various lithographic processes used in solid-state device fabrication. The masks fabricated on these blanks have various advantages like fine resolution, edge acuty and longer shelf-life in repeated process of lithography. Further, where sub- micron geometries are encountered, the use of a mask fabricated on these chrome blanks by  E-beam writing is the only solution.    Chrome blanks are therefore the basic material for directly writing masks by E-beam process.

Chrome blanks are basically glass plates with specific optical transmission  (85 per cent  at   l = 0.35 m), mechanical flatness (2 mm), and parallelism on which a controlled film of chromium (800-1000 Å) is deposited maintaining excellent adhesion and uniformity of thickness (+ 5 per cent) over its extended surface.

Further, since these blanks are to be used for fabrication of masks by E-beam process, film resistivity (20 ohm/sq)  is an essential requirement. In addition, deposited chromium film should have pinhole density of  0.05 Pcs/cm2.

DRDO has been regularly importing these chrome glass blanks  for writing the masks of micron and sub-micron geometries. Appreciating the strategic technological importance of chrome glass blanks, DRDO planned a crash programme to  indigenously develop these chrome blanks using existing facilities and expertise in collaboration with the Department of Electronics.

DRDO has now successfully developed and demonstrated the technology for fabrication of chrome mask blanks for 2.5 and 3.5 in. size. The blanks meet all the required specifications. The performance evaluation of fabricated blanks has also been carried out by Bharat Electronics Ltd, Bangalore and SAC, Ahmedabad, in addition to DRDO.

The following specifications have been achieved:

Film thickness 900 + 70 Å

Film thickness + 5 per cent uniformity

Sheet resistance 20 ohm/sq

Optical density 2.8
at 4000 Å

Pinhole density 0.05 Pcs/
(1-4 mm) cm2

Pattern roughness ~ 0.2 mm