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# Method for Increasing the Depth of Focus by Reducing the Resist Loss After Exposure

IP.com Disclosure Number: IPCOM000115618D
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 70K

IBM

## Related People

Elsner, G: AUTHOR [+2]

## Abstract

Disclosed is a method for increasing the depth of focus by reducing the thickness loss of the resist layer after exposure.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method for Increasing the Depth of Focus by Reducing the Resist Loss
After Exposure

Disclosed is a method for increasing the depth of focus by
reducing the thickness loss of the resist layer after exposure.

For future chip generations with line widths of about 250 nm
the small depth of focus will be the limiting factor in the
manufacturing of storage cells with photolithographical methods.  The
photolithographical processes for structuring the semiconductor
components are reaching the resolution limits of light optics.  The
resolution &Delta.x is defined as:
where K(1) is a constant, &lambda the wavelength and N.A. the
numerical aperture.  The depth of focus D is defined as:
where K(2) is a constant dependent on the required contrast and the
resolution &Delta.x.

In a typical deep UV lithography system with &lambda.= 248 nm and
N.A. = 0.5 the K(1) value for a resolution of &Delta.x = 350 nm is
K(1) = 0.7.  For a resolution of &Delta.x = 250 nm K(1) has the
value of 0.5.  When choosing a contrast of about 70% the K(2) values
are K(2) = 1.0 or K(2) = 0.5 for the &Delta.x values of 350 nm or
250 nm, respectively.  According to the formula given above the depth
of focus D = 1.0 &mu.m for &Delta.x = 350 nm and D = 0.5 &mu.m for
&Delta.x = 250 nm.  With these low depth of focus values also the
layers of the exposed photo resist are very thin.  Thus the
transmission of structures into the layers beneath the photo resist
could cause problems.  The structured photo resist is used as a dry
etch mask for the layers beneath the resist, e.g., quartz.  For
quartz the dry etch selectivity to resist is about 3 : 1 which
requires a resist layer of sufficient thickness.

Assuming a minimal post development resist thickness of about
350 nm would require a starting resist thickness of 0.7 to 0.8 &mu.m
for linewidths of 350 nm and D = 1 &mu.m.  For the 250 nm linewidth
chips the starting resist thickness has...