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Method for Improving Resolution of Focussed Ion Beam Mask Repair Process

IP.com Disclosure Number: IPCOM000119862D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 3 page(s) / 141K

Publishing Venue

IBM

Related People

Harper, JME: AUTHOR [+2]

Abstract

In the preparation and repair of high resolution X-ray lithographic masks, sputtering of features into masking layers is used to fabricate very high resolution shapes. A method is described for improving the resolution of these images where a focussed ion beam is used to delineate the small patterns in a gold absorber. The method induces the creation of a large fraction of negative gold ions, which are ejected electrostatically, instead of purely neutral gold atoms which redeposit on the mask and limit the resolution. Further, the method can also be used to make the sputtering more uniform, thus reducing the unfavorable effects of sputtering variations due to the polycrystalline nature of the gold layer.

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Method for Improving Resolution of Focussed Ion Beam Mask Repair
Process

      In the preparation and repair of high resolution X-ray
lithographic masks, sputtering of features into masking layers is
used to fabricate very high resolution shapes.  A method is described
for improving the resolution of these images where a focussed ion
beam is used to delineate the small patterns in a gold absorber.  The
method induces the creation of a large fraction of negative gold
ions, which are ejected electrostatically, instead of purely neutral
gold atoms which redeposit on the mask and limit the resolution.
Further, the method can also be used to make the sputtering more
uniform, thus reducing the unfavorable effects of sputtering
variations due to the polycrystalline nature of the gold layer.

      Focussed ion beam mask repair uses sputtering of the mask
material to remove unwanted regions from the pattern. Typically, gold
is the mask material, and the focussed ion beam species is gallium.
The sputtering of material by the ion beam leads to ejection of
neutrals in all directions; some of the ejected material comes to
rest on adjacent edges where the spacing is on the order of 0.5 mm or
less.  Since the resolution of resulting images depends greatly on
the steepness of the edges produced during this sputtering removal
process, adjacent edges of a sputtered region will not print the
desired pattern due the added material.  Also, in confined regions,
such as between closely-spaced lines, or in a contact hole structure,
the redeposition of sputtered gold atoms in the region being actively
sputtered prevents the formation of sharply-defined edges.  The
result is a limited spatial resolution and a limited depth for small
contact holes (approximately a 3:1 aspect ratio between the depth and
the size of the opening for 25 keV Ga ions).

      Most of these limitations arise from the broad angular
distribution of the sputtered Au atoms.  To improve the definition of
fine structures, it is helpful if the Au atoms can be ejected out of
the sputtered region at an energy higher than their typical sputtered
energy of several eV, and in a direction substantially normal to the
surface.

      One way to provide additional energy to sputtered Au atoms is
to ionize them and accelerate them electrostatically.  Unfortunately,
most sputtered atoms are ejected as neutrals, and the cross-section
for ionization to a positive ion is small.  The invention here,
however, uses the ability of gold to form stable negative ions, and
to then eject them electrostatically.

      It is known (1,2) that Au negative ions can be formed in
sputtering if the Au atoms are in close proximity to highly
electropositive atoms.  Charge transfer can then occur during the
sputtering process, ejecting a large fraction of Au as negative Au
ions.  The same is true for other electronegative elements such as
Pt, O and the halogens (F,Cl,I, etc.).

      The choice o...