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Efficient Laser Removal of Small Particulates Sticking On a Surface

IP.com Disclosure Number: IPCOM000119998D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 3 page(s) / 138K

Publishing Venue

IBM

Related People

Tam, AC: AUTHOR [+3]

Abstract

There is a rapidly increasing need for techniques to clean surfaces of particulate contaminations. Small particles are notoriously difficult to clean off, particularly submicron ones. However, as technological devices and products get smaller, these particulates cannot be tolerated, as in many cases in VLSI technology, storage technology, surface science, and so on. This critical situation is well summarized in (1). In this article, several recent techniques of cleaning are discussed. Liquid rinses, ultrasound, plasmas, electrostatics, gas jets, and other conventional cleaning techniques are all regarded as totally inadequate for submicron particle removal. Two kinds of "laser cleaning" are considered: direct pulsed laser cleaning (2), and liquid film absorption and explosive removal (3).

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Efficient Laser Removal of Small Particulates Sticking On a Surface

      There is a rapidly increasing need for techniques to
clean surfaces of particulate contaminations.  Small particles are
notoriously difficult to clean off, particularly submicron ones.
However, as technological devices and products get smaller, these
particulates cannot be tolerated, as in many cases in VLSI
technology, storage technology, surface science, and so on.  This
critical situation is well summarized in (1).   In this article,
several recent techniques of cleaning are discussed.  Liquid rinses,
ultrasound, plasmas, electrostatics, gas jets, and other conventional
cleaning techniques are all regarded as totally inadequate for
submicron particle removal.  Two kinds of "laser cleaning" are
considered: direct pulsed laser cleaning (2), and liquid film
absorption and explosive removal (3).

      A pulsed, liquid-assisted, surface cleaning technique has been
invented that can effectively remove particles from a surface and
minimize the possibility of damaging the sample due to thermal
heating or mechanical shocks.  This technique is useful for many
areas of refined cleaning, e.g., cleaning of X-ray lithography masks
or other types of high-resolution masks, optical surfaces, HDA
components, ultra-high vacuum components, and so on.

      The present invention is aimed at improving the efficiency of
laser cleaning of minute particles from delicate surfaces that can be
damaged by excessive laser fluence.  The present technique surpasses
the prior art of laser cleaning in the capability of removal of small
particles more effectively and at a lower laser fluence. The key
invention here is the application of a very thin film of a liquid
(with proper surface tension and thermal properties) onto the surface
momentarily before the "cleaning" laser pulse.  Hence, the liquid
film that is "assisting" the cleaning will be controlled in several
respects:  thickness, physical properties, and deposition timing, as
detailed below.  This is in contrast to the prior art of [3] where an
uncontrolled water film (at least no mention of any control) is used
to assist laser cleaning.

      The liquid used is preferably environmentally safe,
inexpensive, residue free, nontoxic, easy to handle, and satisfies
the following:
      -    Can form a continuous film of thickness on the order of
0.1 to 1 microns on the sample surface to be cleaned;
      -    Has low surface tension so that it goes under the
particles sticking on the sample surface;
      -    Can be quickly brought to a boiling point at a temperature
that would not damage the sample; and
      -    Has heat of vaporization that is not too large.

      Based on these considerations, the liquid of choice is an
alcohol, particularly ethanol or isopropanol.  A water-alcohol
mixture is another possibility.

      The technique of controlled liquid film-assisted las...