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Infrared Laser Interferometric Measurement of Substrate Temperature And Film Growth Rates

IP.com Disclosure Number: IPCOM000101293D
Original Publication Date: 1990-Jul-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 116K

Publishing Venue

IBM

Related People

Cuomo, JJ: AUTHOR [+4]

Abstract

Disclosed here is the use of an infrared (IR) laser for interferometric measurements of films and substrates that are transparent (or at least not strongly absorbing) at the IR laser wavelength. This invention provides a non-contact method of measuring (1) the temperatures of IR transmissive substrates and (2) the growth rates of commonly grown IR transmissive films. This is of particular value for films and substrates of materials that are opaque to visible light but transparent in the IR, such as silicon, GaAs, and Ge/Si alloys.

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Infrared Laser Interferometric Measurement of Substrate Temperature And Film Growth Rates

       Disclosed here is the use of an infrared (IR) laser for
interferometric measurements of films and substrates that are
transparent (or at least not strongly absorbing) at the IR laser
wavelength.  This invention provides a non-contact method of
measuring (1) the temperatures of IR transmissive substrates and (2)
the growth rates of commonly grown IR transmissive films.  This is of
particular value for films and substrates of materials that are
opaque to visible light but transparent in the IR, such as silicon,
GaAs, and Ge/Si alloys.

      To our knowledge, no previous IR interferometric measurements
of film growth have used an IR laser as the light source.  Also, with
one recent exception (1), all previous temperature measurements based
on interferometry (2-6) have been done at visible wavelengths.

      The schematic laser interferometer is shown in the figure.  An
IR laser (e.g., a Melles Groit HeNe at a wavelength of 1.523 mm) is
directed onto the sample at approximately normal incidence.  The
paths of the incident and reflected laser beams are shown by arrows.
The reflected laser light intensity (monitored by a photodetector,
not shown) oscillates with changes in the sample's thickness and/or
refractive index due to the alternating constructive and destructive
interference of the front and back surface reflections.  In the case
of growth rate measurements, material 1 is a smooth substrate and
material 2 is the growing film.  The period of one oscillation
corresponds to a thickness increase WL of g/2n2, where g is the laser
wavelength and n2 is the film's refractive index (at g).  In the case
of temperature measurements, material 1 can be a backing material (if
present), and material 2 is a smooth (on both sides), IR-transparent
substrate of uniform thickness.  In this case, one oscillation
corresponds to a temperature change WT of (g/2n2L)/(a + b), where g
is again the laser wavelength, L is the substrate thickness, n2 is
the substrate refractive index, a is the substrate's thermal
coefficient of linear expansion, and b is the substrate's relative
thermal coefficient of refractive index (i.e., b X (1/n2)(dn2/dT)).

      Lasers have a number of advantages over the IR light sources
previously used for film growth measurements.  For instance, they can
be chopped for phase-sensitive detection, allowing discrimination
against the potentially noisy IR background, a feature clearly not
possible when the IR source is the sample's own "black-body"
radiation.

      IR laser interfero...