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Optimal Laser Pulse Modulation for Sub-Surface Dopant Diffusion into Undoped Epilayers

IP.com Disclosure Number: IPCOM000112495D
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 74K

Publishing Venue

IBM

Related People

Dreyfus, RW: AUTHOR [+4]

Abstract

It has previously been shown that it is possible to obtain nearly uniform local doping of an epitaxial layer as thick as 6 microns from a buried p+ boron substrate [*]. This was achieved by first heating substrate (wafer containing the epi-layer) to 500ºC using a heater stage; then applying a focused laser pulse to the surface of the wafer. The laser absorption is sufficient to melt the epi-layer and about 5-10 microns in depth of the p+ boron substrate to give rise to near uniform doping upon recrystallization. This method is of great utility in providing necessary low resistance pathways between the epi layer and the substrate (ground plane).

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Optimal Laser Pulse Modulation for Sub-Surface Dopant Diffusion into
Undoped Epilayers

      It has previously been shown that it is possible to obtain
nearly uniform local doping of an epitaxial layer as thick as 6
microns from a buried p+ boron substrate [*].  This was achieved by
first heating substrate (wafer containing the epi-layer) to
500ºC using a heater stage; then applying a focused laser
pulse to the surface of the wafer.  The laser absorption is
sufficient to melt the epi-layer and about 5-10 microns in depth of
the p+ boron substrate to give rise to near uniform doping upon
recrystallization.  This method is of great utility in providing
necessary low resistance pathways between the epi layer and the
substrate (ground plane).

      To improve the smoothness of the resulting surface texture it
is desirable to bring the ambient temperature closer to the melt
temperature prior to melting.  This permits a much more gentle and
controllable laser pulse to create the needed heat for melting.
While a steady DC heat cannot be used much more about 500ºC
without causing unwanted boron diffusion, it is possible to provide a
short pre-heat laser pulse to replace the DC heater previously
described in [*].

      What is disclosed here is the use of ~ 1 second
pre-heat laser pulse from a CW argon laser, ~ 1-2 watts,
focussed to a spot 20-50 microns in diameter, onto the epi-layer
surface.  This is achieved using an appropriate acousto-optic
modulator.  The actual laser power and duration are adjusted to
provide a surface temperature just below the melt temperature.  The
modulator is then programmed to provide a second argon laser pulse
within a few milliseconds of the first pulse, 0.1-0.2 s in duration,
slightly higher in intensity to cause melting of the epi-layer and
the substrate to a depth approximately 10 microns.  This second pulse
permits rapid liquid state diffusion of the boron during the molten
phase.  The boron diffusion...