Insulation layer reflow

This disclosure is directed to a method of forming an interlevel dielectric glass layer (16) on a semiconductor device, the layer having a plurality of feed-through apertures (17--17) therein. A CW laser beam (29) is continuously raster scanned over the surface of the glass layer (16) to reflow the layer to densify the material and form a smooth surface topography about the apertures (17--17).

TECHNICAL FIELD 
This invention is related to the formation of glass insulating layers 
during the fabrication of semiconductor devices. In particular, the 
instant technique is directed to the reflowing of such layers to form a 
smooth surface topography. 
BACKGROUND OF THE INVENTION 
In the manufacture of miniature electronic devices, such as Metal Oxide 
Semiconductor (MOS) devices, it is frequently necessary to establish 
electrical connections between two sections of the device by means of a 
conductive film making contact therebetween. Typically, this conducting 
film has a portion that overlies an insulating dielectric layer or film 
and makes contact through small feed-through apertures in that dielectric 
layer to the underlying areas of the device. To accomplish this a thin 
layer of dielectric material is deposited on a semiconductor substrate and 
the feed-through apertures are formed using well known photolithographic 
masking and chemical etching techniques. Such chemically etched 
feed-throughs typically have sharp corners resulting in insufficient 
thickness of conductive material deposited at the corners during 
subsequent processing steps, which can lead to failure of the device under 
high current densities. 
One solution to this problem is to place the device, with the apertured 
insulating layer thereon, in a furnace for a predetermined period of time 
at an elevated temperature until the insulating material reflows to form a 
smooth surface topography around said apertures. Although such a technique 
can be most effective, it has been found that undesirable diffusion of 
dopants within certain devices accompany the high temperature furnace 
reflow operation. 
Accordingly, there is a need for an insulation reflow technique which can 
form a smooth surface topography around the feed-through apertures without 
the attendant undesirable dopant diffusion. 
SUMMARY OF THE INVENTION 
The instant method overcomes the foregoing problem of forming a smooth 
surface topography around apertures in an interlevel dielectric layer in a 
semiconductor device. The method comprises the steps of depositing a layer 
of a dielectric material on a partially completed semiconductor device and 
forming feed-through apertures in the material. The surface of the 
apertured dielectric layer is then exposed to high energy radiation to 
reflow the layer to form a smooth surface topography around the apertures 
without substantially heating the surface therebelow.

DETAILED DESCRIPTION 
FIG. 1 depicts a silicon substrate 5 having a field electrode 10 and an MOS 
gate 11 as well as doped areas 12--12 which serve as source and drain, all 
of which may be formed using well known semiconductor processing 
techniques. A more detailed description of such MOS circuitry is set forth 
in the IEEE Journal of Solid State Circuits, Vol. SC-13, No. 5, page 556 
dated October, 1978 and is incorporated by reference herein. 
FIG. 2 shows an insulating layer 16 deposited on the substrate 5. The layer 
16 is glass, such as a phosphorus doped silicate glass (P-glass) which may 
be deposited using well known Chemical Vapor Deposition (CVD) or other 
deposition processes. 
FIG. 3 shows a plurality of feed-through apertures 17--17 formed in the 
layer 16 to expose portions of the MOS device therebelow. As can be seen, 
the apertures 17--17 have steep sides 18--18 resulting in sharp upper 
corners 19--19. In a subsequent processing step it is necessary to deposit 
a conductive material 20 (e.g., aluminum), as shown in FIG. 5, on the 
layer 16. The material 20 also fills in the feed-through apertures 17--17 
to make an electrical connection with the underlying portions. 
Undesirably, the conductive material 20 in the vicinity of the sharp 
corners 19--19 of the layer 16 is very thin and can cause failure of the 
semiconductor device fabricated using such techniques when high current 
densities pass therethrough, or when the device is subjected to other 
stresses. 
One solution to this problem, once the feed-through apertures 17--17 are 
formed in the layer 16 (see FIG. 3), is to place the processed substrate 5 
in a furnace for a time (e.g., 5 to 10 minutes) and at such a temperature 
(e.g., 1100.degree. C.) so as to cause the insulating layer 16 to reflow 
and form a smooth topography in and about the feed-through apertures. When 
the conductive material 20 is deposited on the smoothed layer 16, as shown 
in FIG. 5, the thickness of the material is substantially uniform and the 
aforementioned failures are avoided. However, the exposure of the device 
to such high temperatures for extended periods of time has been found, in 
some instances, to cause dopants in the areas 12--12, as well as other 
areas, to diffuse into the substrate 5 as well as diffuse laterally, which 
can adversely affect the operation of the device. 
The instant invention precludes such problems by continuously scanning a 
laser beam over the insulating layer 16 having the feed-through apertures 
17--17 therein. FIG. 6 depicts a schematic view of a laser scanning 
system, generally referred to by the numeral 25, for implementing such a 
technique. The substrate 5 with the apertured layer 16 (also see FIG. 3) 
is not drawn to scale for purposes of clarity of exposition. In actual 
practice the apertures 17--17 are approximately 5 .mu.m in diameter, the 
layer 16 is about 1 .mu.m thick and the substrate 5 is 500.mu.m thick. 
The scanning system 25 is comprised of a laser 26, a multi-faceted 
rotatable mirror 27, focusing optics 28 and a movable bed (not shown) on 
which the substrate 5 is positioned. In operation, the substrate 5 is 
placed on the movable bed and the laser 26 activated. A light beam 29 from 
the laser 26 is directed towards the multi-faceted mirror 27 which rotates 
clockwise (see arrow) and the beam is reflected toward the substrate 5 
through the focusing optics 28. The beam 29 impinges on the layer 16 and 
moves laterally thereacross as the substrate 5 moves in the direction 
shown. The rotation of the mirror 27 in concert with the movement of the 
substrate 5 results in the layer 16 being raster scanned by the laser beam 
29. 
In a particular embodiment the laser 26 was a CW CO.sub.2 laser having a 
wavelength of approximately 10.6 .mu.m which had a dwell time on the layer 
16 of about 1 msec. The layer 16 which was P-glass, approximately 1 .mu.m 
thick, was exposed to 8:10.sup.5 watts/cm.sup.2. A Q-switched CO.sub.2 
laser at the same frequency has also been used to reflow a phosphorous 
doped silicate glass layer 16; however, the CW CO.sub.2 has been found to 
be more controllable. The substrate 5 is substantially transparent to the 
beam 29 while the P-glass layer 16 absorbs a substantial portion of the 
light energy at the 10.6 .mu.m wavelength. Such selective coupling of the 
energy causes the P-glass layer 16 to reflow resulting in a smooth 
topography around the apertures 17--17 (see FIG. 4) with little or no 
heating of the substrate therebelow. Hence, minimal interlayer and lateral 
diffusion of the dopants occur. 
Additionally, when the P-glass is vapor deposited on the substrate 5, it is 
in an unconsolidated state. The raster scanning of the P-glass layer 16 
with the laser beam 29 densifies or consolidates the P-glass. Accordingly, 
the instant technique simultaneously densifies the P-glass layer 16 and 
forms a smooth surface topography around the apertures 17--17 upon reflow 
of the layer. 
Once the laser reflow operation has been completed, the conductive material 
20 is deposited on the layer 16 as shown in FIG. 5. The smooth surface 
topography of the feed-through apertures 17--17 results in a substantially 
uniform thickness of the metallic material 20 about the apertures which 
preclude the aforementioned problems. 
It is to be understood that the embodiments described herein are merely 
illustrative of the principles of the invention. Various modifications may 
be made thereto by persons skilled in the art which will embody the 
principles of the invention and will fall within the spirit and scope 
thereof.