Microwave window assembly

A microwave window assembly for transmitting high power microwave energy from microwave propagating means into the interior of a chamber and including first and second windows formed of a dielectric material substantially transparent to microwave energy with the first window sealed in a wall of the chamber and the second window spaced rearwardly from the first window to define a space therebetween. A cooling fluid is circulated in the space between the windows to cool the window positioned in the wall of the vacuum chamber and a waveguide tube extends from the microwave propagating means to the rear surface of the second window to define a waveguide surface extending from the microwave source to the rear surface of the second window. A clamp plate positioned against the forward surface of the second window includes a window which defines a forward extension of the waveguide surface extending forwardly into the space between the windows to a location proximate the rearward surface of the window positioned in the wall of the vacuum chamber. The second window extends radially outwardly beyond the waveguide surface to define an annular outer window portion outwardly of the waveguide surface and the window assembly further includes a seal plate positioned against the rearward surface of the second window and defining an annular groove confronting the rear surface of the outer annular portion of the second window. An elastomeric annular seal is received in the groove and sealingly engages the rear surface of the outer annular window portion.

FIELD OF THE INVENTION 
This invention relates generally to an apparatus for depositing or etching 
film through the use of a microwave initiated plasma and more particularly 
to a microwave plasma deposition apparatus employing an improved window 
assembly adapted to uniformly transmit high power microwave energy from a 
source such as a waveguide into the interior of a vacuum deposition/etch 
chamber. 
BACKGROUND OF THE INVENTION 
This invention window assembly has general applicability to any type of 
apparatus which requires the introduction of high power microwave energy 
from a source such as a waveguide or antenna, maintained at substantially 
atmospheric pressure, into the interior of a vacuum chamber, maintained at 
subatmospheric pressure. The microwave energy is introduced into the 
vacuum chamber for effecting a glow discharge plasma which is utilized to 
either deposit a semiconductor or insulating material onto the exposed 
surface of a substrate or to remove (etch) material from that exposed 
surface. Whereas the invention window assembly has universal applicability 
to microwave apparatus, the invention window assembly is especially 
applicable to the fabrication of photo responsive alloys and devices for 
various photoconductive applications including the fabrication of 
electrophotographic photo receptors. Alternatively, the invention window 
assembly may be employed with equal advantage in association with a vacuum 
chamber adapted to etch or otherwise treat or modify the surface of a 
substrate. 
Regardless of the type of microwave plasma operation (deposition or etch) 
being conducted, the rate at which that operation occurs can be 
controlled, inter alia, by controlling the power at which the microwave 
energy is transmitted into the interior of the vacuum chamber. In order to 
deposit or etch at a high rate, it is necessary to utilize high power 
levels, for example in the kilowatt range and preferably three or more 
kilowatts. However, the use of such high power microwave energy tends to 
cause heating of the dielectric window through which the microwave energy 
is coupled into the interior of the vacuum chamber, and prolonged or 
excessive heating of the dielectric window can cause cracking of the 
window with resultant catastrophic failure of the deposition/etch 
operation. Further, even the introduction of relatively low microwave 
power into the vacuum chamber over a relatively lengthy period of time can 
also cause the dielectric window to overheat and fail. 
In an effort to overcome failure of the dielectric window due to 
overheating, it has previously been proposed to position a second window 
rearwardly of the window in the vacuum chamber wall and pass a cooling 
fluid between the two windows so as to reduce the temperature of the 
window positioned in the wall of the vacuum chamber to an acceptable level 
to allow the introduction of high power microwave energy into the vacuum 
chamber through the window without producing failure of the window even 
over extending periods of operation. 
However, the spaced dual window arrangement creates problems with respect 
to coupling the microwave energy into the vacuum chamber since the 
waveguide surface transmitting the microwave energy from the microwave 
propagating means extends only to the rear or outboard surface of the 
second window so that the microwave energy thereafter moves in an 
uncontrolled manner into the vacuum chamber with the result that the shape 
and dimensions of the microwave energy in the space between the rear 
surface of the second window and the vacuum chamber become promiscuous and 
uncontrolled with the result that the microwave energy spreads out as it 
enters the vacuum chamber. This promiscuous spreading and deterioration of 
the form and dimensions of the microwave energy substantially derogates 
the efficiency of the deposition or etching operation taking place within 
the vacuum chamber and also severely complicates the task of providing a 
seal as between the waveguide surface and the cooling fluid circulating 
between the spaced windows since the randomly and promiscuously moving 
microwave energy will attack and ultimately destroy anything other than 
very expensive and very exotic seal arrangements. 
More specifically, if an elastomeric or O-ring type seal is employed to 
seal the cooling fluid from the interior of the waveguide, the promiscuous 
microwave energy moving between the rear surface of the second window and 
the vacuum chamber causes a capacitive effect to develop in the vicinity 
of the elastomeric seal and the discharge activity resulting from the 
capacitive build-up interferes with the deposition/etching process and 
also derogates the elastomeric seal. 
Accordingly, a need exists for an improved and inexpensive window assembly 
which can efficiently, economically, reliably and safely transmit 
relatively high power microwave energy from a waveguide into a vacuum 
chamber even over extended periods of use. 
SUMMARY OF THE INVENTION 
The invention window assembly is of the type intended for transmitting high 
power microwave energy from microwave propagating means into the interior 
of a vacuum chamber and including first and second windows formed of a 
dielectric material substantially transparent to microwave energy with the 
first window adapted to be sealed in a wall of the chamber and the second 
window spaced rearwardly from the first window to define a space 
therebetween; means for circulating a cooling fluid in the space between 
the windows; and means defining an axially extending waveguide surface for 
transmitting the microwave energy from the propagating means to the window 
assembly. According to the invention, the waveguide surface includes a 
first portion comprising a closed surface of substantially uniform cross 
section extending from a location rearwardly of the second window to a 
location proximate the rearward surface of the second window and a second 
portion, corresponding in size and cross-sectional configuration to the 
first portion, extending from the forward surface of the second window and 
into the space between the windows toward the rearward surface of the 
first window. This arrangement extends the waveguide surface to a location 
proximate the rear surface of the window positioned in the wall of the 
microwave chamber so as to minimize breakdown in the size and shape of the 
microwave energy as the microwave energy moves through the chamber window 
and into the vacuum chamber and thereby minimize derogation of the 
efficiency of the deposition/etching process taking place within the 
chamber and minimize sealing problems caused by promiscuously wandering 
microwave energy. 
According to a further feature of the invention, the second window extends 
radially outwardly beyond the waveguide surface to define an annular outer 
window portion outwardly of the waveguide surface, and the window assembly 
includes annular sealing means which coact with the annular window portion 
to seal the interior of the waveguide surface from the circulating fluid. 
This specific arrangement places the sealing means out of harms way with 
respect to the microwave energy and simplifies the provision of an 
adequate sealing means. 
According to a further feature of the invention, the window assembly 
includes means defining an annular groove confronting a side surface of 
the annular window portion, and the annular sealing means comprises an 
elastomeric annular seal received in the annular groove and sealingly 
engaging the confronting side surface of the annular window portion. This 
specific arrangement allows the use of an inexpensive elastomeric sealing 
member to provide the required sealing action. 
According to a further feature of the invention, the window assembly 
includes a housing structure mounting the first and second windows and a 
seal plate positioned within the housing structure rearwardly of the 
second window, and the annular groove receiving the annular seal is 
defined in the forward surface of the seal plate. This specific 
construction further facilitates the provision of an effective and yet 
inexpensive seal. 
According to a further feature of the invention, the window assembly 
further includes an annular clamp plate positioned against the forward 
surface of the second window and the clamp plate includes a central window 
defining the second waveguide surface portion. This specific arrangement 
provides a simple and effective means for firmly locking the second window 
within the housing structure and concomitantly defining the portion of the 
waveguide surface extending forwardly from the forward surface of the 
second window. 
According to a further feature of the invention, the housing structure 
includes inner and outer telescopically arranged sleeves and the means for 
circulating a cooling fluid between the windows includes means defining a 
cooling fluid path extending axially between the sleeves and communicating 
at its forward end with the space between the windows. In the disclosed 
embodiment of the invention, the cooling path includes first and second 
path portions communicating with the space between the windows 
respectively at generally diametrically opposed locations so as to allow 
the delivery of cooling fluid to the space through one path portion and 
the removal of cooling fluid from the space through the other path portion 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The microwave deposition apparatus, as seen in FIG. 1, includes microwave 
propagating means 10, a vacuum chamber 12, and a window assembly 14. 
Microwave propagating means 10 is of known form and includes a microwave 
energy source 16 and an antennae probe 18. Source 16 may, for example, 
comprise a microwave frequency magnetron having an output frequency of, 
for example, 2.45 GHz. 
Vacuum chamber 12, also of known form, is adapted to deposit successive 
layers of material, preferably amorphous semiconductor alloy materials, 
onto suitable substrate members in response to microwave energy introduced 
into the interior of the vacuum chamber via the invention window assembly 
14. 
The invention window assembly 14 includes a housing structure 19 
constituted by a sleeve assembly including an outer sleeve 20 and an inner 
sleeve 22; a forward or primary window 24; a rearward or secondary window 
26; a seal plate 28; a clamp plate 30; a support plate 32; and a waveguide 
tube 34. 
Outer sleeve 20 is cylindrical and is formed of a suitable metallic 
material. Outer sleeve 20 includes a main body axially extending tubular 
portion 20a, a radially outwardly extending flange portion 20b at the 
rearward end of the sleeve, and a radially inwardly expending, flange 
portion 20c at the forward end of the sleeve. Sleeve main body portion 20a 
is received at its forward end in a suitable aperture 12a formed in a side 
wall 12b of vacuum chamber 12 so as to dispose forward flange 20c 
immediately inwardly of vacuum chamber side wall 12b. 
Inner sleeve 22 is formed of a suitable metallic material and includes a 
main body axially extending tubular portion 22a and a rearward flange 
portion 22b. A pair of diametrically opposed axially extending grooves 22c 
are formed in the outer circumferential surface of main body portion 22a. 
Axial grooves 22c communicate at their forward ends with a circumferential 
groove 22d proximate the forward end of main body portion 22a and groove 
22d in turn communicates with the interior of the sleeve via a plurality 
of radial ports 22e. Inner sleeve 22 is sized to fit snugly and 
telescopically within outer sleeve 20 with grooves 22e and 22d coacting 
with the confronting inner surfaces of the main body portion 20a of the 
outer sleeve to define passages or channels between the inner and outer 
sleeves. A plurality of bolts 36 secure outer sleeve flange portion 20b to 
inner sleeve flange portion 22b to fixedly maintain the sleeves in their 
telescopic relation. 
Primary or forward window 24 is formed of a suitable dielectric material 
substantially transparent to microwave energy and has a generally 
cylindrical configuration. Window 24 is positioned proximate the forward 
ends of the inner and outer sleeves within opening 12a in vacuum chamber 
side wall 12b with the forward surface 24a of the window positioned at its 
peripheral edge against the rearward surface 20d of outer sleeve flange 
portion 20c and the rearward surface 24b of the window positioned at its 
peripheral edge against an annular shoulder 22f defined proximate the 
forward end of the main body portion 22a of the inner sleeve. The extreme 
forward edge of the inner sleeve is chamfered at 22g and acts to sealingly 
squeeze an annular sealing member 40 against the outer periphery of window 
24 and against a rearwardly facing annular shoulder 20e defined by outer 
sleeve 20. 
Secondary or rear window 26 is also formed of a suitable dielectric 
material substantially transparent to microwave energy, has a 
substantially rectangular configuration, and has a thickness significantly 
less than the thickness of primary window 24. For example, primary window 
24 may have a thickness of 1/2 inch and secondary window 26 may have a 
thickness of 1/4 inch. 
Seal plate 28 is formed of a suitable metallic material and has a generally 
cylindrical configuration sized to fit slidably within inner sleeve 22. 
Seal plate 28 includes a main body cylindrical portion 28a and a pair of 
diametrically opposed spacer portions 28b extending forwardly from the 
front surface 28c of the seal plate. A rectangular window opening 28d 
extends through main body portion 28a from rear surface 28e to front 
surface 28c and an annular rectangular seal groove 28f is provided in 
forward surface 28c in surrounding relation to window opening 28d. A 
further circular groove 28g is provided proximate the outer rearward edge 
of main body portion 28a. Seal plate 28 is positioned within inner sleeve 
22 with the outer periphery of the plate contiguous with the inner 
periphery of main body portion 22a of the inner sleeve and with the 
forward surfaces 28h of the spacer portions 28b abutting against the rear 
surface 24b of primary window 24 to space the seal plate rearwardly from 
the primary window by a distance corresponding to the length of the spacer 
portions 28b. 
Clamp plate 30 is formed of a suitable metallic material and has a 
generally rectangular configuration. Plate 30 includes a main body portion 
30a and upper and lower flange portions 30b connected to main body portion 
30a by web portions 30c. A rectangular window opening 30d is formed in 
clamp plate main body portion 30a. Clamp plate 30 is secured to the front 
surface 28c of seal plate 28 by a plurality of bolts 42 with the secondary 
window 26 positioned within flange portions 30b so as to clamp the window 
between seal plate 28 and clamp plate 30. Specifically, the front surface 
26a of the window 26 is positioned against the rear surface of clamp plate 
main body portion 30a and the rear surface 26b of window 26 is positioned 
against the forward surface 28c of seal plate 28 with an annular 
elastomeric sealing member 44 positioned in seal plate annular groove 28f 
sealingly engaging the confronting outer annular portion of the rear 
surface 26b of window 26. 
Support plate 32 has a generally cylindrical configuration and fits 
slidably within inner sleeve main body portion 22a. Support plate 32 
includes a central rectangular window opening 32a conforming in size and 
shape to the window opening 28d in seal plate 28. The forward surface 32b 
of support plate 32 is positioned against the rearward surface 28e of seal 
plate 28 by a plurality of bolts 46 passing through plate 32 for threaded 
engagement with threaded bores in the rear surface of the seal plate with 
an elastomeric seal 50 positioned in seal plate groove 28g to sealingly 
engage the inner periphery of inner sleeve 22. 
Waveguide 34 is formed of a suitable metallic material and has a 
rectangular cross-sectional configuration that is uniform throughout the 
length of the waveguide. Waveguide 34 includes a first portion 34a 
extending from microwave energy source 10 and a second portion 34b 
extending axially and centrally into inner sleeve 22 with its forward end 
portion 34c passing through aligned rectangular openings 32a and 28d in 
support plate 32 and seal plate 28, respectively, to abut the forward 
annular rectangular edge 34d of the waveguide tube against the rear 
surface 26b of secondary window 26. The inner peripheral surface 34e of 
the waveguide tube has a size and cross-sectional configuration precisely 
conforming to the size and cross-sectional configuration of window opening 
30d of clamp plate 30 so that the surface defined by window opening 30d in 
effect forms a forward extension of the surface defined by the inner 
surface 34e of waveguide tube 34. Waveguide 34 is secured to sleeves 20/22 
via an annular flange 52 welded to the outer periphery of the waveguide 
tube and secured by bolts 54 to the rear flange portion 22b of the inner 
sleeve. 
An entry tube 56 extends radially outwardly from main body portion 20a of 
outer sleeve 20 proximate rear flange 20b; a discharge tube 58 extends 
radially outwardly from main body portion 20a of outer sleeve 20 in 
generally diametrically opposed relation to tube 56; and an annular flange 
60 is secured to the side wall 12b of the vacuum chamber 12, in 
surrounding relation to outer sleeve 20, by a plurality of bolts 62 with 
an annular sealing member 64 positioned in the crotch defined between 
flange 60, outer sleeve 20, and side wall 12b. 
It will be seen that the tubes 56 and 58 coact with inner sleeve grooves 
22c and 22d to define a path for delivering a cooling fluid, such as 
water, to the space 66 between the windows 24,26 and for removing fluid 
from the space so as to provide a continuous circulation of cooling fluid 
past the rearward surface of window 24. Specifically, cooling fluid enters 
through tube 56, passes through bore 20f in outer sleeve 20 and into upper 
groove 22c, passes axially forwardly between the sleeves in upper groove 
22c to circumferential groove 22d, passes radially inwardly through ports 
22e into space 66, passes downwardly in space 66 past the rearward surface 
of window 24, passes radially downwardly through further ports 22e into 
the lower portion of circumferential groove 22d, passes axially rearwardly 
in lower groove 22c, and is then discharged through outer sleeve bore 20g 
and through discharge tube 58. 
It will further be seen that the inner surface 34e of waveguide tube 34 
forms a waveguide surface portion extending from energy source 10 to the 
rear surface 26b of window 26 and that the periphery of window opening 30d 
of clamp plate 30 forms a further waveguide surface portion constituting a 
forward extension of the waveguide surface portion defined by waveguide 
tube 34. 
It will further be seen that window 26 extends radially outwardly beyond 
the waveguide surface defined by tube 34 and window opening 30d to define 
an annular outer window portion 26c outwardly of the waveguide surface and 
that the annular elastomeric seal 44 engages the rear surface of this 
annular outer portion 26c of the window 26 so that the sealing occurs at a 
location that is removed from the waveguide surface defined by the 
coaction of the inner periphery of waveguide tube 34 and window opening 
30d. 
It will further be understood that the microwave energy 70 employed in the 
invention apparatus typically has a wave length of approximately five 
inches so that the microwave energy moving down the waveguide surface 
defined by the waveguide tube is unaware of the 1/2 inch gap in the 
waveguide surface defined by the window 26. As a result, the microwave 
energy 70 moves with a constant size and form from the microwave energy 
source 10 to the forward end of the waveguide surface as defined by the 
forward end edge of window opening 30d. Since the forward end edge of 
window opening 30d is only slightly spaced rearwardly from the rear 
surface 24b of primary window 24, for example by 1/8 inch, the microwave 
energy is maintained substantially intact in terms of size and shape from 
the microwave energy source to the rear surface of the primary window 24 
so that the microwave energy passes through the window 24 and into the 
interior of the vacuum chamber substantially intact with respect to size 
and shape. As a result, the efficiency of the deposition/etching operation 
taking place within the vacuum chamber in response to the microwave energy 
is minimally deprecated by derogation in the form and size of the 
microwave's energy and the microwave energy is effectively precluded from 
access to the elastomeric seal 44 so that the problem of dealing with 
capacitive charges created at the seal by promiscuous microwave energy is 
substantially eliminated and so that, accordingly, an inexpensive 
elastomeric seal can be used in place of the expensive and exotic seals 
employed of necessity in the prior art devices. 
The invention microwave window assembly will thus be seen to allow the use 
of spaced double windows in the window assembly to avoid heating and 
failure of the primary window without derogating the size and shape of the 
microwave energy as it enters the vacuum chamber through the window and 
without necessitating the use of expensive and exotic seals to combat 
promiscuous microwave energy movement resulting from the spaced dual 
window construction. 
Whereas a preferred embodiment of the invention has been illustrated and 
described in detail, it will be apparent that various changes may be made 
in the disclosed embodiment without departing from the scope or spirit of 
the invention.