Means and methods for heating semiconductor ribbons and wafers with microwvaes

Means and method including a novel waveguide sample holder for applying traveling microwaves to heat thin low-resistivity semiconductor ribbons and wafers without a susceptor. Traveling microwaves are applied to the semiconductor materials, both with and without a traveling wave resonator. Efficient coupling is obtained by unique placement of the samples in the waveguide.

INTRODUCTION 
The present invention relates to means and methods for manufacturing and 
more particularly, to improved means and methods of applying a traveling 
microwave i.e., a wave traveling in only one direction along the 
longitudinal axis of the waveguide, to heat thin, low-resistivity 
semiconductor ribbons and wafers disposed therein without requiring the 
use of a susceptor therewith. 
BACKGROUND OF THE INVENTION 
A typical procedure in which semiconductor devices are fabricated from 
wafers entails heating the wafers during several process steps. In 
contemporary practice, wafers are routinely heated with resistance 
furnaces, infrared or quartz-halogen lamps, electron beams, and lasers. In 
some applications, radio frequency energy is used to heat a susceptor from 
which the thermal energy is transferred to a wafer by conduction, 
convection, or radiation. 
The principal problem associated with the utilization of an apparatus for 
microwave heating of low-resistivity semiconductor ribbons and wafers 
without a susceptor is the creation of an efficient applicator, i.e., the 
device with which the microwave energy is applied to the sample to be 
heated. Previous attempts to heat ribbons and wafers in this manner failed 
to provide either efficient coupling of the microwave energy into the 
sample or uniformity of heating. 
The present invention relates to innovative means and methods of applying 
traveling microwaves to thin low resistive semiconductor pieces and, more 
particularly, to means and methods of heating semiconductor ribbons or 
wafers directly by microwave energy without using a susceptor. The 
elimination of the susceptor is highly desirable because it obviates the 
need for efficient heat transfer between the susceptor and the wafer and 
eliminates the possibility of wafer contamination by the hot susceptor. 
The realization of this goal thus provides important and unique means and 
methods for the diffusion, drying, sintering and rapid annealing of such 
wafers and ribbons which means and methods are both convenient and cost 
effective. 
Prior attempts, albeit less than highly successful, have been described in 
the literature for somewhat similar problems. For example, Guidici (Siltec 
Corporation, Menlo Park, CA.) described experiments for producing 
photovoltaic devices in which coin-stacked wafers were placed in a 
microwave applicator and heated to 900.degree. C. The absorption of 
microwave radiation near the exterior surfaces of the stack generated heat 
which was transmitted to the interior of the stack by thermal conduction. 
Guidici has used the same apparatus for sintering metallization coatings 
on single wafers. 
Other experiments in which microwave energy was used to heat a small 
silicon sample were recently described by Chenevier et al at CNRS in 
Grenoble. (See: Pulsed annealing of semiconductors by microwave energy, 
Chenevier et al J. Physique-LETTERS, 43 (1982) L-291-294). The principal 
feature of the CNRS method was the use of the small silicon sample as part 
of the wall of a standing-wave resonator made from x-band waveguide. When 
the resonator is excited by the microwave field, the wall currents 
resulting from the microwave field heat the sample because of its non-zero 
resistivity. The procedure is alleged to be energy efficient (up to 30% is 
claimed), and the apparatus required to implement it is quite 
conventional. To facilitate absorption of microwave energy by a cool 
sample of relatively high resistivity, Chenevier et al use an incandescent 
lamp to decrease the resistivity of the sample by photoexcitation of 
carriers. This procedure is, however, suitable only for small samples as 
both thermal and electrical problems occur at the sample edges. 
It is apparent that a clear and present need still exists for the 
development of means and methods of applying microwaves to heat thin 
low-resistivity semi-conductor ribbons and wafers without requiring the 
use of a susceptor therewith. It is toward this need that the present 
invention is directed. 
Accordingly, a principal object of the present invention is to provide new 
and improved means and methods for heating low-resistivity materials, such 
as semiconductor materials with microwaves without a susceptor whereby the 
material being heated is the hottest body within the applicator and the 
possibility of contaminating the sample by a susceptor is eliminated. 
Another object of the present invention is to provide a new and improved 
method of heating semiconductor materials which has a relatively short 
process time because the microwave energy is dissipated directly into the 
semiconductor samples rather than to and through a susceptor. 
A further object of the present invention is to provide new and improved 
means and methods for heating low-resistivity materials such as 
semiconductor materials with microwaves which have substantially enhanced 
energy efficiency. 
These and still further objects as shall hereinafter appear are readily 
fulfilled by the present invention in a remarkably unexpected manner as 
will be readily discerned from the following detailed description of an 
exemplary embodiment thereof especially when read in conjunction with the 
accompanying drawing in which like parts bear like numerals throughout the 
several views.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The key to the present invention resides in the means and methods of 
presenting ribbons and wafers to a microwave heat source for efficient and 
uniform heating to dry or cure the ribbons/wafers and/or diffuse 
impurities thereinto. 
The embodiment herein described and illustrated employs a traveling wave 
wherein the samples are maintained within a sample holder placed in a 
stationary position relative to the wave source. 
As will appear, efficient coupling and uniformity of heating are obtained 
by placing each ribbon in what is effectively a wall of an individual 
waveguide within a composite waveguide arrangement. 
Ideally, the use of a semiconductor sample as part of a wall if a microwave 
structure will not interrupt wall currents of arcing and undesirable 
losses of microwave energy through the openings between the sample and the 
rest of the structure are to be avoided. Note that when a wafer or ribbon 
is used to replace a section of one of the broad walls in a rectangular 
waveguide in which the so-called dominant mode is present, any gap between 
the ribbon and the remainder of the waveguide will, in general, perturb 
the wall currents. As will be shown, the means of the present invention 
eliminates this difficulty by placing the sample so it functions as part 
of a broad wall which is common to two waveguides, each of which supports 
a dominant wave propagating in the same direction. To achieve the desired 
result, the traveling waves in the two waveguides must be in phase. As an 
alternative, standing waves that are in phase in the two adjacent 
waveguides may be used. With either of these arrangements, the wall 
currents circulate around the sample, flowing in one direction on one side 
of the sample and in the opposite direction on the other side. At the same 
time, the currents in other surfaces adjacent to the sample remain 
essentially undisturbed. A similar situation occurs when the planar sample 
is placed in the interior of a rectangular waveguide so that its major 
planar surfaces are parallel to the broad walls of the waveguide. In this 
manner, as many as twenty to thirty uniformly spaced ribbons may be placed 
simultaneously in a single sample holder as is shown in FIG. 1. 
Referring to the drawings and particularly FIGS. 1, 2 and 3, sample holder 
10 comprises a housing 11 formed of brass or aluminum or like alloys which 
is preferably shaped as an open-ended rectangular prism having a first and 
second shelf-like member, 12, 13 respectively, disposed one along each 
side thereof. Each shelf-like member, for example shelf-like member 13 is 
formed of heat resistant ceramic or quartz and comprises a body portion 14 
and a plurality of spaced flange members 15 extending normal from body 
portion 14 and defining a plurality of channels 16 therebetween. As 
illustrated, each flange member 15 has a support surface 17 defined 
thereupon which, in one practice of this invention, will be disposed about 
0.09 inches from the support surface 17 of the adjacent flange member 15. 
As will appear, this dimension is identical to the distance (shown as "d") 
between the center lines of adjacent ribbons 18 and is equal to twice the 
distance (shown as "d/2") between the center line of the outermost 
ribbons, 18a, 18b and the housing wall 11 adjacent thereto. In this 
particular arrangement, the width of each channel 16 will be approximately 
0.045 inches when support surfaces 17 are oriented in a horizontal plane 
and approximately 0.22 inches when support surfaces 17 are oriented in a 
vertical plane. 
In use, a plurality of ribbons or wafers 18 will be positioned within 
sample holder 10 so that the proximal edge 19 of each is disposed upon one 
support surface 17 of shelf-like member 12 within channel 16 and the 
distal edge 20 thereof is disposed in the corresponding channel 16 upon 
the corresponding support surface 17 of shelf-like member 13. 
Ribbons/wafers 18 will be disposed into each tier of support surfaces 17 
until all have a ribbon 18 disposed there upon. Within the preferred 
practice of this invention, members 12, 13 will be configured to provide 
between twenty and thirty pairs of corresponding cooperating support 
surfaces 17 will equally beneficial results. As used herein, ribbons, 
wafers, sheets and the like are used interchangeably to identify the thin 
semiconductor material embraced herein. 
As shown in FIG. 1, housing 11 of the sample holder 10 shown in FIGS. 2 and 
3, has a first flange or waveguide adapter 21 mounted at one end 22 
thereof and a second similar adapter or flange 23 disposed at the other 
end 24 thereof to complete a sample holder assembly 25 which is attachable 
into a circuit which includes, inter alia, a suitable variable power 
source (oscillator) 26 (available as Model GL103 Power Source from Gerling 
Labs, Modesto, CA). Sample holder assembly 25 is connected into the 
desired circuit arrangement by abutting one waveguide flange adapter, 
e.g., 21, with a like annular flange 27 formed upon an adjacent component, 
aligning the several holes 28 which are equispaced about the perimeter 29 
of adapter 21 in spaced inset relationship thereto with the corresponding 
holes 30 in flange 27 and passing suitable fasteners such as bolts, pins 
or the like 31 therethrough to secure sample holder assembly 25 to an 
adjacent component. Directional couplers, terminal loads, isolators and 
circulators, all standard components in microwave circuits, each have 
similar annular flange members formed thereon for convenient assembly to 
complete the microwave circuit. Each flange member is preferably formed of 
brass or similar alloy and the mating surfaces thereof will be machined to 
provide a tight surface-to-surface engagement between adjacent flanges. 
As shown in FIGS. 4 and 5, oscillator 26 can be activated by connection to 
a suitable source of power (such as standard 110V A.C. current) and will 
accomplish its desired effect upon the ribbons or wafers 18 disposed 
within sample holder 10 either with (see FIG. 5) or without (see FIG. 4) a 
resonator. 
When the cost of the initial equipment is secondary to the actual operating 
cost, the circuit with the resonator is recommended because of its 
potential for high process efficiency with low energy loss except for the 
samples. However, where set up costs are more critical than operating 
costs, the circuitry of FIG. 4 which omits the resonantor is highly 
satisfactory. 
One traveling-wave circuit useful in the practice of the present invention 
is shown in FIG. 4 wherein an oscillator is connected in series with a 
loaded circulator (isolater), a directional coupler, the sample holder and 
a terminal load. Both reflected power and incident power in the circuit 
are monitored by the directional coupler and power meters. 
A second circuit configuration useful in the practice of the present 
invention when the traveling-wave resonator is desired is shown in FIG. 5. 
A variable directional coupler is used to tune the resonator to the 
microwave source frequency. The Q of the traveling-wave resonator will be 
in the order of 400 and the microwave source will have a commensurate 
frequency stability. 
The several components of each of the foregoing circuits are clearly 
identified on the circuit of FIGS. 4 and 5 wherein conventional notations 
are employed and need not be further described here. 
With traveling waves, the average power dissipation per unit area of sample 
surface for samples of practical lengths is virtually independent of the 
coordinate corresponding to the direction of propagation if the 
attenuation is not too great. Relatively small attenuations are acceptable 
in the system configuration which includes the traveling-wave resonator. 
Consequently, only the dependence on the transverse coordinates needs to 
be considered. As can be shown, the power dissipation per unit area of the 
sample is essentially independent of the transverse coordinates when the 
broad dimension of the wave guide is chosen so as to make the cutoff 
frequency of the dominant mode equal to about 0.7 of the operating 
frequency. Thus for the dedicated IMR and D band at 2.45 GHz, the optimum 
waveguide width is about 3.41 inches or a multiple thereof. 
It is thus apparent that the present invention comprises an applicator for 
heating low-resistivity semiconductor ribbons, materials and like 
low-resistivity materials; that is, materials having resistivity in the 
range from 0.001 to 1.0 ohm-cm., in thin (e.g., circa 0.020 inches thick) 
ribbons, strips, wafers and like configurations without the use of a 
susceptor. Furthermore, planar samples of any shape may be used here with, 
subject only to the limitation imposed by the waveguide width. Where 
uniformity of heating is a prime requisite, a sample holder having a width 
that is 3.41 inches or an integral multiple of 3.41 in. permits samples 
that are both larger and smaller than 3.41 in. to be processed in the 
applicator with highly successful results. In those applications when 
uniformity of heating is not required or irregularity of heating (e.g., 
hot centers or hot edges) is sought, the specific width relationship 
enumerated above can be ignored. 
Efficient coupling is obtained through placement of the samples in what are 
effectively the walls of waveguides within a composite waveguide in which 
the dominant mode propagates in each. Uniformity of heating, when the 3.41 
relationship is applied, will be assured through the use of 
traveling-waves rather than standing-waves. 
In one practice of the present invention, a sample holder embodying the 
present invention and holding up to 15 samples was fabricated from WR 284 
waveguide. Its width is not the optimum value. Experimental data for 
traveling-wave configurations was obtained by measuring attenuation and 
VSWR (Voltage Standing Wave Ratio) on 0.02 inch thick silicon wafers with 
a nominal resistivity of 0.01 ohm-cm. A summary of the results and a 
comparison of experimental and theoretical values of the attenuation 
constant are shown in Table I, below. 
TABLE I 
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Number of Attenuation Attenuation 
VSWR 
Ribbons dB(Calc) dB(Meas.) (Meas.) 
______________________________________ 
0 -- -- 1.04 
1 .13 .13 1.06 
3 .31 .40 1.06 
5 .67 .67 1.08 
15 2.23 2.24 1.09 
______________________________________ 
The model GL103 Power Source and control console (Gerling, Ibid.) 
combination employed here with provides a completely integrated power 
source for use in either laboratory or production assignments because it 
utilizes three phase input power and has a very low ripple output signal. 
This is accomplished through the use of a power transformer which has 
separate three phase secondaries, one Y connected and one delta connected, 
which are independently rectified and the dc outputs combined in series to 
give a 12 phase output ripple waveform having a very low peak to peak 
ripple with a minimum of filter components. 
In practice, power output of the power source is adjusted by raising and 
lowering the current in the electromagnet surrounding the magnetron, thus 
raising and lowering the level of the magnetic field in the magnetron 
interaction space. If the field is high enough, no electrons will be able 
to cross the interaction space resulting in zero output. As the field is 
reduced, electrons are able to make the transition thus increasing the 
output. The current through the electromagnet is controlled by a solid 
state circuit using the current through the magnetron as a reference 
signal. This allows the output to be smoothly adjusted without waveform 
distortion at all levels from 0 to full power. 
The control system contains two additional circuits which increase the 
versatility of this power source. The first is one which permits the power 
source to be controlled by an analog voltage. In this mode of operation, 
an output signal from 0 to -1 volt will cause the power source to go from 
a preset output to zero output. 
In the second mode, the output can be regulated to an input reference 
voltage anywhere in the range of 0 to -1 volt. Typically, this control 
option allows the power output to be regulated against line voltage 
changes by using the signal from the power output meter as the reference. 
The major characteristics of the power source are summarized as follows: 
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Frequency 2.45 + 30-20 GHz 
Power Output 2.75 kW min - 3.0 kW nom 
Power Control 0 to 3 kW 
Power Waveform Low ripple 
Output Waveguide 
WR284 
Output Flange WR284 Cover Flange w/GL taper 
and pin alignment system for 
use with a V band single screw 
clamp 
______________________________________ 
In one practice of the present invention a plurality of ribbons formed of 
semiconductor material are disposed in spaced parallel relationship to 
each other so as to provide uniform distance ("d") between the axial 
center lines of each pair of adjacent parallel ribbons and a lesser 
proportionate distance (d/2) between the center line of the extreme 
ribbons/wafers and the adjacent housing wall. The adjacent housing walls 
function as an electrical reflector so that microwaves are impinged upon 
both planar surfaces of each ribbon so disposed. As arranged, each ribbon 
functions as a waveguide will within a composite waveguide system defined 
thereby within the sample holder. The ribbons so mounted are then placed 
in the operative traveling-wave field of a microwave generator, the 
traveling microwaves are impinged upon both planar surfaces of each of the 
several ribbons until the desired heat effect is obtained, the generator 
is deactivated and the ribbons unloaded from the sample holder for such 
subsequent handling as the exigencies of their intended use may require. 
A preferred ceramic for use in the fabrication of the shelf-like members 
hereof is hydrous aluminum silicate which is available from General 
Electric under the tradename "Grade A Lava". This material can be readily 
formed prior to curing and thereafter fired to provide a very hard heat 
resistant electrically insulating ceramic shape. Of course, other heat 
resistant insulators such as fused quartz, sapphire, aluminum oxide, and 
like heat resistant ceramics, and even heat resistant Pyrex.RTM. glass 
(Corning) can be used to form the shelf-like member when the intended 
thermal operating conditions are such that the material can survive the 
cycle. 
From the foregoing, it is apparent that means and methods have been herein 
described and illustrated which fulfill all of the aforestated objectives 
in a remarkably unexpected fashion. It is of course understood that such 
modifications, alterations and adaptations as may readily occur to the 
artisan confronted with this disclosure are intended within the spirit of 
this disclosure which is limited only by the scope of the claims appended 
hereto.