System for optimizing process parameters in photoactive semiconductor manufacturing in-situ

Materials and systems substantially having photoactive properties are produced with a high quality output and without time losses in the fabrication process. To determine the quality of the photoactive material in situ, conductivity is induced in the material by exciting charge carriers through irradiation, and an electromagnetic field influenced thereby is measured, with the result of the measurement being evaluated by a computer with a corresponding control of actuating members such as valves and controllers. Optimum process parameters are thus found and used for the process.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates in general to semiconductor fabrication 
techniques and in particular to a new and useful method of making 
semiconductor materials or semiconductor components wherein the setting of 
process parameters is based on material properties which are measured 
during the process. 
Prior art which is relevant to the present invention can be found in the 
publication "Riber R.H.E.E.D. System", October 1975 by the company Riber 
S. A., Ruel-Malmaison, France. That system provides corresponding sensors 
and actuators for pressure, temperature, and other process parameters. To 
what extent a process actually goes or went on in the desired way, may be 
learned, for example in processes such as film formation by growing 
poly-of monocrystaline layers, by in-situ monitoring of the surface 
texture by means of electron beam deflection patterns. Then, upon 
visualizing the results on a luminous screen, the parameters may be varied 
empirically, to optimize the process. 
A substantial problem arising with attempts to make the process automatic 
is to find or select an effect particularly characteristic of the 
intermediate product or the finished part and capable of being measured 
under conditions of automatic operation thus permitting a correct 
interpretation of the relationship between varying measured data and the 
process parameters, and permitting the making of correct decisions. 
Finally, a computer system must be provided and correspondingly 
programmed. 
SUMMARY OF THE INVENTION 
The present invention is aimed at optimizing the fabrication of 
photo-active materials and systems, i.e. particularly, ensuring a higher 
quality and output, and eliminating time losses, by providing in-situ 
measurements. 
Accordingly, an object of the present invention is to provide a method of 
making semiconductor materials and semiconductor components wherein the 
setting of process parameters is based on material properties measured 
in-situ, characterized in that the materials and/or components 
substantially exhibit photo-active properties and, during their process of 
formation, are repeatedly exposed to irradiation to excite charge 
carriers. Variations of an electromagnetic field resulting from the 
conductivity induced in the photo-active material by the charge carriers 
excitation are measured and the measured data are supplied to, and 
evaluated in, a computer. The process parameters are controlled to an 
optimum extent through control and measuring members which are coupled to 
each other through the computer. 
What is substantial and advantageous is that with the inventive method, the 
quality can be monitored without interrupting the fabrication process, 
that the needed corrections of the process parameters become effective 
instantly, and that the specific effects of these corrections are 
automatically and optimally combined with each other to the desired total 
effect. In addition, such combinations are not limited to empirically 
acquired data. The learning and record keeping capability of the computer 
makes it possible to use the available spectrum of parameter combinations 
to automatically determine and select the optimum conditions. 
It is of fundamental importance to the invention, that photoactive 
materials or components are concerned whose photoconductivity can be 
measured without thereby unfavorably affecting the process conditions. 
This is advantageously done with a contact free measuring method. 
Particularly suitable in this respect are microwave measurements known per 
se and recently applied at a growing rate for testing of various 
semiconductor systems. The photosensitivity is measured for example in 
time resolution, also if measuring is effected through electrical contacts 
placed on the substrate before the coating process, by a contactless 
excitation of charge carriers, such as with a small mobility of the charge 
carriers, quality signals of this kind, measured through microwave 
absorption (TRMC) or directly as photoconductivity (PC), are equivalent to 
each other. Which of these measuring methods to apply depends on the 
respective technical conditions. 
Other embodiments of the invention use light radiation. Such use mainly 
relates to in-situ measurements in marginal conditions as well as to the 
general conduction of the process where quite different conditions 
possibly unfavorably affecting each other are to be observed in the 
individual stages. Multichamber systems, for example, may be provided in 
such instances, to create unequal conditions of temperature, pressure, gas 
composition, longer or shorter dwell, local fixing for a certain time of 
the product, rotary motion, etc. The transfer times from one working step 
to the other should be reduced to a minimum, to ensure a continuous 
process with a conveyance which, taking into account the above-mentioned 
variations, might better be termed quasi-continuous. 
Another object of the invention is to provide an apparatus suitable for 
practicing the method. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which 
preferred embodiments of the invention are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An apparatus for carrying out the inventive fabricating method comprises 
the parts shown as blocks in FIG. 1, which are operatively interconnected 
as indicated by arrows. The fabrication of photosensitive materials or 
systems, particularly components, which may also consist of an improvement 
of a given material, takes place in a process chamber P. Even a plurality 
of process chambers P may be provided. The properties of the 
photosensitive material or system in process chamber P are determined as 
quality signals by means of a measuring device M. To this end, the 
material or system in chamber P is irradiated from the outside, in the 
present example by a source of light L. A computer R receives measurement 
signals from both measuring device M and sensors S, evaluates them, 
computes the parameter values for the next operations, and correspondingly 
controls particularly the control member or actuators A. This makes of the 
apparatus a fully automatically operating control circuit closed in 
itself. 
The adjustment or setting of actuators A for an optimum progress of the 
operation is continuously kept up. In particular, new combinations of 
operational parameters may be found during the fabrication of conventional 
or new kinds of photosensitive materials or systems, which then lead to 
higher material qualities. 
The development of a working step within the optimization strategy is 
illustrated in detail in FIG. 2. 
As shown in FIG. 2, a process start signal establishes a process on 
condition. At box 10, variable values of process parameters obtained 
through the optimization strategy of the invention are selected. Initial 
conditions can be used at first. 
The actuators A are then set in accordance with the parameter values of box 
10. Box 12 shows the step of applying sensor signals to the computer 
whereafter coating of the substrate carrier takes place. For the start of 
a new working cycle, box 14 illustrates the step of determining 
conductivity using for example, the TRMC or PC methods as a reference at 
the start of a new working cycle. This for example is after a long 
interruption or after n optimization steps. Illumination and conductivity 
measurement then takes place as shown in box 16, followed by signal 
aquisition evaluation. 
A first interrogation shown at diamond 18 counts the number of measurements 
i which is in integer from 1 to n. If the integer has not yet reached the 
value n the process returns to the step of box 10. n is the number of 
measurements per optimization step or per generation. 
When the integer has reached the value n, the step shown in box 20 is 
executed which is the selection of a parameter combination for the layer 
having best quality signal. 
A second interrogation at 22 determines the difference between the quality 
signals of the optimization steps or generations which establishes 
discontinuance criterian. Once this has been reached, the process stops. 
FIG. 3 shows a basic concept of a test apparatus for a computer aided 
process optimization (CAPO). The individual parts are indicated by the 
reference letters according to FIG. 1, and some peripheral devices of 
computer R are shown. Such an apparatus is suitable particularly for 
determining process conditions in the fabrication of new kinds of 
photosensitive materials. 
FIG. 3 shows a computing assembly 24 which includes computer or CPU R. This 
is connected through interfaces, input or input/output devices to a curve 
drawing recorder, a printer, a display unit which, for example can be a 
CRT, a keyboard and a memory storage facility which may, for example, be a 
floppy disc device. The CPU contains a program for the optimization 
strategy of the invention. 
The computing circuit 24 is connected to a process unit 26 which includes 
the chamber P, the actuators A, the sensors S. Sensors S are connected 
through an analog to digital converter to one of the interfaces of the 
computer R. Actuator A is also connected through this interface to the 
computer. 
Measured data taken from the chamber is applied to the signal for the 
sensors and data for setting the actuators is applied to the actuators for 
modifying their effect on the chamber. 
An illumination and measurement unit 28 is connected to the process unit 26 
and includes the source of irradiation L, the measuring unit M and a 
digitizer for connecting the measuring unit the computer through an 
input/output device. 
EXAMPLE 1 
Producing Amorphous Silicon Layers For Solar Cells 
Amorphous silicon, a-Si, may be produced in a glow discharge by decomposing 
silane, SiH.sub.4. The particular process parameters are: pressure in the 
deposition chamber, substrate temperature, and rate of flow of a gas or 
gas mixture. The actuators A, such as valves and heating controllers, are 
adjusted, and the quality and sensor signals are collected and evaluated 
in or by the computer R. At the same time, the signal and magnitude of the 
variations of the process parameters are determined, and optimized in 
accordance with rules of experimentation of optimizing strategies. The 
corresponding measured quantity is the conductivity variation in the a-Si 
layer. This variation is determined by microwave measurement, for example, 
which is based on the proportionality between .DELTA.P, the microwave 
absorption variation and .DELTA..sigma., the conductivity variation due to 
excitation of charge carriers. 
EXAMPLE 2 
Producing a Photoconductive Polymer Layer 
The process is conducted in an apparatus according to FIG. 4. FIG. 4 shows, 
in a sectional view, a reaction or process chamber P for this example 
serving as a momentaneous receiver during the process. A contacted foil of 
plastic 30 is fixed to a substrate 32 of glass for example. The foil is 
coated with a catalyst 34, such as the Ziegler-Natta-Catalysts. A monomer, 
for example acetylene, is directed at 36 through chamber P. The reaction 
with the catalyst results in polyacetylene which deposits as a polymer 
layer 38. The coating takes place under light irradiation applied at 40. 
To optimize this process, again charge carriers are excited by incident 
light at 42 and microwave measurement (also at 42) is used as in the first 
example. 
EXAMPLE 3 
Improving a Surface 
A photosensitive substrate is to be provided with a surface improving layer 
to satisfy predetermined quality requirements. The optimum composition and 
thickness of this layer are obtained, in accordance with the predetermined 
requirements, by means of an apparatus of FIG. 3. Produced may be, for 
example, an antireflection layer, a passivating layer for an 
optoelectronic component, an anticorrosive layer, etc. 
EXAMPLE 4 
Electrochemical Deposition of a Layer 
Process chamber P is designed as an electrochemical cell. Layers are 
deposited in accordance with the adjusted process parameters, so that they 
form optimally, i.e. satisfy the predetermined requirements. 
EXAMPLE 5 
Producing Energy Converting Boundary Layers, For Example for a P-N Solar 
Cell 
In a process chamber P, an n-type layer is deposited on a p-type layer. The 
variation in time of the microwave absorption is measured. Here again, the 
variation of the induced conductivity caused by irradiation from the 
outside and excitation of the charge carriers in the produced layers, is 
the optimization measure. 
The plasma deposition apparatus shown in FIGS. 5 and 6, are substantially 
identical with each other. Only the measuring equipment connected thereto 
are different. In the apparatus according to FIG. 5, the photoconductivity 
is measured directly through contacts 44, 46 which must be fixed to the 
substrate carrier 48 in advance, prior to the coating. The apparatus 
according to FIG. 6 is equipped with a microwave measuring system. 
In FIGS. 5 and 6 the same reference numerals are used to designate the same 
or similar elements. 
A substrate which is for example, of quartz is provided at 50 on the 
carrier 48. The carrier acts as electrode which cooperates with 
counterelectrode 52 that is powered by a high frequency generator 54. The 
coating is irradiated by a laser pulse at 56 through a window in chamber 
P. Silan is supplied and discharged to and from the chamber at 58. 
Temperature is measured by a thermocouple on a lead 60 and the heating is 
provided by a heating conductor 62. 
In the embodiment of FIG. 5, the conductivity measurement taken across 
contacts 44, 46 are applied to a digitizer 64 which digitizes the analog 
signal and applies it to computer R which generates signals on a line 66 
for application to the actuators. 
In the embodiment of FIG. 6, no measuring contacts are used. Rather, 
waveguide 68 transmits and receives microwaves to and from substrate 50. 
The light irradiation is transmitted from a microwave source 72 through a 
circulator 70 and from circulator 70 to a detector 74. The signal of 
detector 74 is applied through digitizer 64 to computer R which generates 
the actuator signals on line 66. Other elements of FIG. 6 have the same 
function as those shown in FIG. 5. 
Specifically, such an apparatus comprises the following devices: 
______________________________________ 
Computer: micro PDP 11, Company DEC 
Terminal: company DEC 
Process chamber: 
Vacuum system: Balzers TSU 171 
Pumps: Leybold TM 230 
Pressure sensors: PM 410 
HF components: Kenwood TS 530, AT 
230 
Flowmeters: MKS 260, 264A, 260 PS-2 
Temperature controllers: Eurotherm 
Type 820 
Power supply: SM 6020 Electronic 
Irradiation source: 
Nd-Yag-Laser, pulse duration 10 ns 
Wavelengths: 1064 nm and 532 nm 
Measuring equipment: 
Microwave components from the compan- 
ies Hughes, USA; Mid-Centry, GB, 
Waveline, USA 
Digitizer: AD 7912, Textronix 
______________________________________ 
With an apparatus of this kind, a single-dimension in-situ optimization has 
been conducted in accordance with the Fibonacci search, with the 
temperature as the variable. 
Under certain conditions, particularly of a small charge carrier mobility, 
the quality signals obtainable with the microwave measuring apparatus are 
equivalent to those obtained with the photoconductivity measurement 
according to FIG. 5, as may be learned from FIGS. 7 and 8. 
FIG. 9 illustrates the employed strategy. The predetermined search region 
was within the temperature limits of 150.degree. C. and 300.degree. C. 
It was known from the literature that within this interval, the best layer 
qualities can be produced. The other process parameters were maintained 
constant: 
______________________________________ 
Rate of Flow V: 
5 sccm (standard cubic centimeters 
per minute) 
Pressure P: 0.6 mbar = 0.6 hPa 
HF-power P.sub.HF : 
3 watt 
______________________________________ 
By separate consecutive steps, the search region has been narrowed until an 
optimum with a predetermined interval of uncertainty was reached. Within 
the starting interval K=0, the minimum step is Tmin=50.degree. C. The 
measured qualities showed that in any case, temperatures below 207.degree. 
C. can be eliminated. During the next step K=1, a measured quality of 196 
was found. Therefrom it could be next to inferred that below 264.degree. 
C., the qualities are lower, and that also below 243.degree. C., no higher 
quality can be expected than 264.degree. C. What had to be found out 
further was only whether higher qualities than 196 occur above or below 
264.degree. C. In step K=2, at 278.degree. C. the quality of 239, the 
maximum of all measurements heretofore, was measured. At this, the search 
interval narrowing was interrupted by the computer and the temperature of 
278.degree. C. which was already short of the predetermined interval of 
uncertainty, has been accepted as the optimum. 
In FIGS. 10, 11 and 12, the qualities measured within the search steps K=0 
to K=2 according to FIG. 9 are plotted as photovoltage against time at 
temperatures T.sub.1 =150.degree. C. (FIG. 10); T.sub.2 =300.degree. C. 
(FIG. 11); and T.sub.opt =278.degree. C. (FIG. 12). The above indicated 
numbers are readings of the respective scale and correspond to certain 
voltages, the exact values of which, however, are not too important in the 
determination of an optimum. The computer processes the readings of the 
scale. Thus, in the CAPO process, an optimum was found after three cycles 
and six quality measurements. 
A time resolved microwave conductivity (TRMC) measurment is of particularly 
importance in photoelectrochemical process too. Here again, generation of 
charge carriers and their transport, and variation of the conductivity of 
the liquid photosensitive medium are responsive to an irradiation with 
light pulses and are measured through the microwave absorption. What is of 
interest is what happens at the interface between a semiconductor and a 
liquid electrolyte. In accordance therewith, the conductivity variation 
comprises two parts, namely the induced conductivity of the ions and 
carriers of electric charges, and the conductivity due to an absorption of 
electric energy from induced dipoles. To this see "J. Electrochem. Soc." 
Vol. 131, No. 4, April 1984, pages 954 to 956 by Kunst, Beck and 
Tributsch. This article, which is by three of the present co-inventors, 
was published only after the present invention was made. 
From this article it can be seen that the teaching of the present invention 
is applicable not only to a photoactive material in solid state systems, 
but also to the fabrication of photoelectric chemical solar cells. 
In this respect, FIG. 13 shows a design comprising an electrochemical cell 
77 enclosed in a plastic tube 76 which is centrally screwed into a short 
circuited waveguide 78 of the K.alpha. band. A semiconductor working 
electrode 80 forms the bottom cell closing the plastic tube 76. A platinum 
wire 82 serves as the counterelectrode, and a K.sub.2 SO.sub.4 solution is 
employed as the liquid electrolyte in chamber 77. 
Platinum wire 82 also acts as a reference electrode. Wire 82 is connected 
to a potentiostat 88 and electrode 80 is connected through an ohmic 
contact 84 and wire 86 to the potentiostat 88. Microwaves are applied and 
measured at 90, the microwaves being K.alpha. band of 26 to 40 GHz. Laser 
light is applied at 93, in pulses from a Nd-Yag-Laser having a pulse 
duration of 10 ns and a wavelength of 530 nm. 
According to FIG. 14, in the waveguide system, microwaves of the region 26 
GHz to 40 GHz, for example 30 GHz, are directed from the outside against a 
semiconductor electrode 80 of a cell 76, which extends transversely to the 
wave propagation. The microwaves which are reflected from the interface 
between the semiconductor and the liquid electrolyte, pass through a 
directional coupler or circulator 94 to the detector 96. The output signal 
of the detector is proportional to the microwave power variation, at least 
within the low power range, and is recorded after excitation as a function 
of time in triggering oscilloscope 98. 
FIGS. 15 and 16 show curves of such measurments. It appears that with an 
increasing electrode potential, up to 6 volts the photocurrent increases 
progressively, without saturation. The induced conductivity initially 
decreases rapidly (the decay time is partly determined by the laser 
pulse). Then, the conductivity decrease slows down. 
The time scales shown in FIG. 17 are to demonstrate that time can be saved 
with the inventive method, as compared to prior art in-situ measurements. 
This does not yet take into account time reductions resulting from a fully 
automatic optimizing process, thus, for example, such becoming manifest as 
a higher output and a faster sequence of the individual working steps. 
Another advantage is the higher quality standard achievable with the 
inventive method, which is also a weighty factor, even if in another 
connection. 
Typical working steps 1a, 1b, 2a, 2b, for example while forming an a 
Si:H-layer, comprise the following measures: 
Conventional method (1a, 1b in FIG. 17) 
1a: Deposition; typical layer thickness: d=5,000 (.ANG.) used rate of 
deposition r.sub.d for: 
I: Monosilane--SiH.sub.4 : r.sub.d =1 . . . 3 (.ANG./s) results in (strip 
I.1a=) 40 min. 
II. Disilane--Si.sub.2 H.sub.6 : r.sub.d =15 . . . 30 (.ANG./s) results in 
(step II.1a=) 4 min. 
1b. Switching off the apparatus, ventilation, sampling, contacting, 
measuring, evaluation, introducing a new substrate, setting the initial 
conditions, starting a new layer deposition: 
result in (strips I.2a.times.II.2a=) 180 min. 
Present method (2a, 2b in FIG. 17) 
2a: Deposition; typical layer thickness: d=500 (.ANG.) (this is the minimum 
layer thickness to obtain a quality signal) used rate of deposition 
r.sub.d (as mentioned above) for: 
I: Monosilane--SiH.sub.4 : r.sub.d =1 . . . 3 (.ANG./s) results in (strip 
I.2a=) 4 min. 
II. Disilane--Si.sub.2 H.sub.6 : r.sub.d =15 . . . 30 (.ANG./s) results in 
(strip II.2a=) 0.4 min. 
2b: Illumination (i.e. irradiation), determining the quality during the 
flow discharge, transmitting the measurment signal, processing in the 
computer, adjusting the actuators, starting a new layer deposition (i.e. 
continuation of layer deposition with new setted actuators): 
result in (strips I.2b=II.2b=) 2 min. 
(The quality is determined in-situ through a time-resolved microwave 
conductivity measurment--TRMC--or photoconductivity measurment--PC--, as 
noted above.). 
The strips I.1a, I.2a, and II.1a, II.2a in FIG. 17 relate to average 
deposition rates r.sub.d of 2 .ANG./s for Monosilane (I) and of 20 .ANG./s 
for Disilane (II). It can be seen therefrom that: 
EQU t(I.1a+I.1b)=220 min=t(I.1) 
EQU t(I.2a+I.2b)=6 min=t(I.2) 
and 
##EQU1## 
EQU t(II.1a+II.1b)=184 min=t(II.1) 
EQU t(II.2a+II.2b)=2.4 min=t(II.2) 
and 
##EQU2## 
which is, in any case, a remarkable saving in time of the present 
invention over the prior art. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.