Method of measuring the temperature of a photocathode

A method of determing the actual temperature of a layer of an infrared material, especially during heat cleaning, which includes measuring the thickness of the layer and the amount of radiation being emitted from it. An apparent temperature corresponding to a desired actual temperature is found from a curve of apparent temperature, which are derived from the radiation amount, versus thickness. The apparent temperature which corresponds to the desired actual temperature compensates for interference effects on the radiation measurement. A computer may be utilized to calculated the apparent temperature which corresponds to the desired actual temperature and to regulate and maintain the infrared material at the apparent temperature.

BACKGROUND OF THE INVENTION 
This invention relates to image intensifier tubes and more particularly to 
photoemissive cathodes for use in such tubes. 
Image intensifier tubes multiply the amount of incident light they receive 
and thus provide an increase in light output which can be supplied either 
to a camera or directly to the eyes of a viewer. These devices are 
particularly useful for providing images from dark regions and have both 
industrial and military application. For example, these devices are used 
for enhancing the night vision of aviators, for photographing 
extraterrestrial bodies and for providing night vision to sufferers of 
retinitis pigmentosa (night blindness). 
Image intensifier tubes utilize a photoemissive wafer which is bonded to a 
glass faceplate to form a cathode. Light enters the faceplate and strikes 
the wafer, thereby causing a primary emission of electrons. 
After the formation of the cathode, a heat cleaning step is performed to 
remove contaminants, such as oxygen and carbon from the surface of the 
photoemissive wafer. Bringing the cathode to a specific temperature and 
maintaining the cathode at that temperature are necessary in effecting 
proper heat cleaning of the cathode so that its structure and properties 
are not adversely affected. Knowing the heat cleaning temperature is also 
necessary in order to avoid the formation of brush lines in the otherwise 
transparent photoemissive wafer. 
Thermocouples cannot be used to measure the temperature of cathodes because 
they tend to damage the fragile surface of the cathode or give inaccurate 
readings. The most convenient method is radiative measurement of the 
temperature using a thermometer based on detection of blackbody radiation. 
Instruments which detect infrared radiation a wavelengths for which the 
layers are transparent and the glass is opaque sense the radiation from a 
thin layer at the interface of the glass and wafer plus a contribution 
from the wafer. Due to the proximity of the interface to the gallium 
arsenide layer and the good thermal conductivity of semiconductors, the 
measured temperature is a good indicator of the true wafer temperature. 
However, the wafer acts as a thin film which causes the apparent 
temperature to vary by a large amount due to interference effects caused 
by multiple reflection of the blackbody radiation between the internal 
surfaces of the wafer. However, by measuring the thickness of the wafer 
and knowing the indices of refraction, a correction can be made for the 
interference. 
One solution to the problem of accurate photocathode temperature 
measurement during heat cleaning is given in application Ser. No. 814,132, 
filed Dec. 27, 1985, entitled "Method of Measuring the Temperature of a 
Photocathode", in the name of A. Amith. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide a method of measuring 
the temperature of a photoemissive wafer of a cathode. 
It is an additional object of the present invention to provide an accurate 
temperature measurement of a photoemissive wafer during heat cleaning 
which is compensated for cathode interference. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
accomplished by the present invention which provides a method of 
determining the temperature of a layered structure which includes at least 
one layer of an infrared transparent material including measuring the 
thickness of the material, determining the level of radiation being 
emitted or transmitted through by the layered structure, applying a 
correction factor derived from the thickness measurement and calculating 
the temperature of the structure from the correction factor and the 
radiation level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 there is shown a cathode 10 which includes a faceplate 12 and a 
photoemissive wafer 14. The faceplate 12 can be made of a clear, high 
quality optical glass such as Corning 7056. This glass comprises 70 
percent silica (SiO.sub.2), 17 percent boric oxide (B.sub.2 O.sub.3), 8 
percent potash (K.sub.2 O), 3 percent alumina (Al.sub.2 O.sub.3) and 1 
percent each of soda (Na.sub.2 O) and lithium oxide (Li.sub.2 O). Other 
glasses may, of course, be used. In shape, the faceplate 12 includes a 
central, generally circular body portion 12a and a reduced thickness sill 
portion 12b in the form of a flange surrounding the body portion. One 
surface 34 of the faceplate 12 extends continuously across the body and 
sill portions 12a and 12b, respectively, and the portion of this surface 
extending over the sill portion 12b and a small adjacent portion of the 
central body portion 12a fits under a flange 36 and is secured thereto to 
retain the faceplate 12 in a housing (not shown). The remainder of the 
portion of surface 34, that is, that portion surrounded by the flange 36 
is the exposed surface of the faceplate 12 on which input light impinges. 
The faceplate 12 also includes surface portions 38a and 38b which are 
generally parallel to surface 34 and which extend over the body portion 
12a and sill portion 12b, respectively. Because of the difference in 
thickness between the body portion 12a and the sill portion 12b, the 
surface portions 38a and 38b lie in different planes with the portion 38a 
being spaced farther from the surface 34 than is the portion 38b. 
Extending between the surface portions 38a and 38b is a connecting surface 
portion 38c which, in the embodiment disclosed herein, is generally 
frusto-conical. 
The photoemissive wafer 14 is bonded to the surface portion 38a so that 
light impinging on the exposed portion of surface 34 and eventually 
striking the wafer 14 causes the emission of electrons. These electrons 
are accelerated across a gap by an electric field to a MCP 40 causing the 
secondary emission of electrons all in accordance with known principles. 
Connecting the photoemissive wafer 14 to an external biasing power supply 
(not shown) is a coating of conductive material 42 applied to the surfaces 
16b and 16c and also over a portion of surface 16a so that this coating 
makes contact with the wafer 14. 
The photoemissive wafer 14 may be formed in any known manner. One such 
method is described with reference to FIG. 2. A gallium arsenide (GaAs) 
substrate 16 has formed on one of its surfaces a layer 18 of gallium 
arsenide (GaAs) which is identified as a buffer layer. The formation of 
the buffer layer 18 is to facilitate control of a later etching process to 
remove the substrate. An etch stop layer 20 of gallium aluminum arsenide 
(GaAlAs) is formed on top of the buffer layer 18 and an active layer 22 of 
gallium arsenide (GaAs) is formed on the etch stop layer 20. 
The active layer 22 has a layer of gallium aluminum arsenide (GaAlAs) 
formed on its surface and is identified as the window layer 24. Generally, 
formation of the wafer 14 results in a structure which is larger than that 
required for the image intensifier tube. One way of achieving the proper 
diameter for the wafer 14 is to cut the wafer with a saw. If this step is 
to be performed, then cap layer 26 of gallium arsenide (GaAs) is formed on 
top of the window layer 24. This cap layer 26 will provide protection to 
the underlying structure during cutting to prevent chipping of the window 
layer 24. 
Another way of achieving the proper wafer diameter is to carefully chip 
away the excess portions of the wafer 14 after it is bonded to the glass 
faceplate 12. In using this method, a cap layer is not necessary. 
Preferably, the formation of each of the layers is by means of epitaxial 
growth. 
If the cap layer 26 is used, it is removed after cutting, preferably by 
chemical means such as etching. 
On the surface of the window layer 24 is deposited a thin layer 28 of 
silicon nitride (Si.sub.3 N.sub.4). The silicon nitride layer 28 has a 
layer 30 of silicon dioxide (SiO.sub.2) deposited on its surface. Both the 
silicon nitride layer 28 and the silicon dioxide layer 30 are preferably 
formed by sputter deposition. The structure so formed is identified as a 
wafer 32. 
The wafer 32 is positioned with the silicon dioxide layer 30 against the 
surface portion 38a of the faceplate 12. The wafer 32 is bonded to the 
faceplate 12 in a bonding apparatus to form a unitary structure. The 
temperature in the bonding apparatus is raised and pressure is applied to 
the wafer 32 and the faceplate 12 for a length of time sufficient for 
bonding to occur and for a unitary structure to be formed. After bonding, 
the unitary structure is cooled. 
Following cooling, the GaAs substrate 16 removed. This is preferably done 
by lapping off most of the substrate 16 by mechanical polishing. The 
remaining portion of the substrate 16 is thereafter removed by chemical 
etching. The buffer layer 18 and the etch stop layer 20 are also removed, 
preferably by a chemical etching process. The structure is now identified 
as the cathode 10. 
The conductive coatings 42 are applied to the surface portions 38b and 38c 
and a small portion of 38a which is contiguous with 38c. 
The cathode 10 is then heat cleaned to remove contaminants from the surface 
of the wafer 14. The heat cleaning temperature is dependent upon the 
nature of the contaminants and upon the nature of the surface from which 
the contaminants are to be removed; that is, the actual percentage of 
gallium and arsenic at the surface. Once the nature of the contaminants 
and the actual ratio of gallium to arsenic is known, a specific heat 
cleaning temperature is determined. A temperature of approximately 
600.degree. C. is sufficient to free contaminants such as oxygen and 
carbon where the ratio of gallium to arsenic is 1:1. 
Reference will now be made to FIG. 3. In order to perform the heat cleaning 
step, the cathode 10 is placed in a high vacuum chamber 50 and is heated 
to the predetermined temperature. A pyrometer 54 receives radiation 
emitted by the cathode 12 through a window 56 in the chamber 50. At the 
predetermined temperature contaminants are freed from the cathode 10 and 
are removed by the vacuum system. Heat is provided by means of a lamp 52, 
although any other suitable heat source may be used. The embodiment of the 
invention uses a predetermined temperature of approximately 600.degree. C. 
It is important to maintain the wafer 14 at the predetermined temperature 
in order to achieve proper surface cleaning and to avoid shading and 
instability in the wafer. 
Another problem which is related to the heat cleaning temperature is the 
appearance of brush lines or crosshatching marks which result from 
stresses arising between the GaAs substrate 14 and the glass faceplate 12 
during the cooling stage following bonding or heat cleaning. The lines or 
marks appear at a point within the range of temperatures used for heat 
cleaning. Therefore, it is important to maintain the heat cleaning 
temperature below that point. 
Methods of measuring the temperature of the cathode include measurement of 
the peak wavelength being emitted by the cathode. Since the body emits an 
envelope of wavelengths, it is also possible to measure the intensity of 
any wavelength in the envelope. One instrument used to measure wavelengths 
in a specific range is a pyrometer which uses black body radiation to 
measure the peak wavelength being emitted by a body and translating that 
wavelength into temperature. The particular type of pyrometer used herein 
is an IRCON with an operating range of 4.8-5.2 .mu.m. 
However, the wafer 14 is very thin and it is difficult to monitor the 
blackbody radiation of such a thin layer, except if one chose to look only 
at a short wavelength whose absorption length is a small fraction of the 
cathode thickness (e.g. .lambda.&lt;0.6 .mu.m, where absorption length is 
less than 0.25 .mu.m). This fundamental limitation is based on the 
relationship between the emittance and the absorbance. While using an 
IRCON type pyrometer instrument which is tuned to .lambda.=0.6 .mu.m (or 
shorter) would enable one "in principle" to monitor the temperature of the 
cathode itself, this approach has numerous practical pitfalls, one of 
which is the extremely low intensity of 0.6 .mu.m radiation at the 
temperatures of interest and another of which is the large amount of stray 
radiation present if a lamp is used for heating. The pyrometer is 
therefore really looking at the wavelengths being emitted by the faceplate 
and does not see the wavelengths being emitted by the wafer 14. 
In addition, the transparency thickness and index of refraction of the 
wafer 14 cause light which enters one surface of the wafer 14 to be 
reflected one or more times before being transmitted through the other 
surface of the wafer. The result is a fringe pattern or fluctuation of 
energy and output with wavelengths which affect the pyrometer reading. A 
slight variation in thickness of the wafer 14 will affect the readings of 
the pyrometer since the interference phenomenon is very sensitive to 
thickness. Hence totally erroneous temperature readings will result from 
the spectral redistribution of energy caused by the interference. 
It has been found that by measuring the thickness of the wafer 14, it is 
possible to determine the exact temperature using the pyrometer reading by 
applying a compensating factor for the thickness of the wafer 14, which 
factor takes into account the fringe pattern. Since the refractive indices 
of the gallium arsenide and gallium aluminum arsenide layers of the wafer 
14 are almost identical, the wafer 14 may be considered to be a homogenous 
material. Therefore determining the thicknesses of the individual 
component layers of the wafer 14 is not necessary. 
The apparent temperature of several cathodes is measured using an IRCON 
pyrometer under carefully controlled conditions such as by enclosing the 
cathodes in a heated cavity of accurately known and constant temperature. 
Cathodes are used which have the same wafer thickness. After the IRCON 
readings are taken and the apparent temperatures found, the cathodes are 
removed from the cavity and are cooled. Each of the cathodes then has a 
thin layer of the wafer 14 removed, for example, by etching. The thickness 
of the layer which is removed is made exactly the same for each cathode. 
The cathodes are again enclosed in a heated cavity and IRCON readings 
taken. The cathodes are again removed from the cavity and cooled. These 
steps are performed a number of times. At each IRCON reading stage, the 
apparent temperatures of the cathodes are averaged and used to establish 
an apparent temperature which corresponds to the desired actual 
temperature. 
The thickness of each of the wafers 14 is measured either before or after 
heating by nondestructive means and confirmed by destructive means, if 
needed. One method of performing the destructive measurement includes 
cutting a cross section of the wafer 14 and viewing the cut section under 
an electron microscope. A curve of apparent temperatures, corresponding to 
a desired actual temperature, versus thickness is thus generated. This 
curve is also known as a correction curve. 
In order to apply the correction for an unknown cathode, one has only to 
measure the wafer thickness by a nondestructive technique. Once the 
thickness is known, the apparent temperature which is needed to achieve 
the desired actual temperature is determined from the curve. One such 
method of measuring thickness consists of determining the fringe spacings 
in the infrared as the wave number is varied. This measurement can be done 
on a conventional infrared spectrophotometer or on a Fourier transform 
infrared instrument. The thickness is calculated from the fringe spacing 
and used to look up the correction factor on the previously generated 
compensation curve. FIG. 4 shows the correction factor for achieving an 
actual temperature of 600.degree. C. For example, in order to achieve an 
actual heat cleaning temperature of 600.degree. C., a wafer having a 
thickness of 0.3 .mu.m is raised to an apparent temperature of 
approximately 620.degree. C. as determined from FIG. 4. Applying a 
correction factor based on the thickness of the material versus an 
apparent temperature to establish a predetermined actual temperature of 
the structure may also be accomplished by formulating a program which can 
be inserted into a computer. When the correction factor has been 
determined, the heat being emitted by the heat source is adjusted to 
achieve the predetermined actual temperature. A typical interference curve 
is shown in FIG. 5. 
While the methods of this invention have been described with reference to 
temperature measurement of gallium arsenide photoemissive cathode 
structures during heat cleaning, the method is applicable to the 
measurement of temperature of any other layered structures, especially 
those comprised of layers of infrared transparent materials on glass or 
metals. 
While I have described above the principles of my invention in connection 
with specific apparatus, it is to be clearly understood that this 
description is made only by way of example and not as a limitation to the 
scope of my invention as set forth in the objects thereof and in the 
accompanying claims.