1. * GB780089 (A)
Description: GB780089 (A) ? 1957-07-31
Improvements in or relating to measuring instruments and gauges
Description of GB780089 (A)
COMPLETE SPECIFICATION
Improvements in or relating to Measuring Instruments and Gauges
We, MARCONI)S WIRELESS TELEGRAPH
COMPANY LIMITED, a company organised under the laws of Great Britain,
of Marconi
House, Strand, London, W.C.2, do hereby declare the invention for
which we pray that a patent may be granted to us, and the method by
which it is to be performed, to be particularly described in and by
the following statement:-
This invention relates to measuring instruments and gauges and more
specincally to instruments for measuring gauging or testing one
dimension (hereinafter assumed to be the thickness) of a wafer, sheet
or other specimen.
There are numerous known forms of measuring instruments of this nature
and these include, besides purely mechanical devices of the micrometer
type, instruments which operate by measuring the change in capacity
produced by inserting a specimen between two electrodes or measuring
the change of inductance produced when a specimen (in this case a
specimen of magnetic material) is inserted in the field of a coil.
Such instruments have, however, the defect of depending for their
reliable and satisfactory operation, on such physical parameters of
the specimen as its dielectric constant or susceptibility, uni-
formity of thickness, and in the case of very small specimens the
general size of the specimen.
The present invention seeks to provide an improved instrument which is
relatively simple and economical, reliable in use, of very high
sensitivity, and which does not present the defect above mentioned. In
experimental.
practice it has already been found possible to construct instruments
in accordance with the present invention capable of measuring the

2. thickness of wafers up to about 0.020" in thickness with a sensitivity
of the order of 0.0002" and there would appear to be no difficulties
in the way of securing even higher sensitivity of the order of
0.0005".
According to this invention an instrument for measuring gauging or
testing one dimension of a specimen comprises at least one
semiconductor rectifying photo junction, means for projecting a light
pattern slot or spot of light-to illuminate said junction close ro the
interface thereof, means for applying voltage to said junction,
separable members adapted to receive la specimen between them and to
be separated by a distance dependent on the dimension of said specimen
to be measured, means actuated by said separable members for varying
the distance between said tight pattern and said interface in
correspondence with the separation of said members and indicating
and/or responding means responsive to the current flow through the
junction.
The indicating and/or responding means may be simply a suitably
calibrated current indicating electrical meter or it may comprise any
means responsive to current flow through the junction, e.g. relays
connected and adjusted to operate a warning device if the thickness of
an inserted specimen lies outside predetermined limits.
Preferably, instead of using a single photo junction two junctions
connected in a bridge circuit are employed. Either or both of said
junctions may be. arranged to be illuminated, preferably both, in
which case the two june tions are arranged to have the same relative
movement with respect to the illuminating light pattern and said
pattern is arranged to illuminate said junctions on respectively
opposite sides of their interfaces. In this case, since the
illumination is on one side of the interface of one junction and on
the other side of the interface of the other, common movement in
relation to the light pattern will produce a differential effect.
The junctions employed in carrying out this invention may be single a
junctions or double junctions of the p-n-p or n-p-n type.
The underlying physical effect upon which the invention depends is
that if a photo junction, biassed in the reverse direction, is
illuminated, there will flow therethrough a current whose magnitude
depends on the total light flux and on its distribution in relation to
the interface. If therefore, the position of an illuminating light
slot with reference to the interface is varied, the current through
that junction will change.
The invention is illustrated in the accompanying drawing and in the
drawings accompanying the provisional specification. For simplicity
the figures are numbered consecutively, Fig. 6 being the single figure
of the driving accompanying the present specification. In the

3. drawings, Fig. 1 is a schematic view showing the mechanical
arrangement of one instrument in accordance with the invention; Fig. 2
is an enlarged detail of the two junctions employed in the arrangement
of
Fig. 1 showing the illuminating light slot;
Fig. 3 shows the electrical circuit of the arrangement of Fig. i; Fig.
4 is a sectional view shoving a detail modification, this view also
inciuding the circuit of the said modification; Fig. 5 is a schematic
face view of the modification shown in Fig. 4; while Fig. 6 shows a
minor modification d the arrangement of Fig. 4.
Referring to Figs. 1 to 3 the arrangement there sko-cn comprises a
brass or other rigid base plate 1 at one corner of which is a rigid
upstanding pillar 2 soldered or otherwise firmly fixed to the base
plate 1. At a corner adjacent the pillar 2 is an anvil 3 made, for
example, of transparent polymezised plastic and having an accurate
flat surface, said anvil being firmly attached to the base plate 1 as
by screws. A vertical probe 4 having a small captive steel ball 5 at
its pointed base is mounted over the anvil 3 and is capable of up and
down movement under the control of a parallel link mechanism
consisting of two flat strip springs 6 and 7 attached to the probe 4
at one pair of ends and to the pillar 2 at the other. As shown the
springs at 6 and 7 are attached firmly, e.g. by soldering to the top
and bottom faces of spacer bars 8 and 9 fixed respectively to the
pillar 2 and the probe 4.
As will be seen the construction consisting of the parts 2, 4, 5, 6,
7, 8 and 9 is strong and firmly constrained so as to permit movement
of the probe only in the vertical plane.
Passing through a hole in the upper part of the probe 4 and firmly
fixed therein as by soldering is a carrier rod 10 on which is mounted
a carrier block 11 of insulating material, said block being adjustable
in position along the rod 10 and adapted to be locked in any position
by a grub screw 12. The block 11 is formed with two bores 13 and 14 in
each of which is a semi-ccaductor rectifying photo junction of the
germanium eel tyPe. Fig. 1 conventionally indicates the connectors of
the junctions (one pair for each) and Fig. 2, which is a view taken
looking in the opposite direc tion to that for Fig. 1, shows the two
junctions at 15 and 16 with the interface represented in each case by
a horizontal line half way up the junction.
Upstanding from the base 1 is a pair of adjustable telescopic pillars
17, 18 carrying, within a tube 19 ati optical system whose height
above the base 1 can be adjusted by adjusting ule said telescopic
pillars 17 and 18 and v;:lcii can be clamped m any desired position Of
adjus -nent by means of clamp screws 20 and 2i. The optical system
within the tube 19 consists of a small electric lamp 22, a suitable

4. lens system 23, a slotted diaphragm 24 viIiCll can be rotated in its
plane by means of an adjusting handle 25 and a second suitable lens
system 26. By means of this optical system a slot cr line of light is
projected across the pair of junctions as indicated by the chain line
L in Fig. 2 so that it illuminates one junction a small distance to
one side of its fmter- face and the other a small distance to the
other side cf i.s interface, the mechanical adjust r4ent of the whole
arrangement being that this result is obtained when the ball 5 is
resting on the anvil 3. It a wafer or other specimen is inserted
between the ball 5 and the anvil 3 the line of light L is moved nearer
the inter.aee of the junction 15 and further away from the interface
of the junction 16.
Fig. 3 shows a circuit diagram. When the double pole switch 27 is
closed the lamp 22 is energised through the transformer 28 ana at the
same urn me semi-conductor junction circuit is completed. It is
important that the light flux from the lamp 22 shall be constant and
the lamp 4 and tne energising circuit therefor are carefully chosen
with this requirement in mind. Itie junction circuit includes a
suitabie potemial source (not shown) of, for example, 9 volrs,
connected at terminals 29, a potentiometer resistance 30 connected
between one pair of ends of the junctions, a current meter 31
connected between an adjustable centre tap 32 on the resistance 30 and
the other ends of the junctions and an adjustable resistance
consisting of a fixed portion 33 and an adjustable portion 34 in
series with one another across tne instrument 31.
In use the poLentiometer 3S32 is adjusted until, with the on S on the
anvil 3, the meter 31 reads zero indicating the bridge to be balanced.
The resistance in shunt across the meter is then adjusted so that
approximately full scale deflection is given by the meter when a
specimen of the maximum thickness to be dealt witli is placed between
the ball and the anvil. A standard feeder gauge of known thickness is
t':n inserted between the ball S and the anvil and the meter reading
observed for calibration purposes. The instrument is now ready for use
and the feeder gauge may a removed and a specimen to be measured
inserted U112 r tie probe. With the illustrated arrangement the
vertical spacing between the interfaces, one in each junction, should
be about 0.010" more than the maximum specimen thickness to be
measured and the height of the optical system should be so adjusted
that the spacing between the light slot L and the upper interface is
about .005" when the ball 5 is in the anvil 3.
It is not essential for the slot L to be arranged to illuminate both
junctions as illustrated. Only one need be illustrated and the other
used merely to complete the bridge and, being close to said one
junction thus provide a measure of balance as respects temperature and

5. leakage current variations. In this connection it should be noted
that, in addition to the light produced current, there will be an
additional leakage current of magnitude depending on temperature (as
well as treat ment and ageing of the junction) and by using two
junctions having the same reverse characteristics and mounting them so
close together that they are subjected to the same temperature,
variations in leakage currents should be the same for both and should
therefore not contribute to bridge unbalance. However, the optical
response also depends slightly on temperature and the provision of an
adjustment resistance across the meter 31 allows of adjusted ment of
sensitivity. The double illuminated arrangement illustrated is
preferred because of its better sensitivity and linearity. With this
arrangement of course both junctions should be as near as possible
identical in all respects.
Theoretically, of course, a single junction could be used without any
bridge circuit at all but, for in such a case calibration and
stability would not be easy to achieve and the bridge arrangement is
much to be preferred.
In the detail modification shown in Figs. 4 and 5 two double junctions
are used instead of single junctions in order to obtain higher gain.
As shown there is a wafer W, for example, of n type germanium with
dots of
Indium or other suitable material alloyed into opposite sides of the
wafer at 151 and 161 so that a p type material is produced where the
Indium is diffused into the germanium. There are thus provided two
double junctions of the p-n-p type. Obviously however instead of p
type dots in an n type wafer n type dots in a p type wafer could be
used. The dots and wafer at 151 and 161 correspond respectively to the
junctions 15 and 16 of Fig. 2. The slot of light is normally between
the two pairs of junctions as indicated at L in the face view of Fig.
5. This slot of light is obtained from a suitable optical system such
as is shown in
Fig. 1. As in the case of Fig. 1 there is a wide latitude of choice of
shape of the light pattern-it may be a simple line of light, or a
circular spot, for example, or (preferably) it may be of the windlass
shape represented in broken lines in Fig. 5. The electrical circuit
shown in Fig. 4 is practically self-explanatory in view of the
description already given and consists of the potentiometer 30-32, the
meter 31, the sensitivity adjusting resistance 34 and a potential
source (not shown) connected at 29.
Fig. 6 shows a minor modification, which will be practically
self-explanatory from the drawing, of the arrangement of Fig. 4. Fig.
6 again uses two double junctions the only difference in the
arrangement of the junctions of this figure as compared to that of

6. Fig. 4 being that the smaller Indium dots are both on the same side of
the wafer. There is very little to choose as between Figs. 4 and 6
but, in general, the arrangement of the latter figure will give
slightly better linearity.
With suitable design and adjustment a high degree of linearity of
response can be obtained, more particularly with the double
illuminated junction arrangements illustrated.
What we claim is:
1. An instrument for measuring, gauging or testing one dimension of a
specimen said instrument comprising at least one semi-con- ductor
rectifying photo junction, means by projecting a light pattern to
illuminate said junction close to the interface thereof, means for
applying voltage to said junction, separable members adapted to
receive a specimen between them and to be separated by a distance
dependent on the dimension of said specimen to be measured, means
actuated by said separable members for varying the dis- tance between
said light pattern and said interface in correspondence with the
separation of said members and indicating and/or responding means
responsive to the current flow through the junction.
2. An instrument as claimed in Claim 1 wherein the indicating and/or
responding means is a calibrated electrical current indicating meter.
3. An instrument as claimed in Claim 1 or 2 wherein two junctions
connected in a bridge circuit are employed.
4. An instrument as claimed in Claim 3 wherein both junctions are
illuminated said two junctions being arranged to have the same
relative movement with respect to the illuminating light pattern and
said pattern being arranged to illuminate said junctions on
respectively opposite sides of their interfaces.
5. An instrument as claimed in any of the preceding claims wherein
there is at least one double junction.
6. An instrument as claimed in any of the preceding claims wherein the
separable members are constituted, one by an anvil and the other by a
probe, the probe being carried by a parallel link mechanism permitting
movement in one plane only.
7. Instruments substantially as herein described with reference to the
accompanying drawing and the drawings accompanying the provisional
specification.