Spatial characteristic determination

To determine some spatial characteristic, such as size or shape, of an object, the object is viewed with a television camera that employs an image sensor such as a CCD device. An image is formed on a television screen and a set of measurement markers (or cursors) are superimposed on the image at desired positions. The CCD timing signals are used to accurately determine the position of the cursors and hence the desired spatial characteristic.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention concerns spatial characteristic determination, and relates 
in particular to the determination, and measurement, of the size and shape 
of objects. 
It is common in many technical fields to need to know the exact dimensions 
of some object. For example, it may be desirable to check the width of a 
printed circuit board track or the size of structures in a chip-borne 
integrated circuit. Again, it is often desirable to decide whether the 
shape of some article falls into one or other of several allowable 
catagories. Present day methods for measuring object size often make use 
of an optical system--a telescope or microscope, say--with a very 
carefully-prepared and calibrated measuring scale in the field of view 
(functions internally of the system) against which the object's size can 
be determined. While not unsatisfactory, such arrangements require 
considerable precision in their design and construction, and so are rather 
expensive. Because of the flexibility and convenience of electronic 
images, one possible technique for achieving the desired ends is to view 
the object with a television (video) camera, form an image of the object 
on a television screen, superimpose onto the formed image a set of 
measurement markers (or cursors) whose true spatial positions are known, 
and move the cursors about to bracket the object image and thus indicate 
its--and so the object's--real size. This can be done using conventional 
television systems employing standard cameras of the vidicon type. Using 
these, though, there are problems in knowing exactly where a generated 
cursor actually is--what is its true spatial position--and these problems 
millitate against such a solution being of any great value in the 
measurement, or shape determination, of very small objects (where the 
likely positional error may be as big as the object being viewed!). These 
problems can be overcome, however, by using a camera that employs an image 
sensor in the form of an accurately constructed array of very small 
sensing elements--as typified by present-day Charge Coupled Device (CCD) 
image sensors. 
One of the major advantages of image sensor arrays is that their array 
geometry is accurately defined at manufacture. Moreover, because--in 
effect or in actuallity--the output of each array element is examined in 
sequence and at rigidly defined time intervals (defined by the camera's 
master clock signal), in a sensor array camera the contribution to the 
video output of a particular picture element on the array's surface is 
precisely located in time. This is in marked contrast to conventional 
pick-up tubes, where the output from any point on the target is dependent 
on the scan waveforms employed. This feature of array sensor operation may 
be exploited to simplify dimension measurement, particularly in microscope 
systems. In essence, it is possible to mix a signal defining one or more 
marker (cursor) with the array sensor camera output--for example, a pair 
of horizontal and a pair of vertical cursor lines corresponding to the 
array's rows and columns of light-sensitive elements (charge-collection 
sites on a CCD) whose true positions are of course known with great 
accuracy. By having the positions of the cursor(s) adjustable it is 
possible to take advantage of the precise geometry of the array sensor to 
allow measurements to be taken of objects in the view of the camera. 
In one aspect, therefore, the invention provides a method of determining a 
spatial characteristic of an object, in which method: 
the object is suitably viewed with a video camera employing an imaging 
element array sensor to provide an object video image; 
one or more marker image (cursor) is generated for superimposition on the 
video image, the placing of the or each cursor on the video image being 
determined by the array sensor timing signals, and so corresponding to the 
position in the array of one or more particular imaging element; and 
the or each cursor is positioned in the video image to be appropriately 
aligned with the object image as regards the relevant characteristic; 
whereby, from the distance to the or each cursor, known in terms of the 
array sensor timing signals (and thus of the number and size of the array 
elements up to the relevant cursor position), there may be determined the 
required characteristic of the object. 
In another aspect the invention provides apparatus for the determination of 
a spatial characteristic of an object, which apparatus includes: 
a video camera employing an imaging element array sensor, by which the 
object may be suitably viewed to provide an object video image; 
cursor generation means, by which one or more marker image (cursor) may be 
generated for superimposition on the video image, the placing of the or 
each cursor on the video image being determined by the array sensor timing 
signals, and so corresponding to the position in the array of one or more 
particular imaging element; and 
cursor positioning means, by which the or each cursor may be positioned in 
the video image to be appropriately aligned with the object image as 
regards the relevant characteristic; 
whereby, from the distance to the or each cursor, known in terms of the 
array sensor timing signals (and thus of the number and size of the array 
elements up to the relevant cursor position), there may be determined the 
required characteristic of the object. 
The invention concerns the determination of a spatial characteristic of an 
object. The spatial characteristic may be the shape of the object, the 
determination involving a comparison of a "control" shape with the 
object's shape. Alternatively, the spatial characteristic may be some 
physical dimension, such as the length or breadth, of the object, the 
determination involving an actual measurement thereof. 
The object, a spatial characteristic of which is to be determined, can be 
of any sort, large or small, thick or thin. Depending on the optical 
system associated with the camera, the object might be an integrated 
circuit component (using an electron microscope), a bacterium (using an 
optical microscope), a track on a printed circuit board (using an unaided 
camera lens) or a building or geographical feature (using a telescope). It 
might even be a heavenly body, such as a galaxy. 
The object is suitably viewed with the array sensor camera--that is, it is 
viewed in an orientation, and at a magnification, appropriate to the 
characteristic to be determined. Where necessary, then, the viewing is 
effected via the right sort of optical system--ranging from a microscope 
to a telescope. 
In carrying out the invention the object is viewed with a video--i.e, 
television--camera employing a solid state array sensor. This array sensor 
may be of any type--a diode array or an MOS device, for example--but is 
preferably a charge coupled device (CCD) imaging element array. CCD 
sensors of this type are well known. These array sensors may be of various 
different subtypes, sensitive to radiation in the UV, visible or IR 
regions of the electromagnetic spectrum, but all are characterised by 
including a dense and very regular array of photosensitive elements of 
extremely small size. For example, the array may contain over a million 
such elements, arranged as a square of 1000 by 1000 elements, all in a 
physical space less than 1" (2.5 cms) square. A typical CCD sensor is that 
one available from EEV and designated the P8602 (a visible--light, 
frame-transfer device), which is used in EEV's P4310 series cameras 
(including the P4312 remote head camera). This particular CCD sensor is 
about 2/3 inch (roughly 1.6 cms) across, and its sensitive elements 
(pixels) are about 22 micrometers square. 
The viewing with the array sensor camera provides a video image--on 
whatever visual display unit, such as a television set, is appropriate to 
the camera's output. 
Determination of the chosen characteristic involves aligning the object 
image with a marker image, or cursor, superimposed on the object video 
image. There may be one or more cursor, and the or each cursor may take 
different forms depending on the determination being effected. For 
example, each cursor may be a single line (itself aligned with a 
selectable row or column of the array), and there may be two such line 
cursors disposed as a parallel pair, the distance between them being 
variable as each line's position (the array row or column) is varied. 
Indeed, there may be four such lines, disposed as two orthogonal pairs of 
parallel lines, one pair corresponding to two rows and the other 
corresponding to two columns. Alternatively, the or each cursor may be an 
outline of some appropriate shape, possibly variable in both position, 
rotation and size (the shape would be appropriate to the object whose own 
shape was to be determined). 
The placing of the or each cursor on the video image is determined by the 
array sensor timing signals. This is a matter that will be well understood 
in the Art, and need not be discussed further here. Nevertheless, it is 
convenient to observe that one way of achieving this desideratum is 
electronically to count the number of picture element read-out pulses 
since the start of each line and the number of line synchronisation pulses 
since the start of each video field. These two numbers (picture element 
count and line count) are the coordinates of the picture element being 
read out. For example, if the picture element count is five and the line 
count is six then the element being read out is in the fifth column of the 
sixth row in the image sensor array. Thus, if a cursor corresponding to 
this particular element is required it should be placed on the video at 
this point in time. 
When using this invention, the or each cursor is suitably aligned with the 
object image. Thus, if shape is to be determined with an outline cursor, 
then the cursor is perhaps rotated, changed in size, and moved left/right 
or up/down to "fit" over the object. Again, if there is to be measured the 
width of some object (in a standard orientation), then perhaps two line 
cursors are merely positioned aligned with either side of the object. The 
mechanics of this alignment can be any suitable--rotating knobs, 
thumbwheels and so on--appropriately connected to the controlling 
electronics, and these latter may themselves be whatever is appropriate to 
the type of cursor (and will naturally depend on the manner in which there 
is achieved the placing of the cursor as determined by the array sensor 
timing signals, as first mentioned hereinabove). Examples of acceptable 
alignment electronics include counters with comparators and programmable 
down counters. Counters of the former type are connected so as to count 
picture element read-out pulses and line sync pulses (as described above). 
The comparators continually compare the co-ordinates of the required 
cursor with those of the picture elements being read out. When the 
co-ordinates are equal a pulse is generated which is superimposed onto the 
video signal. This modulates the brightness of the video image at this 
point, and produces the required cursor. A counter of the latter type, 
however, is connected so as to be decremented by each picture element 
read-out pulse. The counter is programmed with a number, C, at the start 
of a line. When it has been decremented to zero a pulse is generated which 
is superimposed onto the video signal (in a similar manner to the example 
given above), and this produces a cursor in column C of that line. 
Once the or each cursor is in place the required characteristic of the 
object may be determined. For instance, a width/length measurement may be 
effected simply by observing the distance apart of two cursors-- this 
distance is definable in terms of the timing signals for the two cursors, 
and thus the number of array elements "between" the two. Thus, 
EQU d=(x.sub.2 -x.sub.1).multidot.M/P 
where: 
x.sub.2 and x.sub.1 are the relevant array rows/columns; 
P is the array element size (in the x.sub.2 /x.sub.1 sense); and 
M is the magnification of the system. 
As can be seen, the invention provides a convenient, accurate and 
inexpensive way of determining some characteristic, such as shape or size, 
of an object.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the main components of a measuring system in accordance with 
the invention. 
A CCD camera (1) is mounted at the focal plane of a microscope (11). The 
focussed image of the object (2) to be measured is displayed (here shown 
hatched) on a TV monitor (3). The electronic circuitry (4) is fed with the 
CCD timing signal, and generates two short pulses timed with CCD columns 
x.sub.1 and x.sub.2 (as determined by thumbwheel controls T.sub.L, 
T.sub.R). These are electronically superimposed on the video signal, and 
appear as two cursor lines (C.sub.L, C.sub.R) projected on to the image of 
the object (2). When the cursors are aligned with the structure their 
separation will be x.sub.2 -x.sub.1 picture elements; due to the accuracy 
of the CCD array geometry, this is directly related to the structure 
dimension d by the expression 
EQU d=(x.sub.2 -x.sub.1).multidot.M/P 
(where: 
P=picture element size; 
M=magnification). 
A flowchart of the cursor generator is shown in FIG. 2, and the electronics 
are shown in FIGS. 3 and 4. 
The position of each cursor is determined by a counter consisting of three 
74LS168 TTL ICs (see FIG. 3). The counter is preset to the number 
specified by the BCD switches, and is decremented at intervals until it 
reaches zero and a carry pulse is generated. The carry pulse is used to 
alter the brightness of the video signal to generate the cursor, and also 
triggers the flip flop (74LS74) which inhibits further counting. 
A horizontal cursor is generated by presetting the counter with Field Sync. 
(Field Drive) and decrementing it on subsequent Line Sync. (Line Drive) 
pulses. A vertical cursor is generated by presetting the counter with Line 
Drive and decrementing it on the output register (R.phi..sub.2) pulses. 
When more than one cursor is required it is necessary to combine the carry 
pulses from several of these counters using logic gates. FIG. 4 shows how 
four carry pulses can be combined using 74LS00 gates to allow four cursors 
to be displayed simultaneously. The signal is gated with MVB to ensure 
that the cursors are only injected during picture time and cannot 
interfere with the sync information in the video signal. 
The output of the final gate is added to the video signal through a 
resistor and a preset potentiometer which allows the intensity of the 
cursors to be adjusted (a fainter line may allow the edges of some objects 
to be more easily located). 
Dimension Measurements 
Linear dimensions are determined by placing one cursor on each edge of the 
object or structure which is being measured. The difference between the 
numbers indicated on the BCD switches correspondings is directly 
proportional to the required. The constant of proportionality can be 
determined in two different ways. 
(i) If the magnification of the optical system is M and the picture element 
size on the CCD is P, then the constant is P/M. 
(ii) The system can be directly calibrated by observing an object with a 
known dimension. This technique is particularly valuable for optical 
systems where the precise overall magnification is not known. 
The second method of calibration was used by viewing a diode array, the 
diodes of which were 14 micrometers squares. Using a x100 objective the 
diodes were 79 columns wide and 79 rows high. This implies that each 
column or row moved by the cursor was equivalent to a distance of 
14/79=0.1772 micrometers. 
The prototype was then used to measure the size of structures on a silicon 
wafer of a 2/3 inch Charge Coupled Device, and the following results were 
obtained. 
1. Distance between nitride implants 
x.sub.2 =221, x.sub.1 =96 x.sub.2 -x.sub.1 =125 
distance=125.times.0.1772=22.15 micrometers 
2. Distance between each three polysilicon electrodes 
x.sub.2 =268, x.sub.1 =141 x.sub.2 -x.sub.1 =124 
distance=124.times.0.1772=21.97 micrometers 
These dimensions correspond to the pixel size of the CCD. They are actually 
22 micrometers on the 2/3 inch device, and the measured values are within 
1% of this.