GB2149098A

GB2149098A - Optical metrology

Info

Publication number: GB2149098A
Application number: GB08424997A
Authority: GB
Inventors: Derek Stanley Moore; Christopher Paul Kirk
Original assignee: Vickers PLC
Current assignee: Vinters Ltd
Priority date: 1983-10-26
Filing date: 1984-10-03
Publication date: 1985-06-05
Also published as: GB8328562D0; GB8424997D0; GB8404664D0; GB2149098B

Abstract

Precise measurement of dimensions, e.g. the width of lines on semiconductor wafers, is made using an optical microscope having an objective providing an image of the object to be measured, an optical shearing means arranged to create relative shear between first 18 and second 19 portions of the image, a television camera tube 10 and a television monitor 11 for displaying the sheared primary image received by the camera tube, a video threshold detection system 12 coupled between the camera and monitor to generate thin bright lines along the edges of the image portions by appropriate choice of intensity threshold, and means for measuring the optical shearing movement between one measuring condition in which a thin edge line of the first portion is in visual registry with a thin edge line of the second portion and a second measuring condition in which either the same or the other edge line of the first portion is in registry with the thin edge line of the second portion, thereby to measure the dimension of the object. <IMAGE>

Description

SPECIFICATION Optical metrology This invention relates to optical metrology and is connected with apparatus and method for carrying out the precise measurement of the dimensions of objects using an optical microscope. The invention is particularly, though not exclusively, suitable for use in measuring the width of lines on semiconductor wafers.

During the development and production of integrated circuits it is necessary to be able to measure the widths of lines laid down in various materials on the wafer. This involves measuring lines of the order of only a few microns to a precision of a few hundredths of a micron.

The resolution limit of a high power dry objective lens is typically a few tenths of a micron and the width of the edges of the lines can be of the order of a micron. This results in the image intensity profile having smooth slopes at the points in the image corresponding to the edges of the track.

A commonly adopted approach to producing precise measurements of these lines is to project the image on to the pick-up of a television camera tube and then identify a threshold in the video signal at each edge.

The time taken for the scanning spot to travel from one side to the other is recorded and scaled to provide a measurement of the li- newidth.

This technique suffers from errors due to the geometric distortion of the raster which scans the face of the television pick-up tube.

These distortions vary across the tube face.

They are also a function of light intensity, temperature, ageing and the electrical conditions applying to the tube. This results in a source of error which is unknown and variable.

The present invention has been developed by applying an already established optical image shearing technique to a system of threshold detection in the video signal in order to avoid the errors associated with the geometric distortions of the television camera raster scan.

One optical shearing technique is disclosed in more detail in the specification of UK Patent No. 2027203 (A and B), entitled Optical Metrology, and in the name of Vickers PLC and invented by Dr. F.H. Smith and Mr. D.S.

Moore. This invention is an improvement re- lating to the shearing technique disclosed therein. The established optical shearing technique involves splitting the image from an optical microscope into two, and then displacing these two images relative to each other, and measuring the distance moved. The two images are then recombined, but with a section of one image missing and the equivalent section of the other image in its place. The effect is to produce a resultant image where a strip of the field of view can be sheared relative to the rest.

Referring to Figure 1 of the accompanying drawings, a measurement can be achieved by displacing the two images relative to each other until they touch at one edge, and then measuring the distance required to move them until they touch at the other edge.

This technique does not suffer from geometric distortions, but because of the width of the edges in the image, it is difficult to achieve very precise and repeatable edge to edge setting.

This invention proposes the use of a video threshold detection system to generate thin bright lines along the edges of the image of a track produced on a television monitor. These lines are used in conjunction with an optical shearing technique, in order to achieve a repeatable edge to edge setting.

According to one aspect of the invention there is provided optical apparatus for measuring the dimensions of a small object when viewed by an optical microscope and comprising: an optical microscope having an objective arranged to provide a primary image of the object; an optical shearing means arranged to create relative shear between first and second portions of the primary image; a two dimensional imaging device having its sensitive face arranged to receive the sheared primary image and to generate a corresponding signal; a display device capable of displaying the signal from the imaging device; a signal threshold detection system coupled between the imaging device and the display device in order to generate thin lines associated with chosen intensity thresholds in the sheared primary image;; means for adjusting the optical shearing means in order to displace first and second opposed edges of the first portion relative to first and second opposed edges of the second portion of the primary image; and means for measuring the extent of relative movement of the first and second portions between one measuring condition in which the thin line of the first or the second edge of the first portion is in visual registry with the thin line of the first edge of the second portion and a second measuring condition in which the thin line of the first edge of the first portion is in visual registry with the thin line of the second edge of the second portion, thereby to provide a measurement of a dimension of the object.

According to a further aspect of the invention there is provided a method of measuring a dimension of a small object when viewed by an optical microscope and comprising the steps of:.

forming a primary image of the object in a microscope, the image having relatively sheared first and second portions; applying the sheared image to the sensitive face of a two dimensional imaging device which thereby generates a corresponding signal; transmitting the signal to a display device for displaying a representation of the sheared image; modifying the representation by a signal threshold detection system coupled between the imaging device and the display device which generates thin lines associated with chosen intensity thresholds in the sheared primary image; adjusting the optical shearing means in order to displace first and second opposed edges of the first portion relative to first and second opposed edges of the second portion of the primary image;; and measuring the extent of relative movement of the first and second portions between one measuring condition in which the thin line of the first or the second edge of the first portion is in visual registry with the thin line of the first edge of the second portion and a second measuring condition in which the thin line of the first edge of the first portion is in visual registry with the thin line of the second edge of the second portion, thereby to provide a measurement of a dimension of the object.

In one preferred arrangement, the two dimensional imaging device comprises a television camera tube having a photo cathode arranged to receive the sheared primary image. The display device may then comprise a television monitor for displaying the image received by the camera tube, and the signal threshold detection system will comprise a video threshold detection system.

Conveniently, the measuring means measures the extent of the adjustment movement of the shearing means (coupled with the microscope) necessary to achieve adjustment between the first and second measuring conditions. This could be achieved by optical means. However, other shearing means may be provided, such as displacing a portion of the video signal relative to the remainder.

Conveniently, the video threshold detection system comprises an electronic edge brightener. Devices of this type are well known in providing images on a television monitor.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of optical metrology using an optical shearing technique; Figure 2 is a schematic illustration of optical measuring apparatus according to the invention; and Figure 3 is a schematic illustration of adjustment of a sheared optical image used in a method and apparatus of the invention for measuring the dimensions of small objects.

DESCRIPTION OF PREFERRED EMBODI MENT Referring now to Figure 1 of the drawings, and as indicated above, this concerns an optical shearing technique, used in optical metrology, which is disclosed in more detail in the specification of published UK patent specification No. 2 027 203. Reference is hereby incorporated in the present specification to the various means disclosed therein for optical shearing used in optical metrology. There is shown in the left hand part of Figure 1 a condition of zero shear, in the centre of Figure 1 shearing is to one edge, and in the right hand side there is shown shearing to the other edge.Movement of the sheared portion of the image from the positions shown in the central part of Figure 1 to that shown in the right hand side of Figure 1 will be representative of the thickness of a line of an object which is bordered by two dark edges shown in each view. In the central view of Figure 1, the right hand edge of the sheared portion of the image is in visual registry with the left hand edges of the non-sheared portion of the image. In the right hand part of Figure 1, the left hand edge of the sheared portion of the image is in visual registry with the right hand edge of the non-sheared portion of the image.

In the case as illustrated, the extent of movement of the sheared portion of the image will be double the actual corresponding measurement on the object being measured. Suitable calibration will be provided to take this into account.

There has been described above, by way of example only, shearing movement of a central strip relative to a non-sheared remaining portion. However, it should be understood that the invention is concerned with creating relative shear between the first and second portions (or more portions) of a primary image i.e. each of the portions may be sheared to arrive at the conditions shown in Figure 1.

Referring now to Figure 2 of the drawings, there is shown an embodiment of optical measuring apparatus according to the invention which is capable of carrying out an optical measuring method according to the invention. The apparatus comprises a television camera tube 10, a television monitor 11 for displaying an image received by the camera tube, and a video threshold detection system 1 2 coupled between the camera tube 10 and the monitor 11 and taking the form of an electronic edge brightener.

However, it should be understood that the television camera tube 10 may be replaced by any suitable other form of two dimensional imaging device having its sensitive face arranged to receive the sheared primary image, and to generate a corresponding signal. Correspondingly, the television monitor 11 may be replaced by a display device capable of dis playing the signal transmitted by the imaging device. In such cases, the video threshold detection system 12 will be a signal threshold detection system.

Although not shown, the apparatus will be provided with an optical microscope having a support on which an object can be placed, whose dimensions are to be measured, and an objective arranged to provide a primary image of the object. Furthermore, optical shearing means is coupled with the microscope in order to shear a portion of the primary image.

Any suitable optical shearing means may be provided, such as disclosed in UK Patent Specification 2 027 203. Furthermore, means is provided for adjusting the optical shearing means, to enable displacement of the sheared image, in the manner shown in Figure 1.

Also, means is provided for measuring the extent of movement of the adjusting means, thereby to provide a measurement of a dimen sion of the object being viewed by the micro scope.

The television camera tube is arranged in such relation to the optical microscope that the first and second relatively sheared portions of the primary image are received by the photo cathode 1 3 of the camera tube 10. The primary image displayed at the photo cathode 1 3 is in respect of measurement of the thick ness of a line on a semiconductor wafer, by way of example only, which comprises a por tion 14 displaced relative to a two part por tion 1 5. The extent of displacement of the portion 14 will depend upon the nature and operation of the optical shearing means in the optical microscope, though the initial position taken up by the portion 14 relative to the portion 1 5 is not critical, as far as the subse quent measuring displacements are con cerned.

The television monitor 11 provides a dis play of the image received by the camera tube 10 which, in the absence of the video thresh old detection system 12, would provide some what blurred lines delimiting the edges of the line on the semiconductor wafer which is being viewed and measured. This occurs ow ing to the nature of the object and optical diffraction effects which results in a smooth transition rather than a sharp edge to the image. With known equipment, there tends to be a somewhat imprecise transition between the edge of the line illustrated on the televi sion monitor and the surrounding regions of the television screen.

Since the optical shearing technique in volves relative displacement of the lines of one portion of the image on the television monitor until they come into visual registry with the lines of another portion (s), lack of clarity in the definition of the edges of the lines clearly will have an adverse effect on the precision of measurements being carried out.

In addition, there are errors due to the geometric distortion of the raster which scans the face of the television pick-up tube which vary across the tube face. They are also a function of light intensity, temperature, ageing and the electrical condition applying to the tube. This results in a source of error which is unknown and variable.

The purpose of the video threshold detection system 1 2 is to modify the display image on the television monitor by generating thin lines in the displayed edges of the image. In a preferred arrangement, the lines appear as thin "bright" lines which are readily visible against the surrounding background on the television screen. There is therefore shown in Figure 2, in the screen 1 6 of the television monitor 11 a displayed image designated generally by reference 1 7 which comprises relatively sheared first and second portions 18 and 1 9 respectively.It will be noted that the portions 18 and 1 9 of the displayed image have thin and precise edge lines, in views of the modifications provided by the video threshold detection system 12, which greatly facilitates precise edge-to-edge setting of the two portions of the image 1 8 when measurements of the dimensions of the object e.g. the silicon wafer are required to be taken.

The operation of the system shown in Figure 2 will now be described in more detail.

The illustration of the image on the photo cathode 1 3 shows how a strip sheared optical image is formed with the shearing strip lying parallel to the lines of the television raster scan and in a central band of the raster. The video threshold detection system i.e. the so called edge bright-up electronics unit 1 2 locates the lines and field pulses in the video signal and uses these to position a window in the raster scan. The size and position of this window can be adjusted by controls on the unit, but the circuitry prevents the window from ever including any sychronizing pulses by restricting its movements to an area slightly less than the full area of the raster.

Within this window in the raster, the unit detects the highest and lowest voltages in the video signal and stores these. A third voltage is then calculated by the unit, according to the equation: V3 = V1 + k(V2 - V1) wherein V, is the lowest voltage in the window, V2 is the highest voltage in the window and 0 < k < 1, k is the threshold parameter and its values may be entered into the unit externally. In the particular embodiments, the value of k is set by a control on the front panel. Whenever the video signal rises or falls below this value, this is detected by a comparator circuit and at the moment the video signal crosses V3, the circuit injects a very narrow pulse into the video signal. This pulse is made to have a width of much less than one pixel of the television system.This ensures that the bright lines on the monitor 11 are kept as narrow as possible. The resultant video signal which now contains these pulses is displayed in the image 1 7 appearing on the television monitor 11 Thus, the video threshold detection system generates thin bright lines in the displayed edges of the image, these lines running along the contours of the chosen intensity level.

If the image is of a track on a semiconductor wafer, then a bright line is produced running down each side of the image of the track. The measurement is performed by positioning the shear strip parallel to the direction of the line of the television raster and positioning the object such that the image of the track is perpendicular to the shear strip.

The strip and the track are positioned to lie in the respective central bands of the television screen 1 6. An electronic window 20 is adjusted to enclose an area around where the track and the strip intersect. The window 20 is made to be roughly four times the strip width in height and four times the trackwidth in width. The exact size is not important, provided the window covers the features of interest and excludes other features in the field of view.

Description will now be made of the way in which measurement is carried out, having reference to Figure 3. Figure 3a shows a position of zero shear, in which portion 18 is in registry with portion 1 9. Thus, the portion 1 8 has a first edge line 1 8a and a second edge line 18b, and first line 18a is in visual registry with first edge line 1 9a and second line 1 8b is in visual registry with second edge line 19b of the portion 19.

In Figure 3b, there is shown relative displacement of the portion 1 8 to a position in which the second line 1 8b is in visual registry with the first line 1 9a of the portion 1 9.

The position taken up by the portion 18, shown in Figure 3a or 3b can be considered to represent a first measuring condition of the apparatus, namely in which the line of the first (1 8a) or the second (1 8b) edge is in visual registry with the line of the first (1 9a) edge of the other portion.

A second measuring condition is illustrated in Figure 3c, in which the line of the first edge (1 8a) is in visual registry with the line of the second edge (1 9b). Evidently, movement from the first measuring condition, as shown in Figure 3a to the second measuring condition shown in Figure 3c will be through a distance exactly equal to the distance between the lines 1 spa and 1 8b and 1 9a and 1 9b.

However, the extent of movement between the first measuring condition, as shown in Figure 3b, to the second measuring condition will be double the distance between the lines 1 8a and 1 sub, or 1 9a and 1 9b. This latter measurement mode is the preferred mode, in that measurement of the greater distance with appropriate calibration, should give a more precise measurement.

Thus, to perform a measurement, a particular threshold is selected, to produce a bright line down each side of the track at this level on the television monitor 11. The optical image is then sheared, by operation of adjustment means for the optical shearing means coupled with the microscope (not shown) until the bright line from one edge (1 8b) of the one portion of the image is in visual registry with the bright line of the correspondingly opposite edge (1 9a) of the other portion of the image.

This gives one edge setting. The image is now sheared again in the other direction until the bright line of the opposite edge of the one portion is in visual registry with the bright line of the other of the edges of the other portion (see Figure 3c). The distance moved between the two settings is proportional to the line width measurements. This measurement can be converted to a value for the line width by dividing the measured distance by twice the optical magnification.

If desired, the optical shearing means, the adjusting means therefor, and the measuring means (all not shown) may be combined into a single component and the extent of movement may be measured by any suitable means, such as strain gauges, with appropriate measurement indicators being provided.

The advantages obtainable with the method and apparatus of the invention are that the geometric distortions associated with making the measurement directly on the video signal are avoided and yet, unlike the purely optical strip shearing technique, it has a very precise setting criterion which is less subjective. As is well known, the human eye is extremely sensitive in its observation of lined up edges, and the invention provides very easily observed and sharply defined lines which enable very precise measurements to be taken.

Claims

1. Optical apparatus for measuring the dimensions of a small object when viewed by an optical microscope and comprising: an optical microscope having an objective arranged to provide a primary image of the object; an optical shearing means arranged to create relative shear between first and second portions of the primary image; a two dimensional imaging device having its sensitive face arranged to receive the sheared primary image and to generate a corresponding signal; a display device capable of displaying the signal from the imaging device; a signal threshold detection system coupled between the imaging device and the display device in order to generate thin lines associ ated with chosen intensity thresholds in the sheared primary image;; means for adjusting the optical shearing means in order to displace first and second opposed edges of the first portion relative to first and second opposed edges of the second portion of the primary image; and means for measuring the extent of relative movement of the first and second portions between one measuring condition in which the thin line of the first or the second edge of the first portion is in visual registry with the thin line of the first edge of the second portion and a second measuring condition in which the thin line of the first edge of the first portion is in visual registry with the thin line of the second edge of the second portion, thereby to provide a measurement of a dimension of the object.

2. Apparatus according to claim 1, in which the two dimensional imaging device comprises a television camera tube having a photocathode arranged to receive the sheared primary image, the display device comprises a television monitor for displaying the image received by the camera tube, and the signal threshold detection system comprises a video threshold detection system.

3. Apparatus according to claim 1, in which the measuring means is arranged to measure the extent of the adjustment movement of the shearing means necessary to achieve adjustment between the first and second measuring conditions.

4. Apparatus according to claim 3, in which the measuring means includes optical means.

5. Apparatus according to claim 2, in which the video threshold detection system comprises an electronic edge brightener.

6. A method of measuring a dimension of a small object when viewed by an optical microscope and comprising the steps of: forming a primary image of the object in a microscope, the image having relatively sheared first and second portions; applying the sheared image to the sensitive face of a two dimensional imaging device which thereby generates a corresponding signal; transmitting the signal to a display device for displaying a representation of the sheared image; modifying the representation by a signal threshold detection system coupled between the imaging device and the display device which generates thin lines associated with chosen intensity thresholds in the sheared primary image; adjusting the optical shearing means in order to displace first and second opposed edges of the first portion relative to first and second opposed edges of the second portion of the primary image; ; and measuring the extent of relative movement of the first and second portions between one measuring condition in which the thin line of the first or the second edge of the first portion is in visual registry with the thin line of the first edge of the second portion and a second measuring condition in which the thin line of the first edge of the first portion is in visual registry with the thin line of the second edge of the second portion, thereby to provide a measurement of a dimension of the object.

7. Apparatus according to claim 1 and substantially as hereinbefore described with reference to, and as shown in the accompanyirig drawings.

8. A method according to claim 6 and substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.