GB2112518A - Film thickness gauge - Google Patents

Abstract

A film thickness gauge comprising mounting means for mounting a sample (24) having a film thereon whose thickness is to be measured at a predetermined point (P), said film being such that, when the said point (P) is irradiated with X-rays, it emits fluorescent X-rays; an X-ray tube (20) for irradiating said point (P) with X-rays; a detector (27) for detecting fluorescent X-rays emanating from said point (P); and a mirror (36) disposed on the axis of the X-ray tube (20) and through which the X-rays from the X-ray tube (20) may pass so as to be directed onto the said point (P), the mirror (36) enabling the said point (P) to be visually observed along an optical-axis which coincides with the axis of the X-ray tube (20). <IMAGE>

Description

SPECIFICATION Film thickness gauge The present invention relates to a film thickness gauge.

According to the present invention, there is provided a film thickness gauge comprising mounting means for mounting a sample having a film thereon whose thickness is to be measured at a predeter mined point, said film being such that, when the said point is irradiated with X-rays, it emits fluorescent X-rays; an X-ray tube for irradiating said point with X-rays; a detector for detecting fluorescent X-rays emanating from said point; and a mirror disposed on the axis of the X-ray tube and through which the X-rays from the X-ray tube may pass so as to be directed onto the said point, the mirror enabling the said pointto be visually observed along an optical axis which coincides with the axis of the X-ray tube.

The gauge may be provided with viewing means for viewing the mirror so as to view the said point.

Thus the viewing means may comprise a microscope.

The X-ray tube preferably has a shutter which is provided with said mirror.

The shutter may be provided with a collimator for collimating the X-ray beam from the X-ray tube.

The detector may be provided with a cobalt filter.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a diagrammatic view of a conventional film thickness gauge, Figure 2 is a plan view of a sample irradiated by an X-ray beam produced by the gauge of Figure 1, Figure 3 is a side view of a case for a film thickness gauge according to the present invention, Figure 4 is a front view of the case shown in Figure 3, Figure 5 is a diagrammatic view of the complete construction of a thickness gauge according to the present invention, Figure 6 is a diagrammatic view of the main part of the gauge shown in Figure 5, Figure 7 is a perspective view of a shutter forming a part of the structure shown in Figure 6, Figure 8 is a front view of the film thickness gauge according to the present invention, Figure 9 is a side view of the structure shown in Figure 8, Figure 10 is an enlarged cross-sectional view of the main part of the structure shown in Figure 8, Figure 11 is a cross-sectional view taken on the line A-A of Figure 10, Figure 12 is a plan view of the main part of the structure shown in Figure 10, Figure 13 is a fross-sectional view taken on the line B-B of Figure 10, Figure 14 is a simplified diagrammatic view of a film thickness gauge according to the present invention having a detector provided with a filter, Figure 15 is a perspective view of the said detector provided with a filter, and Figure 16 is a simplified diagrammatic view of another embodiment of a film thickness gauge according to the present invention.

Figure 1 illustrates diagrammatically a convention al film thickness gauge in which the thickness of a film (not shown) on a sample 1 is measured by employing an incident X-ray beam 3 from an X-ray tube 2 positioned above the sample 1 to irradiate a point P on the sample 1. The sample 1 may, for example, be an integrated circuit (IC), and the point P may be a limited area of the IC which is provided with a metallic film whose thickness is to be gauged.

A fluorescent X-ray 4 is emitted from the point P and is directed onto a detector 5 which measures the amount of fluorescent X-rays. The position of the point P, which is irradiated by the X-rays from the X-ray tube 2, is determined by viewing the sample 1 by means of a viewing means 6 comprising a microscope and projector positioned above the sample 1, the position of the point P being visable through the microscope by reason of the provision of a fluorescent screen (not shown). However, since the optical axis 7 of the viewing means 6 does not coincide with the X-ray axis 3 of the X-ray tube 2, i.e.

since the surface of the sample 1 is not irradiated at right angles thereto but is irradiated at an angle which is inclined with respect to the X-ray axis 3, the shape of the area on the sample 1 which is irradiated by the X-rays is not circular but elliptical, as indicated by a dotted line in Figure 2. For instance when one employs the gauge of Figure 1 to measure the plating film thickness of very small plating areas provided on an exceedingly precisely made member such as an integrated circuit (IC), the shape of the irradiated area on the sample is ellipitical even if the X-ray beam is a thin mean, and the portions of the sample 1 adjacent the desired measuring point P are also irradiated unnecessarily. As a result, the film thickness cannot be measured accurately.Further, if the sample 1 moves vertically, the shape of the irradiated area becomes elliptical and the position of the irradiated area also moves so that the measuring accuracy is poor.

Figure 5 therefore shows diagrammatically the complete structure of a film thickness gauge according to the present invention. The film thickness gauge of Figure 5 comprises an X-ray tube 20 provided with a shutter 21 for preventing transmission of X-rays when the gauge is not in use and a collimator 22 which is mounted below the shutter 21.

The shutter 21 is mounted for movement in the horizontal direction. A sample 24 is positioned on a sample holder 23 which is mounted below the collimator 22.

The point P on the sample 24 is irradiated by X-rays 25 from the X-ray tube 20. Fluorescent X-rays 26 are therefore emitted from the sample 24 and are directed onto a detector 27. The fluorescent X-rays 26 are converted into electrical signals by the detector 27 and are fed to a pulse height discriminator 30 of a controller 29 by way of an amplifier 28.

The controller 29 controls and operates a measuring head 31. A high voltage power source constituting an X-ray power source 32 is connected to an X-ray tube 20. Output signals from the pulse height discriminator 30 are fed to a scaler timer 32a, micro computer 33 and a digital display 34 to display data relating to the thickness of a film on the sample 24.

The controller 29 is constructed as shown in Figures 3 and 4, and has a keyboard portion 35 for the keying in of operational information for the X-ray tube 20. The shutter 21, which is disposed below the X-ray tube 20, is provided with a miror 36 at one end thereof as shown in the diagram of Figure 6. The shutter 21 is mounted for horizontal sliding movement as shown in Figures 11 and 12. Moreover, the collimator 22 is formed as a common unit with the shutter 21 at the bottom of the latter as shown in Figures 7 and 11. The shutter 21 is slidably mounted on a guide rail 38 (Figure 10) provided at the bottom portion of a tank 37. A guide plate 39 of the shutter 21 is arranged to be moved horizontally by a rotary solenoid 40 which is fixed at the bottom of the tank 37, whereby the shutter 21 may be slidably set in a desired position.The shutter 21 is engageable with a stopper screw 41 whose position may be adjusted, whereby the position of the shutter in the horizontal direction is restricted.

The sample 24 is not irradiated by the X-rays from the X-ray tube 20 when the shutter 21 is closed. The sample 24 is, however, irradiated by the X-rays by way of the collimator 22 when the shutter 21 is opened by being caused to slide horizontally by the rotary solenoid 40.

A holder 42 (Figure 10), which is secured to the side wall of the tank 37, is provided with a viewing means constituted by a microscope 42. When the shutter 21 is positioned as shown in Figure 10, the upper surface of the sample 24 is viewed through the mirror 36. As shown in Figure 13, the holder 42 is slidably mounted on guide shafts 45 formed on a base plate 44 at the side of the tank 37 to enable the holder 42 to slide in the directions of a double headed arrow A. The holder 42 may thus be moved to an arbitrary position in the directions of the arrow A by means of an adjusting screw 46. Consequently, the X-ray axis of the X-ray tube 20 can be made to coincide with the optical axis of the microscope 43.

The X-ray tube 20, which is disposed inside the tank 37, is provided with a high voltage power source 47, and the tank 37 is filled with an insulating oil 48, a cover plate 50 being mounted on an upper opening 49 of the tank 37. The detector 27 is disposed at the bottom of the tank 37, and a filter 53, Figures 10, 14 and 15, which is secured to a filter guide 52, is mounted below the tank 37 and faces an opening 51 of the detector 27. The filter 53 is made of cobalt and is adapted to suppress a fluorescent X-ray of copper but to transmit a fluorescent X-ray in the nickel spectrum so as to direct the latter onto the detector 27 which counts the amount of fluorescent X-rays in the nickel spectrum out of the fluorescent X-rays emitted from the sample 24.Thus the film thickness of a nickel plate 24b (Figure 14) on a copper base 24a is measured by the amount of the fluorescent X-rays counted by the detector 27.

The microscope 43 projects from a central portion of a casing 54 (Figure 10) and a sample holder positioning means 55 (Figures 8 and 9) is located beneath the sample holder 23. The portion of the sample 24 which is viewed by the microscope 43 is enlarged thereby. Accordingly, the X-rays can be caused to irradiate accurately a levelled portion of the sample 24 since the optical axis of the microscope 43 coincides with the axis of the X-ray tube 20.

The microscope 43 can be adjusted to view a smail area such as about 0.3 mmM.

As indicated above, in the embodiment of Figure 14, a cobalt filter 53 is employed, but otherwise the construction of the gauge is similar to that of Figure 6. In orderto simplify Figure 14, however, the mirror 36 and shutter 21 have been omitted, although these parts would in fact be provided.

In the embodiment ofthe present invention shown in Figure 16, a microscope 43 is not employed but the mirror 36 is made of a material which both reflects light and transmits the X-rays, such as an aluminium layer evaporated onto an SiO2 plate or an aluminium layer evaporated onto an organic film.

Accordingly, the X-rays are directed onto the sample 24 through the mirror 36, while the sample 24 is visible to an eye 60 by reason of the light reflected by the mirror 36. In orderto simplify Figure 16, the shutter 21 is not shown therein although it would in fact be provided.

In the film thickness gauges described above, the axis of the X-ray tube coincides with the optical axis of the viewing means, and consequently the point on the sample onto which the X-rays are directed can be observed from the same direction as the X-ray axis of the X-ray tube. Therefore, a small measuring area about 0.3 mmM is measurable. The shape of the X-ray beam irradiated on the sample becomes circular, whereby the mis-irradiation and mismeasurement are completely eliminated.

Claims (7)

1. A film thickness gauge comprising mounting means for mounting a sample having a film thereon whose thickness is to be measured at a predetermined point, said film being such that, when the said point is irradiated with X-rays, it emits fluorescent X-rays; an X-ray tube for irradiating said point with X-rays; a detector for detecting fluorescent X-rays emanating from said point; and a mirror disposed on the axis of the X-ray tube and through which the X-rays from the X-ray tube may pass so as to be directed onto the said point, the mirror enabling the said pointto be visually observed along an optical axis which coincides with the axis of the X-ray tube.

2. A gauge as claimed in claim 1 comprising viewing means for viewing the mirror so as to view the said point

3. A gauge as claimed in claim 2 in which the viewing means comprises a microscope.

4. A gauge as claimed in any preceding claim in which the X-ray tube has a shutter which is provided with the said mirror.

5. A gauge as claimed in claim 4 in which the shutter is provided with a collimatorforcollimating the X-ray beam from the X-ray tu be.

6. A gauge as claimed in any preceding claim which the detector is provided with a cobalt filter.

7. Afilm thickness gauge substantially as hereinbefore described with reference to and as shown in Figures 3-16 of the drawings.