GB2190487A - Optical profilometer - Google Patents

Abstract

An optical profilometer which is suitable for measuring the profile of a curved surface uses a beam of light which is focussed on and reflected by the surface to determine variations in the surface position. As the beam of light intercepts different regions of the surface, the optical axis of the focussing system is arranged to remain along the radius of curvature of the surface. Light from laser 1 is directed via polarising beam splitter 5 and quarter-wave plate 7 onto objective lens 9 which focusses the beam onto curved surface 17. The reflected beam passes to quadrant detector 13, and any focus error produces a signal from generator 15 which moves objective 9 to correct the error; the error signal providing the measurement. The surface 17 is mounted on a mechanical handling system which moves the surface so that the light beam remains along a radius of curvature. <IMAGE>

Description

SPECIFICATION Profilometers This invention relates to profilometers. In particular the invention relates to optical profile-meters.

Optical profilometers provide a convenient means for measuring the surface profile of a surface of an object without it being necessary to contact the surface. Known optical profilometers comprise a lightsourcewhich is arranged to directa light beam through a first focussing system onto a point on the surface of an object whose profile it is required to measure. A light detector is arranged to detect light which has been reflected by the point backthrough the first focussing system, and through a further focussing system. Whether the beam is focussed on the point on the surface will depend on the distance between the point and thefirstfocussing system measured along the optical axis of the firstfocussing system.Thus by monitoring the amount by which the first focussing system must be displaced along its optical axis with respect to the point such that the detector detects a focussed image of each point on the surface which intercepts the beam as the beam is scanned overthe surface, an indication ofthe surface profile of the surface may be obtained.

Such optical profilometerssufferthe disadvantage, however, that whilsttheyare particularly suitable for measuring the surface profile of substantially flat surfaces, for example the planar surfaces of silicon wafers, they are not suitable for measuring the surface profile of curved surfaces, for example the internal surfaces ofthe spherical metal components employed in the construction of microwave amplifier tubes. This is because the local tilt of the surface at any point along the surface will result in a variation of the direction of the reflected light from the surface, in extreme cases the reflected light being so displaced from the optical axis of the focussing system that it does not enterthe focussing system.

It is an object of the present invention to provide an optical profilometer suitable for measuring the surface profile of a concave surface.

According to the present invention an optical profilometerfor measuring the surface profile of a surface comprises: meansfordirecting a light beam through a focussing system onto a point on the surface; means for directing light reflected back through the focussing system by the point onto a detector means arranged to determine whetherthe beam is focussed on the point; means for moving the focussing system and the surface relative to each other along a curved path such that the beam scans across successive points on the surface, the optical axis ofthefocussing system remaining along the radius of curvature of the surface at each point ofthe surface onto which the beam is directed and the distance between each said pointandthefocussing system remaining substantially constant; and means for obtaining an indication of any changes in said distance between each said point and the focussing system from the light detected by the detector means.

The detector means suitably includes a further focussing system through which said reflected light is arranged to pass.

Preferablythe profilometerincludes means for adjusting said distance between each said point and the first focussing system dependent on the output of said detector means such that the beam is focussed on each said point.

One optical profilometer in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure lisa schematic view of the profilometer; Figure2 illustrates the scanning operation ofthe profilometer on a first mode; Figure 3 is a plan view ofthe surface being scanned during the scanning operation illustrated in Figure 2; Figure 4 illustrates the scanning operation of the profilometer in a second mode; and Figure 5 is a plan view of the surface being scanned during the scanning operation illustrated in Figure4.

Referring firstly to Figure 1 ,the profilometerto be described comprises a compact HeNe laser 1 arranged to direct a light beam through a lens 3 onto a polarizing beam splitter 5. The beam splitter5 is positioned so as to direct light from the laser 1 through a quarter-wave plate 7 onto an objective lens 9. The profilometerfurther includes a cylindrical lens 11 and a quadrant detector 13 whose output is connected to a focus-error signal generator 15, the functions of these components being described hereafter, together with a position sensing system 16.

In use ofthe profilometer a spherical concave surfacel7whosesurface profileitisrequiredto measure, for example the spherical surface of a spider-grid for use in a microwave amplifiertube, is placed on a mechanical handling system (not shown), and light from the laser 1 passing through the grid to the sensor 16 is monitored and used to position the surface 17 such that light from the laser 1 is approximately focussed on a spot on the surface and is reflected back up through the objective lens 9 and the quarterwave plate 7. The double pass of light from the laser 1 through the quarter-wave plate 7 causes the reflected light to pass through the polarizing beam splitter 5. The reflected lightthen passesthroughthecylindrical lens 11 onto the quadrant detector 13.

If the light point on the surface 17 from which the light is reflected is exactly atthe focus ofthe objective lens 9, the quadrant detector 13 will detect the focussed "circle of least confusion" ofthe reflected light. If however the point is slightly displaced from the focus of the objective lens 9,the presence of the cylindrical lens 11 will cause the detector 13 to detect a light pattern comprising a spot elongagated in one of two perpendicular directions dependent on the direction of displacement ofthe point from the focus.If the point is displaced from the focus ofthe objective lens 9, the output signal of the quadrant detector will trigger the focus-error signal generator 15 to generate a focus-error signal which is fed to an objective drive circuit, indicated as 19, effective to adjust the position ofthe objective lens 9 such thatthe errorsignal produced bythe generator is minimised. The point on the surface is thus placed at the focus of the objective lens, the amount of movement of the lens 9 required to achieve this being logged by a controller indicated as 21.

Referring now also to the other figures in orderto measure the surface profile across the surface 17, the surface is rotated by the mechanical handling stage with respect to the objective lens 9 abouttwo orthogonal directions, the0 and G scan directions.

In the scan mode illustrated in Figure 2 the mechanical handling stage is arranged to rotate the grid such that the surface 17 is rotated in an arc centred on the centre of curvature P of the surface, the centre of curvature Plying on the optical axis of the objective lens 9. The focussed light beam from the laser 1 will thus be directed onto successive points along a diameter of the surface, as indicated in Figure 3the surface at each point being normal to the beam,the distance between the lens 9 and each point on the surface remaining constant in the absence of any irregularities in the surface. In the other scan mode 8 illustrated in Figure4and Sthe beam is arranged to hit a point on the surface away from the centre of surface, the grid being positioned such that its central axis is parallel to the optical axis ofthe lens 9.The handling stage is arranged to rotate the surface such that the locus of the points onto which the beam is directed is a circie centred on the centre of the surface.

Thus bya combination ofthetwo scan modes, the surface profile ofthe surface may be measured along every direction along the surface, this being partially shown in Figure 3 where three different diameters of the surface 17 have been traced bythe beam, these diametrical IZi mode paths being linked by rotation in the &commat; mode. It will of course be appreciated thatfor measurement of concentric members as are present in a spider grid, the scan mode (3 is also useful by itseif.

It will be appreciated that whilst the profilometer described above is designed forthe measurement of the surface profile of spherical concave surfaces, a profilometerin accordance with the invention may 3a designed forthe measurement of the surface profile of otherthan spherical concave surfaces.

It will also be appreciated that whilst it is particularlyconvenientto move the objectwith respect to the focussing system in order to cause the light beam to scan across a surface of the object, in an alternative profilometer in accordance with the invention the focussing system may be arranged to move ratherthan the object.

Itwill also be appreciated thatwhilstthe profilometerdescribed hereabove is particularly useful for the automatic checking of the surface profile of, for example, a spider grid, the objective lens being arranged to move automatically in dependance onthefocus error signal, in a simpler profilometer, the objective lens may be arranged to be moved manually.

It will also be appreciated that whilst the optical profilometer described hereabove is particularly useful for measuring the surface profile of a curved surface, the 8 mode scan also finds application in the measurement of the surface profile of a planar surface.

Claims (6)

1. An optical profilometerfor measuring the surface profile of a surface comprising: means for directing a light beam through a focussing system onto a point on the surface; means for directing light reflected backthrough the focussing system by the point onto a detector means arranged to determine whether the beam is focussed on the point; means for moving the focussing system and the surface relative to each other along a curved path such that the beam scans across successive points on the surface, the optical axis of hefocussing system remaining along the radius of curvature of the surface at each point of the surface onto which the beam is directed and the distance between each said point and the focussing system remaining substantially constant; and means for obtaining an indication of any changes in said distance between each said point and the focussing system from the light detected by the detector means.

2. An optical profilometeraccording to Claim 1 in which said detector means includes afurther focussing system through which said reflected light is arranged to pass.

3. An optical profilometer according to Claim 2 in which said further focussing system comprises an astigmatic lens.

4. An optical profilometer according to Claim 3 in which the detector means includes a quadrant detector arranged to intercept said reflected light after it has passed through the astigmatic lens.

5. An optical profilometer according to any one ofthe preceding claims including means for adjusting said distance between each said point and the focussing system dependent on the output of said detector means such that the beam is focussed on each said point.

6. An optical profilometerfor measuring the surface profile of a surface substantially as hereinbefore described with reference to the accompanying drawings.