Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1998-03-11
2001-01-30
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
C356S370000, C369S044240, C250S201300, C250S201500
Reexamination Certificate
active
06181422
ABSTRACT:
The present invention relates to apparatus and methods for measuring the positions of points on sample surfaces. Those measurements may be used to calculate the numerical values of such characteristics as the flatness, form and roughness of the sample.
According to a first aspect of the invention there is provided an apparatus for measuring the position of a point on the surface of a sample comprising,
means for producing a spot of light to illuminate said point on the surface of the sample,
a focus detector,
a focusing element system, comprising a movable focusing element and at least one other focusing element, arranged to project onto the focus detector an image of a plane beyond the focusing element system in the region of the sample surface,
a focusing element system drive means for translating the focusing element system,
a movable focusing element drive means for moving the movable focusing element within the focusing element system so as to vary the distance between the plane imaged on the focus detector and the focusing element system,
a control means responsive to the focus detector for so operating the focusing element system drive and movable focusing element drive that an image of the illuminated point on the sample surface is kept in focus, or in a desired state of defocus, on the focus detector.
The focus element system drive may be arranged to move the focus element system mechanically and the movable focusing element drive may be arranged to move the movable focusing element electromagnetically.
The control means may be arranged to operate the movable focusing element drive in preference to the focusing element system drive to achieve focus.
The control means may be arranged to operate the focusing element system drive when the movable focus element drive approaches the end of its range.
The control means may be arranged to operate, when the movable focus element drive approaches the end of its range, the focusing element system drive further than is necessary to achieve focus and to operate the movable focusing element drive so that it moves back towards the centre of its range.
The movable focusing element drive may comprise a coil for producing a field for moving the movable focusing element and may comprise a driver circuit having a low output impedance.
The control means may be arranged, in response to the focus detector indicating that the apparatus is a long way from focus, to fix the position of the movable focusing element and to move the focusing element system with its drive towards the sample from a predetermined point until focus is achieved, having first withdrawn the focus element system if it is closer to the sample than the predetermined point.
According to a second aspect of the invention there is provided an apparatus for measuring the position of a point on the surface of a sample comprising,
means for producing a spot of light to illuminate said point on the surface of the sample,
a focus detector,
a focusing element system, comprising a movable focusing element, arranged to project onto the focus detector an image of a plane beyond the focusing element system in the region of the sample surface,
a movable focusing element drive means for so moving the movable focusing element as to vary the distance between the plane imaged on the focus detector and the focusing element system,
a control means responsive to the focus detector for so operating the movable focusing element drive means that the position of the movable focusing element oscillates about the position at which an image of the illuminated point on the sample surface is in focus on the focus detector, the control means being arranged to record a measurement of the position of the movable focusing element at times when the image of the illuminated point is in focus on the focus detector.
The apparatus may comprise means for moving the spot across the sample and the control means may be arranged to keep the oscillation of the movable focusing element generally centred about the point of focus as the spot moves across the sample or the control means may be arranged to move the centre of the oscillation of the movable focusing element when the focus point approaches the end of the range of the oscillation. The control means may be arranged to control the frequency of oscillation to correspond to the desired spatial frequency of measurement points across the sample, or to a multiple thereof.
According to a third aspect of the invention there is provided an apparatus for measuring the position of a point on the surface of a sample comprising,
means for producing a spot of light to illuminate said point on the surface of the sample,
a focus detector comprising a transducer having at least two light sensitive areas,
a focusing element system, comprising at least one focusing element, arranged to project onto the focus detector an image of a plane beyond the focusing element system in the region of the sample surface,
the focus detector and the focusing element system being so arranged that a first one of the light sensitive areas receives more light when the illuminated spot is out of the said plane to one side and that a second one of the light sensitive areas receives more light when the illuminated spot is out of the said plane to the other side,
a drive means for so moving the focusing element system, or one of its elements, as to vary the distance between the plane imaged on the focus detector and the focusing element system,
signal generation means for adding signals from the said first and second light sensitive areas of the focus detector to provide a sum signal and comparing the level of the sum signal, or the level of a signal derived from the sum signal, with a threshold level for providing a gating signal indicating whether or not that sum is greater than the threshold, and also for providing a difference signal being the difference, or a function of the difference, of the signals from the said first and second light sensitive areas, and
a control means responsive to the gating and difference signals controlling the drive means.
The said function of the difference signal may have the sum signal in its denominator.
The focus detector may comprise a first plurality of light sensitive areas that receive more light when the illuminated spot out of the said plane is to one side and a second plurality of light sensitive areas that receive more light when the illuminated spot is out of the said plan e to the other side, the signals from the first plurality may be added together before being passed to the signal generation means and the signals from the second plurality may be added together before being passed to the signal generation means.
According to a fourth aspect of the invention there is provided an apparatus for measuring the position of a point on the surface of a sample and for imaging an area of the surface, the apparatus comprising,
spot producing means for producing a spot of light to illuminate said point on the surface of the sample,
an objective focusing element or system of focusing elements for collecting light reflected or scattered from said illuminated point and for collecting light from the area to be imaged, and
a plate arranged to receive the light collected by the objective focusing element or system of focusing elements and orientated at an angle inclined to the direction of that light, the plate transmitting part of and reflecting part of that light,
the apparatus further comprising sensing means sensitive to and arranged to receive light reflected from the plate.
The said plate may be such that it transmits a substantial portion of the collected light scattered or reflected from the illuminated point and reflects a substantial portion of a band of wavelengths other than that emitted by the spot producing means and to which the sensing means is sensitive.
The said plate may also reflect a portion of the collected light scattered or reflected from the illuminated point and the sensing means may also sensitive to that light.
The sensing means may be a camer
Brown & Sharpe Limited
Pham Hoa Q.
Wolf Greenfield & Sacks P.C.
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