The invention relates to a device for optically measuring the height of a surface of an object, which device comprises a radiation source unit for generating a radiation beam and for forming an essentially stationary radiation spot on the surface by means of said beam, means for selecting a reflected beam from the radiation reflected by the surface in the radiation spot, and a position-sensitive radiation detection system arranged in the radiation path of the reflected beam. An essentially stationary radiation spot is understood to mean that movement of the spot during a measurement is not necessary.
Such a device is used, for example, to check whether a surface has the desired profile, for example, whether a flat plate is really flat and does not have any grooves or projections. The device may also be used to check whether holes and elevations are provided at the correct positions on a substrate plate and it may further be used, for example, for inspecting electronic circuits comprising electrically insulating plates provided with conducting metal strips and electronic components.
A device as described in the opening paragraph is known, inter alia, from EP-A 0,134,597. This document describes a device for performing a triangulation so as to determine the distance between the surface of an object and a reference level. The known device comprises a radiation source from which a narrow radiation beam is incident on the surface where it forms a radiation spot. The radiation spot is focused on a linear radiation-sensitive detector surface by means of one or more objective lenses. The pupil of the objective system selects a reflected beam coming from the radiation spot. The position where the radiation spot is imaged on the detector surface indicates where the radiation beam incident on the object surface impinges upon the latter surface, and hence indicates the height of this surface. A displacement of the position of the radiation across the detector surface involves a change of the surface height of the object to be measured.
The sensitivity of the height measurement, i.e. the ratio between the change of the surface height and the displacement of the position of the incident radiation across the detector surface is dependent, inter alia, on the focusing of the image of the radiation spot on the surface of the position-sensitive radiation detection system. In order to maintain this image focused at any height, the detector surface should be tilted with respect to the direction from which radiation is incident on the surface. Due to such a tilt the fraction of the radiation which contributes to the electric output signal of the detector will decrease, thus rendering the detection system more insensitive.