Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1990-09-14
1992-08-18
Turner, Samuel A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356359, G01B 902
Patent
active
051393360
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical measuring apparatus and methods and, in particular, to scanning optical profilometers for the simultaneous measurement of amplitude and phase variation during the measurement of a surface.
A scanning optical profilometer has distinct advantages over contact methods such as the stylus probe which can be difficult to operate. In addition optical techniques allow the possibility for remote measurement and are consequently more flexible in their usage. Conventional optical profilometers, however, lack sensitivity: they rely on interfering two light beams which follow separate paths and thus are susceptible to mechanical vibrations. One method to overcome this problem is by operating an optical profilometer in the differential mode. Such systems respond to the variation instead of the absolute phase of the returning light beam. Differential optical profilometers have been shown to have a phase sensitivity of 3.times.10.sup.-3 mrad in a 1 KHz bandwidth (equivalent to 1.5.times.10.sup.-5 .ANG. in the context of topography).
We have devised a new technique which in addition to monitoring phase variation, allows the simultaneous measurement of the differential of the amplitude of the reflected light. This may be related to the reflectivity of an object. The sensitivity for this mode of operation is similar to existing systems, which is 3 in 10.sup.5 a 1 KHz bandwidth.
According to the present invention there is provided optical apparatus for measuring the topographical properties of a surface compising means for splitting a beam of radiation into two component beams, means for focussing said two component beams on to a surface under test, means for recombining said two component beams after reflection from said surface to cause interference between said two component beams and means for measuring the differential of at least one parameter of the interference products of said two component beams.
The invention will now be particularly described with reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram illustrating the principle of the invention;
FIG. 2 is a more detailed schematic diagram illustrating a practical embodiment;
FIG. 3 depicts differential optical phase and amplitude line traces across an etched silicon wafer; and
FIG. 4 depicts differential optical phase and amplitude line traces across a partly ion-implanted silicon wafer.
Referring now to the drawings, we use a heterodyne interferometer to achieve simultaneous and independent measurements of amplitude and phase. The principle of the system is outlined in FIG. 1. The main component is the beam splitting device, BS which has the following properties: amplitudes and an angular deviation, .alpha.; beams; f.sub.s.
The two beams emerging from the beam splitter BS are focused by a lens L1 normally on to the surface of an object O. The lens L1 is positioned in the system so that the point of apparent splitting F is at the back focal point of the lens L1 and the object is at its front focal plane. The two beams will then focus normally on to the object with the distance between the two focal spots determined by the value and the focal length of the lens. Upon reflection from the object surface, the two beams traverse through the lens and beam splitter a second time, where they are recombined. Both the frequencies and the amplitudes of the two beams will be shifted (or modulated) again after the second passage. The recombined beam is then diverted towards a photodetector PD via the beamsplitter.
After the first passage through the beam splitter BS the states of the two beams can be written as +.omega..sub.1)t+.phi..sub.1 ]} (1) +.omega..sub.2)t+.phi..sub.2 ]} (2) frequency, .omega..sub.1 and .omega..sub.2 are the frequency shifts of the two beams, and .phi..sub.1 and .phi..sub.2 represent the average optical path lengths. After reflection from the object surface, both the amplitudes and the phases of the beams are modified by the object structure. The beams, in front of the photodetector, can then be
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See Chung W.
Vaez-Iravani Mehdi
National Research Development Corporation
Turner Samuel A.
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