SU1755050A1 - Method of measuring surface relief - Google Patents

Abstract

The invention relates to optics. The purpose of the invention is to improve the accuracy and sensitivity of the method, which is to register the interference pattern of the pulsed reference beam and the object beam repeatedly reflected from the test surface, and the registration is carried out only at the moment of mixing of the beam pulses. 2 Il.

Description

The invention relates to optics, namely, optical methods for measuring the surface topography, and can be used in technological processes in the construction of profilometric control systems.

The known method of measuring the surface asperities, which consists in mixing the reference laser beam and the object laser beam repeatedly reflected from the test surface, by spatially separating the rays of the subject laser beam that have experienced a different number of reflections from the test surface, registering the interference pattern of the reference and subject beams, and determining the surface micro irregularities by registered picture. The disadvantage of this method is the limited scope due to the fundamental impossibility of measuring the relief with irregular discontinuities.

There is also known a method for measuring the surface topography consisting in mixing the reference laser beam and the object laser beam, repeatedly

reflected from the monitored surface (besides, mixing is achieved as a result of the restoration of a two-exposure hologram), recording the pattern of interference of the reference and object beams by matching the optical path length of the reference laser beam and the subject laser beam that experienced a specified number of reflections from the monitored surface with a coherent laser radiation length and determining surface relief on the registered picture.

This method has a wide range of applications, as it allows to measure reliefs with any spatial configuration of microresistances. However, its disadvantages are low accuracy, due to the low diffraction efficiency of the hologram recovery, as well as the background illumination of the recorded pattern of interference; low sensitivity due to the fundamental impossibility of achieving high values of the amplification ratio of phase inhomogeneities, due to the severe restriction on the optical path length of the reference laser beam and the subject (/)

WITH

Xi

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a laser beam that has experienced a given number of reflections from the test surface due to the fixed value of the laser radiation coherence line.

The aim of the invention is to improve the accuracy and sensitivity of the method of measuring the surface topography,

The goal is achieved by the method of measuring the surface topography, which consists in mixing the reference Azeri beam and the object laser beam, polyshaft about reflected from the test surface, registering the interference pattern of the reference and object beams, and determining the surface relief from the registered pattern, and the reference and object beams are formed as pulses whose duration does not exceed the time between the two successive reflections of the object beam from the test surface, and the interference pattern is recorded at the time points corresponding to the mixing of the pulse of the reference beam with the pulse of the object beam.

Figure 1 presents the bio-diagram of the system that implements this method; on ig.2 - timing charts of the system.

The system contains a laser 1 with a short pulse pulse shaping unit, as well as a telescopic expander located on the same optical axis, consisting of a micro-lens 2, a microdiaphragm and a collimating lens A, in addition to the wind splitters 5 and 6 and a resonator consisting of a beam splitter 7, and Reflecting object 8, the surface relief of which is to be measured. The beam splitter b is optically connected to the reference mirror 9, and the beam splitter 5 is optically connected to the electro-optical shutter (EOZ) 10, the recorder 11, as well as a lens 12 and a photodetector 13, which in turn is electrically connected to the circuit: pulse generator 14 - generator time shifts 15 - voltage amplifier 16 - EOS 10.

A serially produced picosecond laser MSU-NPP Alkor, YAGUAR was used as a laser 1 with a unit for forming short pulses in an experimental test bench.

Optical elements 2,3,4,12 used from the set of optics serial installation UIG-22. M. The standard Kerr cell is used as an EOZ 10. The recorder 11 can serve as a domestic Zenit-12 type SD camera camera or an ordinary matte fiber. The FD-256 high-speed photodiode, supplied with a YAGUAR type 1 laser, can serve as a photodetector 13. The pulse generator 14, the time shift generator 15 are serial devices of the type G5-72, I1-8, respectively. As amplifier 16, a standard key circuit can be used.

The reflection coefficients of the air splitters 5, 6, and 7, as well as the reference mirror 9, are calculated from the condition of equality of intensity on the recorder 11 of the subject and reference laser pulses and are determined by the specified number of reflections from the object. In the experimental bench for five reflections from the object, the reflection coefficients of the beam splitters 5, b and 7 are 0.5; 0.1 and 0.45, respectively, whose values are formed by vacuum sputtering according to standard technology. The reference mirror 9 has a reflection coefficient of the order of 0.98 and is selected from the set of metal mirrors attached to the UIG-22 installation M. The distance between the beam splitter 7 and the reflecting object 8 in the experimental bench being manufactured is 4 mm.

The method is implemented as follows.

A laser 1 with a short pulse generation unit generates radiation in a pulse-periodic mode, with

the duration of the laser pulse is less than the time it takes for the implementation of two consecutive reflections from the test surface. The time taken by the laser pulse to effect two consecutive reflections from the test surface is determined by the time of complete passage of the resonator 7,8 and is calculated by the formula

gpr 2 L / C,

where С 3x108 m / s is the speed of light (figure 1).

In the experimental stand L 4 cm and the time of one pass is gpr 0.3 ns. The number of reflections from object 8 in the experimental bench was set equal to five. Consequently, the time during which the laser pulse makes five full passes of the resonator 7, 8 will be tnpz 5 Gpr 1.5 ns. The duration of the pulse train of the YAGUAR laser pulses in the mode-locking mode

simultaneous Q-switching is a stable value of T 4 not. In each train, five picosecond pulses are emitted with an interval of 1 np. The first pulse of the train, reflected from the beam splitter 5, is focused by the lens 12 onto the high-speed photodetector 13, the time constant g of which is less than the interval rp and in the experimental test bench is r 1 not (Figure 26). The signal from the photodetector 13 is fed to a pulse generator 14, which forms a sync pulse (Fig. 2c), which enters the time shift generator 15, shifting the sync pulse by an interval of GdW "T-rn (Fig. 2d). Next, this pulse arrives at the amplifier 16 and the voltage pulse generated in this way opens the EOZ 10 exactly at the moment when the last laser pulse of the train completes a specified number of reflections or a controlled surface (a given number of passes through the resonator 7, 8). 4 Thus, all the pulses entering the laser radiation train 1, except the last, passed the elements of the optical circuit 2-8, do not reach the recorder 11, since the EOZ 10 is closed (Fig. 2) The last pulse of the laser radiation train 1 by the pad on the telescope 2-4 expands and, having passed through the semitransparent beam splitter 5, is divided by the beam splitter 6 into a reference laser pulse that hits the reference mirror 9 and is reflected from it, and the subject laser pulse entering the resonator formed by the beam splitter 7 and the object 8, the profile of which is to be measured (figure 1). The arms of the interferometer And and h are matched so that the reference laser pulse, reflected from the mirror 9, meets on the beam splitter 6 with the object laser pulse that performs the specified number of reflections from the object 8 In the experimental bench And and 2 are matched so that the reference laser pulse meets on beam splitter 6 with a subject laser pulse that made five full passes through the resonator 7, 8. The resulting interference of two laser pulses, reflected from the translucent beam splitter 5, falls on EOZ 10 ko ory at this point briefly opened (according to the scheme described synchronization) and transmits it to the receptionist

11. With the generation of the following laser radiation train 1, the operation of the circuit is repeated. Thus, the opening of the EOZ 10 occurs only at the moment when the interference pattern from the reference laser pulse and

a subject laser pulse that has completed a predetermined number of reflections from the monitored surface enters the recorder 11. The closed state of the EOZ 10 at other points in time eliminates the background illumination of the recorded interference pattern from the subject laser pulses that have made the number of reflections from the monitored surface less than the specified Zame

Timing, the intensity of all predmg.tst reflections from the test surface above the intensity of the specified reflection The exclusion of the background recording of the recorded interference pattern makes it possible to increase the accuracy and sensitivity of measuring the surface profile of the object O without using holographic methods for recording the interference pattern, which is important when measuring small amplitudes of micro-asperities topography, in particular, when visualizing surface acoustic waves and

Claims (1)

Invention Formula The method of changing the surface relief, which consists in mixing the reference laser beam and the object laser beam repeatedly reflected from the test surface, registering the interference pattern of the reference and subject beam and determining the surface relief according to the registered pattern, different in order to improve the accuracy and sensitivity , reference and subject the beams form in the form of pulses, the durability of which does not exceed the time between two successive reflections of the object beam from the monitored surface, and the registration of the pattern Interferences are carried out at times corresponding to the mixing of the pulse of the reference beam with the pulse of the object beam. l vs -ixi- CM “V 5; at Outputs: