SU1723435A1 - Device for radiometric differential thickness measuring - Google Patents

Abstract

The invention relates to radiometric methods for measuring thickness and can be used in microelectronics and instrument making. The purpose of the invention is to improve the accuracy by providing simultaneous measurement of the intensity for both non-attenuated radiation and radiation transmitted through the sample with the same coordinate-sensitive detector (PSD). The device uses a multicollimator 3 with multiple parallel channels as a collimator, a signal extraction unit 10 contains a threshold discriminator 12, a comparison unit 16, an upper level selector 13, a maximum signal extraction unit 14, ADC blocks 17, 18. The processing of the measurement information is performed by a unit comprising two storage devices 20, 21, a computing unit 23, a central processor 22, and an information output device 24. 3 il. W Ё

Description

The invention relates to a measurement technique, namely, radiometric methods for monitoring the thickness of objects, and can be used to control the thickness of foil, thin films and coatings in various industries.

A device for radiometric measurement of the thickness of silicon membranes is known, comprising a radiation source, a point collimator, a radiation receiver with a power and control unit, a timer, a signal extraction unit including a threshold discriminator and an information processing unit. The membrane to be measured is located between the collimator and the radiation receiver. The thickness of the membrane in the surface area, determined by the size of the collimator, is calculated by the formula

In

NX

No

where dx is the sample thickness;

c-linear attenuation coefficient in the membrane material,

NX is the intensity of the transmitted radiation.

No is the intensity of non-attenuated radiation, measured before and after measuring an object and recorded in the memory of the microcomputer.

The formula assumes that the receiver registers both attenuated and non-attenuated radiation.

The drawbacks of the described device are low accuracy, since the measurement of the intensity of un-attenuated radiation cannot be carried out simultaneously with the measurement of the radiation transmitted through the test object; low productivity, since the thickness of different parts of the membrane surface is measured sequentially and the results are processed in real time.

The closest in technical essence to the described (selected as a prototype) is a differential device for radiometric measurements. A standard differential radiometric measurement device contains two. a collimator located on opposite sides of the radiation source. One of the collimators is located between the radiation source and the object to be measured, the other between the radiation source and the input window of one of the two radiation receivers. Both receivers are simultaneously measured as un-attenuated radiation.

and radiation transmitted by the controlled object.

The device contains a power supply and control unit for radiation receivers, a timer

to determine the signal accumulation time, including the threshold discriminators, as well as the information processing unit, which compares the radiation intensities recorded simultaneously by two receivers, and calculates the thickness of the object on the surface area determined by the size collimator. However, the principle of standard differential measurement itself contains a physical contradiction — with an increase in accuracy at the signal processing stage, accuracy is lost due to the presence of two radiation detectors differing from the original

0 sensitivity and stability.

Another disadvantage is the restriction in choosing a radiation source that must provide the same radiation flux to both receivers located

5 on opposite sides of the source. This requirement cannot be realized technically for widely used radionuclide sources.

The purpose of the invention is to increase the measurement accuracy, which is achieved by using a coordinate-sensitive detector (PSD) as a receiver in a known differential thickness measurement device, a multicollimator with multiple parallel channels as a collimator5, each channel being coaxially aligned with receiving elements the sensitive detector, the signal extraction unit contains the comparison unit and the first analog-to-digital converter (ADC), as well as the series-connected threshold iskriminator whose input is connected to the output of the preamplifier and the second output - to

5 to the first input of the comparison unit, the upper level selector, the second output of which is connected to the second input of the comparison unit, the maximum signal extraction unit, the scaling amplifier and the second

0 ADC, the output of which is connected to the first input of the information processing unit, and the output of the comparison unit through the first ADC is connected to the second input of the information processing unit, information processing unit

5 contains two memory devices, a central processor and a computing unit, the inputs and outputs of which are connected to the central processor, the third input-output pair of the central processor is connected to an external information output device, the fourth input-output pair of the central processor is connected to the control unit of the device, the outputs of which connected to the coordinate-sensitive detector and the input of the power supply unit, the output of which is also connected to the input of the coordinate-sensitive detector.

FIG. 1 is a schematic diagram of a radiometric differential thickness measurement device and a block diagram of the main electronic components; in fig. 2 - pulse waveforms at different stages of signal processing; in fig. 3 is a schematic diagram of the organization of the internal structure and transfer of signals in a radiation detector based on a charge-coupled device (CCD).

At the base 1 there is a source of radiation 2, on one side of it is a multi-collimator 3 in the form of a system of collimation holes. Controlled membrane 4 is placed in the holder 5, in the center of which a hole is made, limiting the working field of the membrane. The holder with the membrane is set so that at least one collimator is not blocked by the object. The receiving elements 6 of the coordinate-sensitive detector 7 are oriented with collimation holes.

The PSD 7 input is connected to a power supply unit (for example, a clock generator) 8 and a device control unit 9. The output of PSD 7 is connected directly to signal extraction unit 10.

The signal isolation unit 10 comprises a preamplifier 11, a threshold discriminator 12, a high-level selector 13; A maximum signal extraction block 14, connected to a scaling amplifier 15, and a comparison block 16 are connected in parallel to the output of the upper level selector. The input of the comparator unit 16 is also connected to the output of the threshold detector 12. The outputs of the scaling amplifier 15 and the comparator unit 16 are connected via two units — ADC 17 and 18 — to the information processing unit 19.

The information processing unit 19 comprises two storage devices 21 and 22 connected to the central processor 22, as well as a computing unit 23 and an information output device 24. The processing unit 19 exchanges information with an installation control unit.

The device operates as follows.

The controlled object in the holder 5 is installed in the gap between the system of collimators 3 and the input window 6 of the PSA 7 so. that part of the collimators were not blocked.

On the Start signal, the built-in 5 timer of control unit 9 is started and the output of power supply unit 8 (synchronous generator) is enabled. As a result, power supply voltage pulses are applied to PSD 7 and the process of signal accumulation begins in the detector. In this case, the receiver registers radiation attenuated by the material of the object at the control points specified by the spatial arrangement of the collimators, as well as non-attenuated radiation (neglecting absorption in the air gap), which falls on the receiving elements of the detector through collimators not covered by the object.

In the case of using the CCD as

0 PSD absorbed in a semiconductor radiation causes the generation of electron-hole pairs. In the depleted layer, under the action of an electric field, these pairs are separated: the electrons are localized at potential max, and the holes are carried to the neutral region of the semiconductor. The amount of charge accumulated in this element is proportional to (in the first approximation) the flux of energy F and the accumulation time tH averaged over the area S of the receiving element:

Q q F in S tH

5 where 0 is the integral conversion efficiency;

q is the electrode charge. A CCD consists, as a rule, of two identical sections (regions) - accumulation area A and storage area (memory) B, as shown in FIG. 3. The accumulation time of charges (exposure time) is determined by the timer of control unit 9 and is determined by the intensity of the incident radiation. AT

5, depending on the measurement conditions, the accumulation time of charge in section A can vary from not to units. minutes After the accumulation is completed, the clock pulses of the discharge voltage Fsh are supplied to the phase electrodes of the accumulation section; Fan; FSCH, and information in the form of charge packets is moved from the elements of the accumulation section to the storage section B, from which, during the time in the accumulation section

5, the formation of a new information structure is output via the output register BP to the output device of the slave and then to the preamplifier of the signal isolation unit. The output of information from

the storage sections control the flash pulses; Ф2п; Fzp: output information from the output register BP pulses F2p; FZR. The speed of information output from the CCD reaches tens MHz.

The oscillogram of the output signal from the PSD device (Fig. 2a) shows two types of pulses exceeding the background signal level: 11 — a pulse from receiving PSD elements spatially corresponding to the i-th collimators and proportional to the intensity of the radiation attenuated by absorption in the monitored object; lka are the pulses from the receiving cells of the short-wave function, spatially corresponding to the kth collimators and characterizing the intensity of the unmitigated radiation of the source.

Using a threshold discriminator 12, pulses are exceeded that exceed the background level; The waveform of the signals at the output of the threshold discriminator is shown in FIG. 26. From the output of the threshold discriminator, the signals arrive at the upper level selector 13 and the comparison unit 16. The high-level selector selects only amplitude signals that exceed

(imax- d), i.e. corresponding JMSKC

intensity of non-attenuated radiation; The waveform of the signals at the output of the upper level selector is shown in FIG. 2c.

From the output of the upper level selector 13, the signals in parallel arrive at a maximum signal extraction unit 14 (in this case, a peak detector) and a comparison unit 16.

Comparison unit 16 is an electronic mismatch circuit, to the input of which pulses are received both from the threshold discriminator 12 and from the output of the upper level selector 13 (taking into account the discrimination time delay). At the output of the comparator unit 16, in this case, only pulses corresponding to the intensity of the radiation transmitted through the measured sample will be.

The oscillogram of the pulses at the output of the comparison unit is presented in fig. 2d.

After passing the block of ATSP 18, the pulses go to the information processing block 19 and are recorded in 3Vi 21.

To the output of the selector 13 of the upper level, a maximum signal extraction unit is attached — in this case, a peak detector 14, which extracts a signal of maximum amplitude corresponding to the intensity of the un-attenuated radiation.

Using the scaling amplifier 15, the initial adjustment and calibration of the processing system is performed, as well as the signal amplification during the operation, which is necessary due to the presence of losses during discrimination. The information from the output of the amplifier 15 is fed to the ACGI unit 17 and is also recorded in the CPU 20.

The work unit 19 information processing 0 (BOI) occurs according to the following algorithm:

1. Waiting and starting from the control unit,

2. Waiting for the readiness of external devices to release information.

3. Enter information from external devices (ZU1 and ZUA timer BU).

4. Calculation of the program based on the use of the formula. Information output on UNI 24.

5. Return to original condition.

As UNI 24 a digital display is used, the calculation results are output simultaneously with the coordinates of points.

5 controls, i.e., the topography of the control object is provided.

Due to the time sharing operation, a high processing speed is achieved and the thickness control performance is significantly improved.

Claims (1)

The invention The radiometric differential thickness measurement device containing a collimated ionizing radiation source located on one side of the test object and on the other side serially connected radiation detector with a power supply unit, a preamplifier, a signal extraction unit and an information processing unit that differs from that, in order to increase the measurement accuracy, the detector is made coordinate-sensitive, and the collimator is in the form 5 of the multicollimator, with parallel channels, each of the channels is located coaxially with the receiving elements of the coordinate-sensitive detector, the signal extraction unit is designed as 0 of the comparison unit and the analog-to-digital converter (ADC), as well as the successively connected threshold discriminator / input of which is connected to the output of the preamplifier, and the second output to 5 to the first input of the comparison unit, the upper level selector, the second output of which is connected to the second input of the comparison unit, the maximum signal extractor, the scaling amplifier, and the second ADC, the output of which is connected to the first the input of the information processing unit, and the output of the comparison unit through the first ADC is connected to the second input of the information processing unit, the second input and output of the information processing unit are connected to the control unit of the device, the outputs of which are connected to the control inputs of the coordinate-sensitive detector and the unit input power supply, the output of which is connected to the power input of the coordinate-sensitive detector. iW / WrWrV WaWfVfWz at and LL LM LLL t t2 V; Yes Yes 2 BUT g if F1NF2IPZN Fig.Z