RU173996U1 - DEVICE FOR CONTROL OF PARAMETERS OF SEMICONDUCTOR LAYERS ON A DIELECTRIC SUBSTRATE - Google Patents

Abstract

The utility model relates to the field of measurement technology, namely to non-destructive methods for controlling the structural perfection of epitaxial silicon layers grown on dielectric substrates, and can be used in microelectronics technology to control the quality of epitaxial silicon layers in silicon-on-sapphire structures. The technical result is to increase the reliability of the measurement method in the control of semiconductor layers with a high concentration of charge carriers. In the device for controlling the parameters of semiconductor layers on dielectric substrates, which includes a measuring unit consisting of a metal ring electrode with an aperture, a metal sample holder, the holder being able to move in horizontal and vertical directions, an irradiation source of the sample with IR pulses and a reflected intensity detection device from the surface of the sample induced by the irradiation of the signal, according to a utility model, the measuring unit additionally contains t needle metal electrode passing through a hole in the ring electrode, and the distance from the surface of the sample to the needle electrode is from 100 to 200 microns. 1 ill.

Description

The claimed technical solution relates to the field of measurement technology, namely to non-destructive testing of the electrophysical parameters of the epitaxial layers of semiconductors grown on dielectric substrates, and can be used in microelectronics technology to control the parameters of the epitaxial layers of silicon in "silicon on dielectric" structures and in particular in structures Silicon-on-sapphire (KNS).

The technology of KNS structures is one of the most dynamically developing areas of semiconductor materials science. However, the problem of ensuring high electrophysical and functional parameters of devices based on them, as well as their radiation resistance and reliability, is largely determined by the high defectiveness of the silicon instrument layers. For silicon-on-sapphire structures, this defect is due, in particular, to the difference in the crystallographic structure of silicon and sapphire, as well as to the self-doping of the silicon epitaxial layer with aluminum from the sapphire substrate to concentrations of 10 ¹⁸ -10 ²⁰ cm ^-3 .

Currently, microelectronics technology is almost universally used to evaluate the defectiveness of silicon epitaxial layers in SSC structures using indirect control meds based on the use of test MOS structures (in particular, MOS capacitors and MOS resistors), which is an expensive and not very informative procedure .

In this regard, the solution to the problem of quality control of epitaxial silicon layers in the structures of the SSS and in particular the assessment of the level of defectiveness of silicon layers using non-destructive and operational control is of particular relevance.

This control, if possible, should injure the epitaxial layer as little as possible (that is, the method should be non-destructive) or even non-contact.

A device for controlling the imperfection of epitaxial silicon layers on dielectric substrates using the microwave method for detecting the photoconductivity of a silicon layer [1].

In the device in question there is a measuring chamber with a holder on which the sample is attached - a silicon-on-sapphire (SSS) structure. Microwave radiation with a frequency of 36.4 GHz and a power of at least 50 mW from the generator on the Gunn diode through the waveguide valve and circulator is introduced into the measuring device in which the sample is located. The microwave radiation reflected from the sample through the same circulator is fed to a microwave detector operating in the linear detection mode. The signal allocated by the detector enters the broadband amplifier, which cuts off the constant component of the signal and amplifies only the pulse signal due to non-stationary photoconductivity. According to the picture of the change in the amplitude of the signal and the nature of its temporal shape - the form of the decrease in photoconductivity - assess the quality of the structure of the SSC in comparison with a similar test sample.

The device has the following disadvantages:

- it is very difficult to reliably assess the quality of the silicon layer, since the amplitude and nature of the decay of unsteady photoconductivity depends not only on the defectiveness of the silicon layer, but also on other processes that control the charge change on the surface and the silicon-sapphire interface;

- the device does not even allow you to approximately evaluate the nature of the distribution of defects in the silicon layer.

A device for controlling structural defects in silicon epitaxial layers is also known, in which an object placed in a measuring chamber is irradiated with probing infrared radiation with a wavelength of λ = 1.1–5.0 μm, the intensity of probing infrared radiation transmitted through the object is recorded, and the calibration dependence of the intensity of the probe radiation transmitted through the object on the concentration of structural defects determines the integral estimate of the concentration of structural defects in the object [2].

The magnitude of the flux of infrared radiation passing through an object (silicon wafer) has a correlation with a number of electrophysical parameters that affect the percentage of suitable test elements (transistor assemblies or integrated circuits) made on these wafers, and in particular, with the concentration of structural defects .

Such a correlation is due to the fact that the absorption of infrared radiation with an energy less than the semiconductor band gap (E <E _{> g} = 1.12 eV, which corresponds to a wavelength of λ = 1.1 μm), characterizes the structural-impurity state of the silicon wafer .

The absorption of infrared radiation with a wavelength of λ≥1.1 μm occurs as a result of ionization processes occurring in the bulk of a silicon wafer. The presence of defects and the entire complex of impurities, including those with a low concentration in the structure of the semiconductor, lead to the appearance in the band gap of energy levels with an ionization energy E≈1 / 2 Eg that can absorb infrared radiation with λ≥1.1 μm. In addition, these levels after ionization become centers providing an indirect “zone-zone” transition, which also leads to an increase in the absorption of infrared radiation.

The disadvantages of the device include the impossibility of direct visualization of the distribution of structural defects: the method only allows you to make an integrated assessment of the level of structural defects based on a comparison of the intensity of the infrared radiation transmitted through the object with the output of suitable devices for this area of the measurement object.

In addition, the registration results turn out to be ambiguous, since the yield of suitable test elements is affected not only by the structural perfection of the volume of the object, but also by the quality of the preparation of the plate surface, as well as the parameters of the technology for the formation of test elements.

Closest to the claimed technical solution is a device for monitoring the parameters of semiconductor layers on dielectric substrates, including a measuring unit consisting of a metal ring electrode with a hole, a metal sample holder, and the holder has the ability to move in horizontal and vertical directions, the irradiation source of the sample is IR pulses and devices for detecting the intensity of the signal reflected from the surface of the sample induced by irradiation [3].

This technical solution provides for the registration of the amplitude U _O induced IR pulse signal (photo-electromotive force), the calculation of the relative nullity epitaxial layer N _DEF and comparing the calculated value N _def with the known value N _{DEF (et)} comparing the reference signal is used, the relative defectiveness the semiconductor layer is calculated from the ratio:

U _o / U _{o (min)} = N _{def (et)} / N _def ,

where: U _{o (min)} - the minimum of the recorded values of U _ox .

The essence of the considered technical solution is as follows. When exposed to the structure of the SSC by a pulsed light flux with a wavelength of λ = 380–630 nm from the side of the epitaxial layer, the latter causes excess generation of minority charge carriers (NEC). At the same time, the surface of the epitaxial layer should be clean and not contain a natural oxide layer in order to avoid uncontrolled recombination of NNZ during the control process. During periods of pulsed illumination, the generation and recombination of NSWs on defects of the epitaxial layer occurs until the establishment of the stationarity regime. Under certain conditions, the duration of the exposure pulses and their duty cycle, the amplitude of the induced photo-emf and the photoconductivity decay curve can be measured and recorded.

The specific wavelength of the light flux λ is selected from the condition that radiation penetrates to the entire depth of the epitaxial layer. Since the typical thickness range for silicon epitaxial layers in the SSS structures is from tenths to units of microns, it is quite sufficient to use a source that generates radiation with a wavelength of λ in the range of 380 ÷ 480 nm to control the proposed method.

It was experimentally established that when exposed to the SSC structure by radiation pulses with a wavelength of λ = 380 ÷ 630 nm with a pulse duration of τ ₁ = 50 ÷ 100 μs and a duty cycle of τ ₂ = 250 ÷ 500 μs, it is possible to isolate and measure with sufficient accuracy the amplitude of the induced photo-emf.

When the duration of the exposure pulses τ ₁ <50 μs, the photoconductivity decay curve does not reach the stationary state, and the value of the recorded signal U _o becomes ambiguous.

When the duration of the exposure pulses τ> 100 μs, the reliability of the control sharply decreases, since in this case the process of exponentially decreasing the amplitude U _o due to self-discharge of the capacitance formed by the ring electrode and the epitaxial silicon layer begins. Taking into account such a decrease in the amplitude U _out requires the use of special methods for recording and processing the recorded signal, which leads to a significant hardware complication of the control, an increase in the duration of the control, without leading to an increase in the accuracy and reliability of the control results.

The range of pulse duty cycle values τ ₂ = 250 ÷ 500 μs is selected from the condition of ensuring the complete recombination of low-voltage objects on the defects of the epitaxial layer even at maximum relative defectiveness N _def .

When the duty cycle of pulses τ ₂ <250 μs, the process of recombination of NNS proceeds in a mode that does not correspond to the stationary one, i.e. minority carriers does not have time fully to recombine at defects of the epitaxial layer, and this leads to an accumulative effect at registration amplitude U _O induced photovoltage and distort test results.

With an increase in duty cycle τ _{2 of} more than 500 μs due to an increase in the influence of transients in the capacitance formed by the ring electrode and the epitaxial silicon layer, the shape of the photoconductivity decay curve is distorted, which makes it difficult to record the amplitude of the output signal U _o .

In the stationary mode, the value of the output signal U _o for the SSC structures can be expressed as:

≈φ _O U _{T *} ln (1 + Δn / n _i),

where: ϕ _Т - temperature potential;

n _i is the intrinsic concentration of charge carriers in silicon due to the high concentration of defects at the interface "epitaxial layer of silicon - sapphire";

Δn is the excess concentration of NNC generated by a pulsed light source.

In turn, the excess concentration of charge carriers Δn can be expressed as:

Δn≈G _* τ _eff ,

where: G is the rate of generation of electron-hole pairs;

τ _eff is the effective lifetime of the NSC in the epitaxial silicon layer.

The effective lifetime τ _eff for epitaxial silicon layers in the SSC structures can be expressed in terms of the surface τ _S and volume τ _V time components as follows:

τ _eff = (τ _{S *} τ _V ) / (τ _S + τ _V ).

Since the value of the surface component τ _S of the NSC lifetime is always much larger for the SSS structures than the volume component τ _V , the effective lifetime τ _eff is a value that directly depends on the relative defectiveness N _def in the volume of the silicon epitaxial layer near the silicon-sapphire interface, those.:

_eff τ ≈1 / N _{O DEF} ≈U.

Thus, the claimed technical solution allows you to indirectly assess the quality of the silicon epitaxial layer (the relative concentration of defects in the volume of the epitaxial layer at the silicon-sapphire interface) by the magnitude of the amplitude induced in the epitaxial silicon layer of photo-emf.

The disadvantages of the prototype include the lack of reliability of the measurement results when monitoring semiconductor layers with a high concentration of charge carriers.

This is because when measuring layers with high carrier concentration NCC does not have time fully to recombine at defects of the epitaxial layer, and this leads to an accumulative effect at registration amplitude U _O induced photovoltage and distortion measurement.

The problem to which the claimed technical solution is directed is to increase the reliability of the measurement method in the control of semiconductor layers with a high concentration of charge carriers.

This goal is achieved by the fact that in the device for controlling the parameters of semiconductor layers on dielectric substrates, which includes a measuring unit consisting of a metal ring electrode with a hole, a metal sample holder, the holder being able to move in the horizontal and vertical directions, the source of irradiation of the sample is IR pulses and devices for detecting the intensity of the signal reflected from the surface of the sample induced by the radiation, in order to increase the sensitivity STI technique measuring unit further comprises a metal needle electrode extending through an aperture in the annular electrode, the distance from the sample surface to the needle electrode is 100 to 200 microns.

The minimum distance of the needle electrode from the surface of the sample is due to the exclusion of the possibility of a corona discharge between the sample and the electrode, and the maximum distance is determined experimentally and is limited by the upper boundary of the surface of the ring electrode.

An example of a specific implementation of a utility model

An example of a specific implementation of the proposed technical solution is illustrated in FIG. 1, where:

1 - radiation source (photodiode);

2 - light flux;

3 - ring electrode;

4 - metal ring holder;

5 - epitaxial layer of silicon;

6 - sapphire substrate;

7 - metal base;

8 - needle metal electrode;

h is the distance from the ring electrode to the surface of the epitaxial layer, microns;

U _o - recorded signal, mV.

At the installation, the measuring unit of which is shown in FIG. 1, we measured the surface photo-emf in the structures of the SSS. As a sample, KNS ∅ 100 mm structures were used, the epitaxial silicon layer of which had n-type conductivity and a thickness of 0.3 μm.

The structure was placed on a special metal ring holder 4. The holder itself was located on the metal base surface 7 of the device. Above the epitaxial layer 5, at a distance of h ~ 200 μm from its surface, a ring electrode 3 with an aperture was located to provide illumination of the surface of the epitaxial layer by a pulsed light flux 2 from photodiode 1, which generated radiation with a wavelength of λ = 420 nm.

The measurement process was carried out as follows.

The sample was irradiated with a light flux of 2 pulses with a duration of τ ₁ = 50 μs and a duty cycle of pulses of τ ₂ = 250 ÷ 260 μs, and an electrical potential of ~ 3 kV was applied to the needle electrode 8 in the intervals between τ ₁ and τ ₂ .

Under these conditions, the light flux induced NCC uspevyut completely recombine at defects of the epitaxial layer, and this does not lead to an accumulative effect at registration amplitude U _O induced photovoltage and distortion measurement.

The photoelectric processes occurring in the epitaxial layer 5 upon pulsed irradiation of the sample by means of capacitive coupling were converted into a time-varying electric potential U _o on the ring electrode 3, which was recorded by the electronic circuit, and the recorded amplitude of the output signal U _o surface photo-emf was subjected to subsequent digital processing.

The range of recorded values of U _o at various points of the sample varied from 0.1 to 10.0 mV.

The measurements were performed on the surface of the epitaxial layer of the SSS structure with a locality of up to 2 mm with a locality step of up to 10 mm (excluding the edge zone of 3 mm). At the same time, the measurement time at one point on the surface (including the movement of the ring holder 4 with the sample inside it from point to point) did not exceed 0.2 seconds, and the number of measured points on the surface of the sample was 60.

Thus, the entire process of controlling the structure of the SSC, including chemical processing of the sample before control, installing the sample in a ring holder, controlled movement of the sample, recording and processing the recorded signal on a computer and visualizing it in the form of a topogram of the distribution of relative defectiveness over the surface of the structure, took no more than 5 min

The measurement process was completely non-contact and non-destructive.

The measurements showed good convergence of the measurement results with the data obtained by the method of absolute assessment of the level of defectiveness of the epitaxial silicon layer [4].

Thus, the claimed technical solution provides an increase in the reliability of the measurement method when monitoring semiconductor layers with a high concentration of charge carriers, in particular when monitoring the defectiveness of the silicon epitaxial layer at the silicon-sapphire interface.

Information sources

1. P.A. Bordovsky, A.F. Buldygin, N.I. Petrov, S.N. Rechkunov, V.A. Samoilov.

Quality control of KNS structures by the microwave method. - "Microelectronics", 2008, v. 37, No. 2, p. 101-110.

2. RF patent No.2009573 dated 04/22/1991

3. RF patent No. 2515415 of November 30, 2012 - (prototype).

4. RF patent No. 2436076 dated 04/28/2010.

Claims (1)

A device for monitoring the parameters of semiconductor layers on dielectric substrates, which includes a measuring unit consisting of a metal ring electrode with an aperture, a metal sample holder, the holder being able to move in the horizontal and vertical directions, an irradiation source of the sample with IR pulses and a device for detecting the intensity reflected from the surface of the sample induced by irradiation of the signal, characterized in that in order to increase the reliability of the method from Measurements in the control of semiconductor layers with a high concentration of charge carriers, the measuring unit further comprises a needle metal electrode passing through a hole in the ring electrode, and the distance from the surface of the sample to the needle electrode is from 100 to 200 μm.

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