SU1737261A1 - Method of contact-free determining of thickness of epitaxial semiconductor layers - Google Patents

Abstract

The invention relates to a control and measuring technique. The purpose of the invention is to expand the field of application by making it possible to measure the thickness of epitaxial layers that coincide in optical characteristics with the substrate. The goal is achieved by passing a probe radiation with a photon energy smaller than the width of the forbidden zone of the epitaxial layer under study, simultaneously illuminating it with an optical injector with a quantum energy greater than the width of the forbidden zone of the test layer, measuring the modulation M of the probe radiation and the thickness of the epitaxial film is determined by the spectral dependence of the magnitude of the modulation on the wavelength of the injector, since, due to the spectral dependence Light absorption coefficient of the wavelength of the radiation injector having different wavelengths generate the radiation penetrates the sample to a different thickness. 5 il. (L vj cj xi yo

Description

The invention relates to a measuring and measuring technique and can be used in the measurement and investigation of semiconductor structures representing an epitaxial layer on a semiconductor or dielectric substrate and differing from it by electrophysical parameters, as well as in the manufacture of devices based on such structures, in particular for measuring thickness epitaxial layers, when making contacts to the sample is impossible.

Methods are known for contactlessly determining the thickness of epitaxial layers deposited on a substrate.

However, according to these methods, X-rays are used to measure thickness, which has a detrimental effect on personnel. In addition, X-ray equipment is expensive.

There is also known a method for measuring the thickness of a semiconductor layer, which consists in placing an object close between a metal probe and a metal base, irradiating it and measuring the current through the pn-junction.

The disadvantage of the method is that the sample contacts the probe and the base.

Closest to the proposed interference method of contactless measurement of the thickness of semiconductor epitaxial layers, according to which the monochromatic radiation with a quantum energy less than the band gap of the forbidden semiconductor zone is directed to the sample epitaxial structure on the semiconductor substrate under investigation, the sampling radiation with a quantum energy less than the band gap of the semiconductor is sent to the sample structure. register the intensity of this radiation after it interacts with the measured structure when the length changes in lny (photon energy) of the probing radiation and a spectral distance between the probe radiation ex- extrema determined thickness of the epitaxial layer. The order P of the observed interference extremum at the wavelength I is equal to

(nri2-sinV) 1 / 2U + -G,) 2Jr,

where W is the desired thickness of the epitaxial layer;

Pg1 is the refractive index of the film; p is a dip angle;

I is the wavelength of the incident light;

Q2 - change in the phase of the beam upon reflection at the boundary with the substrate.

For two values of the wavelengths AI and R2 (A- | Yaa), the difference of the interference order. The ratios written for extrema at wavelengths AI and H2, based on the formulas given, allow us to obtain a system of three equations with three unknowns W, Pi, and P2, the solution of which gives an expression for the thickness of the epitaxial layer

W ()

I

2 (pg1 -sin p}

1/2

or

W - m + 021 / (2lg) -022 / (2n :)

2 (n i-sin2p) 1/2 (t / A2-1 / ai)

where Q21 and 0.22 are the values of Q2 for the Yati J2 wavelengths.

five

10 15 20 25

thirty

35

40 5

0

five

When implementing this method, you can use a tunable wavelength source of probe radiation with a quantum energy smaller than the band gap of the semiconductor forming the epitaxial layer, the sample holder and the receiver detecting this radiation after it interacts with the measured structure (i.e., after passing through the structure or reflection from it).

However, this method allows one to measure the thickness of the epitaxial layer only if its refractive index nri differs from the refractive index of the substrate G2 and the interference of the probing radiation occurs in the epitaxial layer. At the same time, if the structure is homoepitaxial, then the epitaxial layer differing in electrophysical parameters from the substrate may have the same optical characteristics as the substrate. In this case, interference in the film is not observed and the method is not applicable.

The purpose of the invention is to expand the field of application by making it possible to determine the thickness of epitaxial layers that coincide in optical characteristics with the substrate (). This occurs, in particular, in homoepitaxial structures.

According to the proposed method, probing radiation with a photon energy smaller than the band gap of the semiconductor forming the epitaxial layer is directed onto the structure under study, and the intensity of this radiation is recorded after it passes through the structure under study or reflects from it. In addition, the structure of monochromatic radiation with a photon energy greater than the band gap of the semiconductor forming the epitaxial layer is additionally illuminated. This radiation generates non-equilibrium charge carriers in the structure, which leads to a change in the intensity of the probe radiation transmitted through the structure and reflected by it. The magnitude of these changes varies depending on whether non-equilibrium charge carriers are generated — in the epitaxial layer or in the substrate, due to the difference in their electrical properties. The region of generation of nonequilibrium carriers depends on the depth of penetration of the injecting radiation into the structure, i.e., on its wavelength. The spectral dependence of the change in the intensity of the probe radiation transmitted through the structure or reflected by it from the wavelength of the additional

radiation, fix the wavelength I p, which determines the spectral position of the transition of the magnitude of the intensity change from the value characteristic of the substrate to the value characteristic of the epitaxial layer, and determine the thickness of the epitaxial layer W by a predetermined calibration curve.

FIG. 1 shows the epitaxial structure; in fig. 2 - dependence of the electrophysical parameter — the lifetime of charge carriers in the epitaxial structure — on the x coordinate, directed perpendicular to the surface of the structure; in fig. 3 - spectral dependence of the absorption coefficient I of monochromatic radiation with a photon energy greater than the width of the forbidden band for one of the semiconductors (silicon); in fig. 4 shows the dependence of the modulation value, i.e., the relative change in the intensity of the probing radiation M} transmitted through the epitaxial structure, on the wavelength An of the optical injector; in fig. 5 shows the calibration curve for the dependence of the spectral absorption I n of the transition region of the magnitude of the change in the intensity of the probing radiation from the value characteristic of the substrate to the value characteristic of the epitaxial layer on the thickness of the epitaxial layer W.

The method is implemented as follows.

The probing radiation of a source with a quantum energy smaller than the width of the forbidden band is directed to the sample under study and after passing the sample (or reflections from it) is recorded by a photo-receiver.

The parameters of the probing radiation (e.g., intensity) after its interaction with the semiconductor depend on the magnitude of the absorption and refractive indices. Upon additional irradiation of a semiconductor by monochromatic radiation of an optical injector with a quantum energy greater than the band gap of the semiconductor, it is absorbed in it and generates non-equilibrium charge carriers - electrons and holes, which affect the refractive and absorption indices of the semiconductor. This leads to a change in the parameters of the probe radiation interacting with the semiconductor, in particular, to a change in the intensity of the probe flux. This change can be characterized by the modulation value, the magnitude of which depends on the electrophysical parameter of the semiconductor - the lifetime of charge carriers in it

 . L. t, (1)

where k is a combination of world constants, parameters of the semiconductor under investigation and a measurement setup;

ARP

- number absorbed in

the semiconductor layer of the short-wavelength photons of the injector, which is equal to the ratio of the photons paved in the power layer to the photon energy W;

t is the lifetime of the carriers in this semiconductor layer. For a structure consisting of an epitaxial layer and a substrate, the expression (1) takes the form

 h G 1 + K2 - 1 g T 2. (2)

Here, index 1 refers to epitaxial

layer, and index 2 to the substrate. For homoepitaxial structures.

The epitaxial layer of the structure and its substrate differ in their electrophysical properties, in particular, the lifetimes of the carriers are charged. Therefore, the number of photons of the optical injector 4 is absorbed in the epitaxial layer (Li) and which in the substrate () depends on the amount of modulation recorded by the photo-receiver 3 M. The depth of penetration into the monochromatic radiation structure of the optical injector 4 depends on its wavelength (I and). This leads to a spectral dependence of the magnitude of M on the wavelength Yap of the optical injector 4. The form of this dependence is shown in FIG. 4 for different thicknesses of the epitaxial layer W. With a small depth of penetration of the injector radiation into the structure (i.e. with small I c), the value

m is determined by the value of t h h in the epitaxial layer, and with a large penetration depth (i.e. with large Yi), by the value of g of the g in the substrate of the structure Depending on the wavelength of the optical injector, the value of M is measured from the value Mi, substrate, to the value of Ma characteristic of the epitaxial layer. The spectral position of the jn transition region depends on the thickness

epitaxial layer W.

The spectral position of the transition region is fixed by some characteristic point of the dependence M (I and), for example the point In, where the value M (Mi + M2) / 2, as shown in FIG. 4. Then, the thickness of the epitaxial layer W is determined by a pre-constructed calibration curve (Fig. 5). The graduation curve is constructed experimentally from measurements of samples with a known thickness of the epitaxial layer or is calculated theoretically.

Example. The epitaxial layer of silicon with a lifetime of μs is grown on a silicon substrate with a lifetime of 0 μs. The wavelength I of the probe radiation is 10.6 microns. The dependence of the modulation of the probe radiation M on the wavelength of the injector An is removed. The values of Mi and M2 are determined from it. Fix the value of the nd for which M (Mi + M2) / 2. For example, 67 microns. The value of 9 µm is determined from the calibration curve (Fig. 5).

Claims (2)

The invention The method of contactless determination of the thickness of epitaxial semiconductor layers on substrates, consisting in the direction of the probing radiation structure under study with a photon energy smaller than the band gap of the semiconductor forming The epitaxial layer, and recording the intensity of this radiation after it has passed the structure under study or reflection from it, characterized in that, in order to expand the field of application by making it possible to determine the thickness of the epitaxial layers coinciding in optical characteristics with the substrate is additionally irradiated by the investigated epitaxial layer with radiation with a photon energy of greater width the band gap of the semiconductor forming this layer removes the spectral dependence of the change in the intensity of the probe radiation transmitted through the structure or reflected by it from the length additional radiation waves with a photon energy greater than the band gap forming the epitaxial semiconductor layer fix the spectral position of the transition region of the change in the intensity of the probe radiation from the value characteristic of the substrate to the value characteristic of the epitaxial layer and use the obtained value when determining the layer thickness. W; FIG. i LМКС about yu tf /ABOUT ( / 0f-5 2.O 2.5 30 FIG. five FIG. 2 hl) iB M Oh h, 6 Oh, & /, Oh / lm, fiff. Jl / um & og ° Q6 m, fiff.t FIG. C