JPH02170008A - Measuring method for film thickness of semiconductor multilayered thin film of heterojunction thin film multilayered structure - Google Patents

Abstract

PURPOSE:To easily measure the film thickness of heterojunction of any combination of materials without breaking a sample by irradiating an epitaxial thin film slantingly with laser light, detecting reflected light from each junction surface, and finding the film thickness of a thin film from the interval of the reflected light. CONSTITUTION:A sample wafer 2 which has an epitaxial grown thin film is mounted on an XYZ stage 5 and a thin light beam is made incident slantingly on the surface of the sample wafer 2 from a laser light oscillator 6. The reflected light of the incident light 1 from the wafer 2 is plural reflected light beams reflected by the top layer of the sample 2 and the heterojunction surfaces of respective thin films and the respective reflected light beams 3 are guided to a photodetector 4, whose output is led to a computer 7 and converted into a digital value, which is analyzed to find the thickness of each thin film. Thus, the sample is not broken and the reflection is caused on each junction surface, so the thickness values of the plural thin films are found at a time and the time required for the measurement is shortened.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field The present invention relates to a method for measuring the thickness of a semiconductor heterojunction.

Heterojunctions are often used in semiconductor devices such as light emitting diodes and semiconductor lasers.

This may be a combination of GaAs and A,dGaAs layers. Or InP layer and InGaPA
It may also be a bonded layer with an s layer.

Furthermore, a GaAs layer may be grown on a Si substrate or a Si thin film.

It is called a heterojunction because the elements that make up the crystal are different.

In the case of a pn junction, since the host element is the same, the chemical and optical properties are almost the same on both sides of the junction.

However, in the case of a heterojunction, the composition of the host elements is different, so the chemical and optical properties change.

Heterojunctions can be made using liquid phase epitaxy, gas phase epitaxy,
It is made using epitaxy techniques such as molecular beam epitaxy and organometallic pyrolysis. The thickness of the film can be controlled primarily by the length of the growth time.

There are already several methods for measuring the thickness of thin films separated by heterojunctions after growth.

(b) Conventional technology Examples of conventional techniques include the following.

For example, when measuring the thickness of the GaAs layer of a GaAs/AlGaAs semiconductor hetero thin film epitaxial wafer,
A resist pattern is formed on the surface of the wafer, and the etching speed is extremely different for GaAs, A, and dGaAs.
Dry etching (RIE) using a reactive gas such as Cl2F2. The portions not covered by the resist are etched, but because the etching speeds for the two layers are different, the etching occurs in a stepwise manner. By measuring the height of the stairs, the thickness of the GaAs layer can be determined.

This will be tentatively called the selective etching method here.

Alternatively, there is a method in which a part of the wafer is folded, a cross section of the thin film layer is taken out, and the cross section is observed using a scanning electron microscope (SEM). Since this is done through observation, it is possible to clearly see not only the thickness but also the unevenness and distribution of the thin film.

Furthermore, a part of the wafer is shaved at a certain oblique angle to expose the boundaries of the thin film, and the horizontal length between the boundaries is determined using a microscope. The thickness is obtained by multiplying this by the tangent tan θ of the oblique angle θ.

(1) Problems to be Solved by the Invention These methods can measure the thickness of an epitaxial thin film in a heterojunction, but they all have the disadvantage of being destructive tests. Sometimes the whole Ueno is destroyed, sometimes only a part of it is destroyed. At least the part where the film thickness is to be measured is destroyed. This makes it impossible to directly measure what should become a product.

The selective etching method can only measure the thickness of a thin film on the surface. The thickness of the second and third thin films cannot be determined with this method. Furthermore, there are not always reactive gases that etch significantly different rates for the materials on either side of the heterojunction. Can only be used for special heterojunctions.

Furthermore, these methods are not easy to perform. This is because pretreatment such as etching and interfolding of the sample is required, and furthermore, microscopic observation is required.

It is not possible to measure the thickness of many wafers at many measurement points in a short period of time.

Purpose It is an object of the present invention to provide a simple method for measuring the thickness of a hetero thin film that can be applied to any combination of materials for heterojunctions without destroying the sample.

(k) Structure The present invention applies a laser beam to an epitaxial thin film from an oblique direction, detects reflected light from each bonding surface, and determines the film thickness of the thin film from the interval between the reflected lights.

Since the film thickness is determined using reflected light, the sample is not destroyed. This is a non-destructive test. Furthermore, since reflection occurs at each bonding surface, the thicknesses of multiple thin films can be determined at once. Furthermore, the time required for measurement is short, making it an extremely economical method.

FIG. 1 is a schematic diagram of the measuring device of the present invention.

A sample wafer 2 having an epitaxially grown film is placed on the XYZ stage 5.

A laser oscillator 6 is provided so as to make an extremely narrow beam of light obliquely incident on the surface of the sample wafer 2.

Incident light 1 is reflected obliquely by sample wafer 2 .

A photodetector 4 is provided to receive this light.

The computer 7 receives the output from the photodetector 4, converts it into a digital value, and analyzes it to determine the thickness of the thin film.

The results are displayed on the display 8 and output to the printer 9.

The computer 7 can move the xyz stage 5 in the xyz directions and measure the film thickness at different points on the film.

The position of the laser oscillator 6 may be fixed, or may be changed according to instructions from the computer 7.

(2) Production The measurement principle will be explained with reference to FIG.

Incident light 10 impinges on the surface of the sample wafer. It is assumed that the epiaxial thin films are labeled LII and III from the top, and have thicknesses of dl, d2, and d3, and refractive indices of nl, n2, and n3, respectively.

The incident light AO is first reflected at a point Po on the surface p of the layer (2) and becomes reflected light 11.

Let the angle between the first layer and the incident light 10 be 0. Reflected light 11 and 1
The angle formed by the layers is also O.

The light that enters one layer is refracted and reaches 91 points on the interface q between the layers 1 and 2. Some light is reflected at 91 points and exits at 21 points on the surface of one layer. This is called reflected light 12.

The light that further enters the ■ layer at the 91st point is the boundary surface r between the ■ and ■ layers.
reaches the R1 point. Here, some light is reflected. This is refracted at the boundary surface q (point Q2) and exits from point 22 on the surface r. This is called reflected light 13.

The light refracted at point R1 is reflected at eight points on the boundary surface S between the (1) layer and the layer or substrate below it. The reflected light is at point R2,
Exit through point 93 and point R3. This is called reflected light 14.

The reflected lights 11, 42, 13, and 14 are parallel lights. The distance between them is extremely narrow. However, if the incident light 10 is sufficiently narrow, these reflected lights can be distinguished.

If the photodetector 4 is a large number of independent light-receiving elements arranged one-dimensionally, the intervals W1, W2, and W3 between these reflected lights can be determined. As it stands, one unit of the subdivided light receiving element must be smaller than the thickness of the thin film.

Since this is difficult in practice, it is best to use a concave (actually convex) lens system to expand the distance between the parallel beams and then make them incident on the -dimensional light receiving element.

The interval between the parallel reflected lights is determined by the thickness and refractive index of the film on which the light is reflected, and the incident angle θ.

Since the refractive index is known, the film thickness can be determined from the interval of reflected light.

For example, the interval W1 between 11 and 12 is determined as follows.

Let the refraction angle in one layer be Φ1.

The formula cose = nI CO3Φ1 OPI 2 d cotΦ□ P(IP1s1nθ holds true. Here, the angle of incidence and the angle of refraction are different from the usual definitions, and are defined as the angle between the ray and the surface. From these formulas, Since nl, θ, and Wl are known, dl can be found.
That's what it means. Similarly, the interval W2 between 12 and 13 is as follows. W3 is. The boundary between the i-th layer and the (i-1) layer, and (
The distance Wi of the light reflected at the boundary between the i+1 )th layer and the ith layer is generally given by.

The distances W1, W2, . . . between the reflected lights are obtained by the photodetector 4. The refractive index nl, n2, . . . of each layer is known in advance. Since it is a Peter junction, the oil tangibility is different and the refractive index of each layer is known.

O is a preset angle of incidence and is known. From these relationships, the film thicknesses d1, d2, . . . can be determined.

This method must distinguish only the light that is reflected once at the boundary from the light that is reflected multiple times. Since this is possible, it is better to make the reflected light smaller than the refracted light.

In order to reduce the ratio of reflected light, it is better to bring θ closer to 90° and closer to normal incidence. However, this reduces the distance between the light beams, making measurement difficult.

In order to widen the interval W, it is better to bring θ closer to 00. However, this will make the reflected light stronger. In particular, reflection on the surface increases. Furthermore, the intensity of the multiple reflected light may be about the same as that of the single reflected light from the deep layer.

Taking these things into consideration, the incident angle θ is appropriately determined.

(→Example: By molecular beam epitaxial growth method (MBE method), G
0.3ttm thickness +7) A7Ga on aAs substrate
An As film and a 1.0 μm thick GaAs film were grown. 10 μm is the designed thickness.

Then, the thickness of the GaAs film was changed using the method of the present invention and CC1.
It was measured by reactive ion etching and microscopy using 12F2. Five measurement points were taken on the diameter of the wafer. The results are shown in FIG.

The black circles are the measurement results according to the present invention, and the ridges are the results obtained by etching and microscopy.

The same wafer was measured using the present invention and then the same diameter was measured using etching microscopy.

The film thickness is about 1 μm, but it is close to 1.1 μm at the center and about 1.05 μm at the periphery. No matter which method is used, the results of film thickness measurement are almost the same. Since the results of both methods match, it can be seen that the method of the present invention is effective.

(i) Effects The present invention has the following advantages as a method for measuring the thickness of a semiconductor thin film crystal having a petrojunction made by epitaxial growth.

(1) It is a non-destructive inspection. There is no need to etch or crease the semiconductor wafer. It can be used to inspect wafers that will become products.

(2) It is possible to measure not only the thickness of the surface layer but also the second layer, third layer, etc.

(3) Unlike selective etching, it only needs to be a thin crystal film that allows light to pass through. There are few restrictions on target crystals.

(4) Easy to measure. It is convenient because it is measured by optical means and there is no need to operate moving parts.

(5) It is effective when applied to evaluation of structures of high electron mobility transistor (HEMT) wafers, thin film multilayer epitaxial wafers, etc.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the method for measuring the thickness of a hetero thin film multilayer structure according to the present invention. FIG. 2 is a ray diagram showing how light is reflected at the boundary surface of a thin film. Figure 3 shows AlGaAs and GaAs on a GaAs substrate.
(7) A graph showing the results of measuring the thickness of a GaAs thin film using the method of the present invention and the conventional etching microscopy method for a fully grown thin film. 1...Incoming light 2...Sample wafer 3...Reflected light 4...Photodetector 5...xyz stage 6... ... Laser oscillator 7 ... Computer 8 ... Display 9 ... Printer

Claims (1)

[Claims] A thin beam of light is applied in an oblique direction to a sample wafer 2, which has one or more single crystal thin films epitaxially grown on a semiconductor substrate and has one or more heterojunctions, and is reflected from the top layer of the sample and the heterojunction surface of each thin film. A semiconductor multilayer thin film having a hetero thin film multilayer structure, characterized in that a plurality of reflected lights are detected by a photodetector 4 to determine the interval between the reflected lights, and the film thickness of each thin film is determined from the refractive index of the thin film and the value of the interval. Film thickness measurement method.