JP2608132B2 - Crystal growth monitoring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an apparatus for monitoring a growth process without affecting the growth itself in the growth of a thin film semiconductor crystal by a molecular beam epitaxial growth method, a vapor phase epitaxial growth method, or the like. It is about.

[Prior art] Molecular beam epitaxy (MBE), in which material molecules are supplied on a substrate crystal heated in an ultra-high vacuum vessel and an epitaxial thin film is grown on the substrate, or a crystal to be grown is grown. Chloride containing constituent elements, organic compounds,
Vapor phase epitaxy (VP) in which hydrides and the like are introduced onto a heated substrate crystal and epitaxially grown on the substrate crystal by thermal decomposition or chemical reaction of those compounds
E) Growth is widely used as an epitaxial growth technique for III-V compound semiconductors such as GaAs and Si. Although these methods can control the growth layer thickness at the atomic layer thickness level in principle, in order to measure the growth thickness and feed back the growth rate, the state of the growth layer and the growth layer It is necessary to monitor the thickness. Conventionally, there are two monitoring methods, a reflected electron beam method and an optical measurement method.
Two methods were used.

1) Reflection Electron Beam Method In the molecular beam epitaxy method, a method of measuring a reflection signal of a high energy electron beam (RHEED method) has been conventionally used (JHNeave, BA Joyce, PJDobson, and N. Nor).
ton, Applied Physics. A31, 1 (1983)).

An outline of monitoring by the reflected electron beam method will be described with reference to FIG. A substrate crystal 4 is fixed to a substrate holder 3 provided in an ultrahigh vacuum vessel 1 having an observation window 2. Molecular or atomic beams 5A and 6A of the crystal material to be grown by evaporating the material evaporation sources 5 and 6 arrive on the substrate 4, and the crystal grows. The growing crystal surface is irradiated with an electron beam 8 from an electron gun 7 and receives a reflected electron beam 9 on a fluorescent screen 10.
The diffraction image of the reflected electron beam is observed through the observation window 2.

As shown in the figure, the electron beam is incident on the substrate surface at a shallow angle and is reflected, so that the growth process on the surface can be observed in very detail by the diffraction image and reflection intensity of the reflected electron beam.

FIG. 6 is an example in which the intensity of the reflected electron beam was observed during growth. After the growth starts (t = 0), the intensity starts oscillating,
It has been clarified that one cycle of this oscillation (T ₀ ) corresponds to the growth of one atomic layer, which is an extremely effective method for observing the growth process in detail.

However, this method has the following two major problems. One is that since it uses an electron beam, it can be applied only in an ultra-high vacuum. Therefore, this method cannot be used for a gas phase method other than the molecular beam epitaxy method, a plasma CVD method, or the like. Another is that the reflected electron beam method usually uses high-energy electrons of 10 kV or more, which affects the surface being measured. For this reason, understanding of the actual growth surface state was beyond speculation.

2) Optical measurement method Recently, an optical monitoring method has been developed as a method for solving these problems. The outline is DEAspens, JPHarbi
son, AAStudna, and LTFlorez; J.Vac.Sci.Technol.A6
(3) (1988) 1327. According to this method, the growth process can be carefully observed by measuring the reflected light on the substrate surface of the light introduced from outside the device.

FIG. 7 shows an outline of the measuring apparatus. Light 12 from an external light source 11 (xenon lamp) passes through a reflecting mirror 13, a condensing reflecting mirror 14, and a depolarizer 15 for extinguishing polarized light generated upon reflection by these, and then passes through the observation window 2 and onto the substrate 4. Incident on the crystal growth surface. Light reflected from growth surface
16 is converted into two polarized light beams orthogonal to each other by the rotating polarizer 17, enters the 0.1 m spectroscope 20 via the condensing reflecting mirror 18 and the reflecting mirror 19, is monochromatized and enters the photodetector 21, Output as a signal. The output of the photodetector 21 is
The difference is input to a phase discriminating amplifier 22 synchronized with the rotation of the polarizer 17, and the difference between the intensities of different polarization components of the reflected light 16 is amplified and recorded by the recorder 23. The polarization caused by the reflection at the growing crystal surface changes depending on the growth state of the crystal. Taking GaAs growth as an example, Ga
The polarization is maximized when an atomic plane (or As) is formed, and the polarization decreases when As (or Ga) molecules are deposited on it, and after the minimum value, the As (or Ga) atomic plane is formed Then, the polarization becomes maximum again. Therefore, by taking out orthogonal polarization components and measuring the difference, the growth process on the substrate crystal surface can be directly observed. It was thought that this method would solve all the disadvantages of the backscattered electron beam method.

[Problems to be Solved by the Invention] However, it has been found that the above-described optical measurement method has many problems. The main ones are:
(I) In order to facilitate the measurement, a form is adopted in which the polarization plane is modulated by rotating the analyzer or by another modulation method.
However, due to the birefringence based on the distortion of the observation window 2 and the polarization remaining in the beam splitter, light source and monochromator,
When the polarization plane is modulated, in addition to the polarization derived from the crystal plane, the polarization derived from the observation window or the optical system is mixed, resulting in a large measurement error. To minimize this effect, special attention must be paid to the mounting method of the observation window and the adjustment of the optical system, and additional optical components are required.
(Ii) This complicates the optical system and increases its size. Therefore, even if it is possible in principle, it cannot be directly applied to a growth that must be performed in a limited space and environment, such as a molecular beam epitaxial growth apparatus or a vapor phase epitaxial growth apparatus. It is possible.
(Iii) The configuration of this method requires a precise optical system as described above and also requires that the output of the photodetector be amplified by a phase discriminating amplifier, which makes the system extremely expensive. (Iv) In this method, the difference between the outputs of the phase discriminating amplifiers that appear at different timings is calculated, so that the system is easily affected by external disturbance.

For the above reasons, the above-mentioned optical measurement method is a very good method, but it is practically impossible to directly apply it to a currently operating apparatus.

An object of the present invention is to solve the problems of the conventional method described above, and to provide a monitoring device for a growth process that is stable, simple, and applicable to any growth method.

Means for Solving the Problems The present invention relates to an apparatus for monitoring the growth state of a crystal to be epitaxially grown on a substrate crystal placed in a vacuum vessel. A light source system for irradiating light, a spectral unit for monochromaticizing the reflected light from the crystal surface, a polarizing unit for separating the monochromatic light into two different polarization components, and one and the other of the two polarized lights, respectively. It is characterized by comprising two light detecting means for detecting and outputting an electric signal, and amplifying means for outputting a difference signal between output signals of the two light detecting means.

[Operation] In the present invention, the rotation of the polarizer and the rotation of the polarization plane by the modulator are not performed at all, and the whole system is completely static. In this way, the sum of the birefringence of the observation window and the residual polarization of the optical component is always constant and does not affect the measurement result. For this reason, the optical system can be extremely simplified, and is hardly affected by other external light. Furthermore, since the two polarized waves are separated by the Lochon rhythm and detected by independent photodetectors, and the difference is used as an output, there is no need for a phase discrimination amplifier, and only one simple small DC amplifier is used. Is enough S / N
Is obtained. Due to such features, the measuring system according to the present invention (i) can be mounted on a molecular beam epitaxial growth apparatus, a vapor phase epitaxial growth apparatus, a plasma CVD apparatus, etc., which is very easy to use, because the whole system is reduced in size. It is possible. (Ii) Since only two photodetectors (photodiodes) and a small DC amplifier are necessary in an electrical detection system, the entire system is extremely inexpensive.
(Iii) It is stable because it does not involve modulation of the polarization plane or rotation of the polarizer. In addition, since the two photodetectors (photodiodes) are connected in opposite polarities, external noise light is easily canceled, and the system becomes extremely stable against disturbance of external light.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention applied to molecular beam epitaxial growth. In FIG. 1, reference numeral 24 denotes a light source (in this embodiment, a 20 W halogen lamp is used); 26, a half mirror for separating reflected light from a substrate; 27, a condenser lens (in this embodiment, f = 200 mm). , 50 mm diameter), 28
Is an aperture. The light 25 emitted from the light source 24 is reflected by the half mirror 2
6. The light is guided to the growing crystal surface through the observation window 2 through the condenser lens 27 and the stop 28. The reflected light from the crystal surface is received by the half mirror 29, and by observing the half mirror 29 with the eyepiece 30, it is checked whether the light has reached an appropriate position on the substrate surface, and the optical path is adjusted.

On the other hand, the reflected light 31 from the crystal surface is reflected by the half mirror 26, enters the spectroscope 32 (in this embodiment, a 15 cm grating spectroscope is used), and is converted into monochromatic light. This monochromatic light is guided to the Rochon prism 34 through the condenser lens 33, and is separated into light having two orthogonal polarization planes. The light is guided to PIN photodiodes 35 and 36 having similar characteristics. Photodiodes 35 and 36
Are connected in series with opposite polarities, and the difference output is amplified by the DC amplifier 37 and recorded by the recorder 23. In this way, a change in polarization corresponding to the state of the growing crystal surface can be observed. This system does not rotate or modulate the polarizing prism at all. For this reason, the whole system is static and extremely stable measurement is possible.

FIG. 2 shows the results of observation of the growing GaAs crystal surface by the present apparatus. In the figure, the horizontal axis is the growth time and the vertical axis is the output of the DC amplifier 37, which corresponds to the intensity difference between the orthogonal polarization components. The output of the DC amplifier 37 shows a damped oscillation, and one cycle of this oscillation corresponds to the growth of one atomic layer of the crystal, similar to the cycle of the RHEED intensity shown in FIG.

FIG. 3 shows another embodiment of the present invention. In this embodiment, the bandpass filter 38 (λ _p ; 80 in this case) is used without using the spectroscope.
0 nm Δλ; using 50 nm) to obtain monochromatic light. The other configuration is exactly the same as that of the embodiment shown in FIG. 1, except that a concave lens 39 is used as a condenser lens. With this method, the equipment became extremely small, and the S / N was greatly increased.

FIG. 4 shows still another embodiment of the present invention. This embodiment is an example in which the incident light on the substrate crystal surface is made monochromatic by the band-pass filter 38. The difference from the device shown in FIG. 3 is that the filter is located on the incident light side. Instead of the bandpass filter 38, it is also possible to provide a spectroscope on the incident light side.

Observation results as shown in FIG. 2 can also be obtained by the apparatus shown in FIG. 3 or FIG.

The above embodiment is an example of application to the MBE method, but it is clear that the apparatus of the present invention can be applied to other vapor phase growth apparatuses.

[Effects of the Invention] As described above, in the present invention, since the rotation of the polarizer and the modulation of the polarization plane by the modulator are not performed, the birefringence due to the distortion of the observation window, the spectroscope, the lens, the mirror, and the polarizer Is constant throughout the system and over the entire measurement time. For this reason, all the optical systems introduced in the prior art to eliminate the adverse effect of the residual polarization can be eliminated. More importantly, significant stability improvements are obtained, such as an increase in S / N against external optical noise. Further, since the light irradiation energy on the substrate crystal surface is extremely small, it is effective when applied to a strict study of the surface.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, FIG. 2 is an observation diagram of polarization of a GaAS crystal growth surface by the apparatus shown in FIG. 1, and FIGS. FIG. 5 is a schematic diagram of a conventional crystal growth monitoring device, FIG. 6 is a diagram showing RHEED oscillation observed by the conventional device, and FIG. 7 is a block diagram of a conventional optical crystal growth monitoring device. FIG. DESCRIPTION OF SYMBOLS 1 ... Vacuum container, 2 ... Observation window, 3 ... Substrate holder, 4 ... Substrate crystal, 5, 6 ... Evaporation source, 7 ... Electron gun, 10 ... Fluorescent screen, 11 ... Light source, 15 …… Depolarizer, 17 …… Polarizer, 20 …… Spectroscope, 21 …… Photodetector, 22 …… Phase discrimination amplifier, 23 …… Recorder, 24 …… Light source, 26 …… Half mirror, 28 …… Aperture , 30 ... eyepiece, 32 ... spectroscope, 34 ... Roshon prism, 35,36 ... photodiode, 37 ... DC amplifier, 38 ... bandpass filter.

Claims (3)

(57) [Claims] 1. An apparatus for monitoring a growth state of a crystal to be epitaxially grown on a substrate crystal placed in a vacuum container, wherein a non-deflected light is applied to a growth surface of the crystal from a window provided in the vacuum container. A light source system, a spectroscopic unit for monochromaticizing the reflected light from the crystal surface, a polarizing unit for separating the monochromatic light into two different polarization components, and detecting one and the other of the two polarized lights, respectively. A crystal growth monitoring device, comprising: two light detection means for outputting an electric signal by means of a light source; and amplifying means for outputting a difference signal between output signals of the two light detection means. 2. The crystal growth monitoring device according to claim 1, wherein the non-polarized irradiation light is white light. 3. The crystal growth monitoring device according to claim 1, wherein the non-polarized irradiation light is monochromatic light.