RU2011973C1 - Method and device for measuring growth of semiconductor films - Google Patents

Abstract

FIELD: semiconductor engineering. SUBSTANCE: generator 9 produces pulses synchronously with rotation of substrate holder 4. These pulse unlock fast-moving electron gun 5 which produces pulses with the period being equal to period of rotation of the substrate. Diffracted electron beam is recorded by detector 6. Filter 10 extracts low-frequency component which carries information of changes in intensity of diffracted electron beam from the signal recorded by detector 6. Rate of growth of semiconductor film is judged from the diffracted electron beam. EFFECT: improved precision. 2 cl, 3 dwg

Description

 The invention relates to a technology for growing thin films and can be used in molecular beam epitaxy (MBE) to control the growth rate of semiconductor films.

 One of the leading trends in the modern stage of the development of physics and technology of semiconductors is an ever-wider study and application of semiconductor film structures with ultrathin constituent layers produced by MBE, for example, heterostructures and superlattices. Further progress in MBE technology is due to the possibilities of reproducible growing homogeneous in properties and thickness of ultrathin semiconductor layers on substrates of maximum area.

 For reproducible growth, operational control of the film growth rate over the entire substrate area should be ensured at the level of ≈1 monolayer / s. The uniformity of the properties and thickness of the grown layers over the area of the substrate provides the MBE process on a rotating substrate.

 There is a method of measuring the growth rate of semiconductor epitaxial films, which consists in irradiating the film growth surface with a high-intensity light beam with quantum energy near the photoemission threshold and measuring the period of oscillations of the photoemission current from the illuminated epitaxy surface [1].

One period of photoemission current oscillations corresponds to the growth of one monolayer of atoms on the surface of a semiconductor. If the duration of one oscillation period is T s, then the growth rate is V _p = 1 / T monolayer / s. This method allows you to measure the growth rate of MBE on a fixed substrate or in the center of a rotating substrate.

 The disadvantages of the described method are the difficulties of measuring the distribution of the growth rate over the surface of a fixed substrate, associated with the need to obtain a highly focused, high-intensity short-wavelength optical beam moving along the surface of the substrate. In the case of a rotating substrate, measurements are made in the center of the substrate due to an interfering signal due to the rotation of the substrate.

 Also, the photostimulated adsorption of impurities from the atmosphere of the residual gases of the vacuum chamber of the MBE installation causes a negative effect on the epitaxy process.

 Closest to the invention is a method for measuring the growth rate of semiconductor films, which consists in irradiating the growth surface with a fast electron beam, detecting a diffracted electron beam reflected from the growth surface, and measuring the oscillation period of the electrical signal obtained by detection [2].

 A device for implementing the known method is a vacuum chamber with molecular sources, shutters and a substrate holder with a substrate, in which a fast electron gun is connected to the control unit, and a reflected electron beam detector (for example, a fluorescence screen and a photodetector), the gun and detector being placed relative to the substrate so that the electron beam falls on the growth surface at a sliding angle and ensures the fulfillment of the Wulf-Bragg conditions, and the reflected diffracted electron beam hits the detector input (the image of one of the diffraction patterns reflexes fast electrons (RHEED) on the fluorescent screen, the photodetector is converted into an electric signal proportional to the intensity of the reflected electron beam). The detector output is connected to the input of the registration unit [2].

 During epitaxy, the intensity of the reflected electron beam and, therefore, the signal at the output of the detector oscillate with a period T equal to the build-up time of one monolayer of semiconductor atoms. The growth rate is determined in the same way as in the first measurement method, using the registration unit (which can be used as a recorder, digital oscilloscope or computing device).

The disadvantages of the known method and device are the limitation of the scope, due to the inability to measure the growth rate on the surface of a rotating substrate (since during rotation of the substrate, different parts of the growing film are exposed to irradiation with a beam of fast electrons), and contamination of the grown film due to uncontrolled introduction of impurities (oxygen and carbon) in the place of exposure from the atmosphere of the residual gases of the vacuum chamber of the MBE installation,
The purpose of the invention is the expansion of the scope of the method by providing measurement of the growth rate on a rotating substrate and improving the quality of the grown structures by reducing the uncontrolled introduction of impurities into the growing layers during the measurement process.

;
F_b = V_{р макс} , где d - ширина отраженного от поверхности роста электронного пучка в плоскости детектирования;
l - расстояние от центра подложки до плоскости детектирования;
V_{р макс} - максимальная скорость роста пленки.This is achieved by the fact that in the method for measuring the growth rate of semiconductor films, which consists in irradiating the film growth surface with a fast electron beam, detecting a diffracted electron beam reflected from the growth surface, measuring the oscillation period of the electric signal obtained by detection and determining the film growth rate from the measured period , irradiation of the film growth surface is performed in a pulsed fashion with a period equal to the period of rotation of the substrate, at times corresponding to s rapid penetration of the electron beam on the surface of the rotating portion controlled growth substrate, and the electrical signal is limited by the width of the frequency spectrum band ΔF = 0 - F _b, wherein irradiating the pulse duration t _u and the cutoff frequency F _b is selected from the conditions:
t _u ≅

;
F _b = V _{p max} , where d is the width of the reflected electron beam reflected from the growth surface in the detection plane;
l is the distance from the center of the substrate to the detection plane;
V _{p max} - maximum film growth rate.

 A device for implementing the method, comprising a vacuum chamber with molecular sources located in it, provided with gates, a substrate holder with a substrate, a fast electron gun connected to the control unit, and a reflected diffracted electron beam detector connected to the registration unit, equipped with a pulse generator, the input of which is connected to the synchronizer connected to the substrate holder, and the output to the fast electron gun, and a low-pass filter, the input connected to the detector output, and you connected to the input of the registration unit.

In FIG. 1 shows a functional diagram of the proposed device; in FIG. 2 - time dependence of the intensity I _{e of the} reflected diffracted beam; in FIG. 3 - change in time of the voltage of the signal U _with after filtering.

 The method is as follows.

The growth surface at a given location of the rotating substrate is irradiated by a beam of fast electrons at discrete time instants corresponding to the fulfillment of the Wulff-Bragg conditions. The time dependence of the intensity I _{e of the} reflected diffracted electron beam and the corresponding electric signal after detection in this case is a sequence of pulses whose low-frequency component of the spectrum carries information about the growth rate similarly to the case of continuous irradiation of the growth surface.

The resulting electrical signal in discrete form is limited by the width of the frequency spectrum, as a result of which it is converted to a continuous shape and is similar to the electrical signal U _c with continuous irradiation of the growth surface.

, где Т - период колебаний континуального электрического сигнала, измеренный после фильтрации.The growth rate is determined by the formula:
V _p =

where T is the oscillation period of the continuum electric signal, measured after filtering.

≥ 2V_{p макс}

(1)
Величина V_{р макс} имеет физические ограничения, вытекающие из принципа МЛЭ, и определяется особенностями конструкции конкретной установки МЛЭ - диаметром подложки и расстоянием ее от молекулярных источников. Величина V_{р. макс} обычно не более ~ 1

.The frequency of the irradiation pulses is determined on the basis of Kotelnikov's theorem (4), which determines the condition for the exact restoration of the continuum signal represented by its reference at discrete time instants. In particular, for accurate restoration of a sinusoidal signal with a period T, samples should be taken at least 2 times per oscillation period. Therefore, at the maximum speed V _{p max, the} sampling frequency and, therefore, the frequency f of the irradiation pulses must satisfy the condition
f

≥ 2V _{p max}

(1)
The value of V _{p max} has physical limitations arising from the MBE principle, and is determined by the design features of a particular MBE installation — the diameter of the substrate and its distance from molecular sources. The value of V _{p. max} usually not more than ~ 1

.

Moreover, during rotation of the substrate for any point of the substrate, the diffraction conditions can be satisfied at least once per revolution of the substrate, therefore, the rotation frequency of the substrate f _BP should be equal to the pulse repetition rate, i.e. f _BP. = f.

или t_u≅

. (2) Нижний предел длительности импульсов облучения t_u ограничивается чувствительностью и быстродействием детектора электронов.The duration t _u pulse irradiation is set the time at which during rotation of the substrate the reflected diffracted beam intersects the receiving area electron detector, and which is determined from the corner of the substrate rotational velocity ω = 2nf _vr, the distance l from the center of the substrate to the detector receiving pad, the width d of the reflected electron beam in the detection plane (the width of the detector receiving area in the azimuthal plane is assumed to be equal to the width of the reflected electron beam), in other words, the duration t _{u of} pulses it is set as the time during which, when the substrate rotates with an angular velocity ω = 2π f _{bp, the} end of the reflected electron beam at the detector receiving area travels along an arc of radius l equal to twice the width of the reflected electron beam (double the width of the reflection in the RHEED pattern); t _u =

or t _u ≅

. (2) The lower limit of the duration of the irradiation pulses t _{u is} limited by the sensitivity and speed of the electron detector.

 A device for implementing the method (Fig. 1) comprises a vacuum chamber 1 with molecular sources 2 placed in it, shutters 3, a substrate holder 4 with a substrate, a fast electron gun 5 and a reflected diffracted beam detector 6. The control unit 7 is connected to the fast electron gun, the synchronizer input 8 is connected to the substrate holder, the output of which is connected to the input of the pulse generator 9, the output of which is connected to the control electrode of the gun 5. The low-pass filter 10 and the registration unit 11 are connected to the output of the detector 6.

 The device operates as follows. After pregrowth preparation of the substrate, the rotation of the substrate holder 4 is turned on and the shutter 3 is opened to start growth. The synchronizer 8, coupled to the substrate holder 4 (optically or mechanically, etc.) at times corresponding to the arrival of the fast electron beam on the controlled portion of the growth surface, once per revolution of the substrate generates synchronization pulses, which are fed to the input of the 9 pulse generator. The latter simultaneously with the rotation of the substrate holder 4 generates pulses unlocking the gun 5, which at discrete moments in a selected location of the substrate irradiates the growth surface with a beam of fast electrons.

 The reflected diffracted electron beam (corresponding to the reflection of the RHEED pattern on the fluorescent screen) at the moment of the action of the unlocking pulse enters the input of detector 6. From the output of which a discrete electrical signal is fed to the input of a low-pass filter 10, which converts the signal from discrete to continuum. The signal from the output of the low-pass filter is fed to the input of the registration unit, with the help of which the growth rate is measured over the period of oscillations of the continuous signal.

, где V_{р макс} - максимальная скорость роста в процессе эпитаксии.The low-pass filter 10 serves to isolate a low-frequency informative component from a discrete electrical signal. In accordance with the Kotelnikov theorem (4), its cutoff frequency is selected from the condition:
F _{in (u)} = V _{p max}

where V _{p max} is the maximum growth rate during epitaxy.

,
l = 250 мм.Setting the MBE installation parameters:
V _{p max} = 1

,
l = 250 mm.

 d = 1 mm.

=

≈ 0,4·10^-3 (3)
Частота
среза фильтра
нижних частот F_b = Vр_.макс. = 1 (Гц)
Минимальное
время роста од-
ного монослоя T_мин=

= 1 (c)
Время действия импульса облучения занимает лишь малую часть времени наращивания одного монослоя
δ ≅

=

= 8·10^-4 (4)
Поэтому влияние внедрения примесей из атмосферы остаточных газов вакуумной камеры МЛЭ на качество выращиваемого полупроводника в месте облучения уменьшается.Find:
Frequency
impulses
irradiation f ≥2V _r.max
Choose f = 3 (s ^-1 )
Number
revolutions
subcoder
resident f _bp = f = 3 (s ^-1 )
Duration
impulses
irradiation t _u =

=

≈ 0.4 · 10 ^-3 (3)
Frequency
filter cutoff
low frequencies F _b = _{Vр. max.} = 1 (Hz)
Minimum
growth time one
monolayer T _min =

= 1 (c)
The exposure time of the irradiation pulse takes only a small part of the build time of one monolayer
δ ≅

=

= 8 · 10 ^-4 (4)
Therefore, the influence of the introduction of impurities from the atmosphere of the residual gases of the MBE vacuum chamber on the quality of the grown semiconductor at the irradiation site decreases.

 Thus, the proposed method and device for measuring the growth rate of epitaxial films allows, due to pulsed irradiation of the growth surface, to ensure the measurement of the growth rate on a rotating substrate and to reduce the uncontrolled introduction of impurities into the growing layers during the measurement process, which ultimately improves the quality of the grown thin-film epitaxial structures .

Claims (2)

1. A method for determining the growth rate of semiconductor films during their growth on a substrate, which consists in irradiating the film growth surface with a fast electron beam, detecting a diffracted electron beam reflected from the growth surface, measuring the period of the electric signal at the output of the detecting device, and determining the growth rate from the measured period film, characterized in that, in order to ensure measurement of the film growth rate on a rotating substrate and reduce uncontrolled the introduction of impurities into the growing layers during the measurement process, the growth surface is irradiated pulsed with a period equal to the period of rotation of the substrate, the electrical signal at the detector output is filtered, limiting it by the width of the frequency spectrum to the range ΔF = 0 - F _c , and the duration of the irradiation pulses t _and and the cutoff frequency is selected from the conditions
t _and ≅

; F _in = v

,,
where d is the width of the electron beam reflected from the growth surface in the detection plane;
l is the distance from the center of the substrate to the detection plane;
v _pmax is the maximum film growth rate.  2. A device for determining the growth rate of semiconductor films during their growth on a substrate containing a vacuum chamber in which molecular sources are provided with gates, a substrate holder, a fast electron gun connected to the control unit, and a reflected diffracted electron beam detector connected to the unit registration, characterized in that it is equipped with a synchronizer associated with the substrate holder, a pulse generator, the input of which is connected to the synchronizer, and the output to eye of fast electrons, and a low pass filter having an input connected to the output of the detector, and an output coupled to the recording unit.

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