Crystal growing apparatus

Abstract

A crystal growing apparatus comprises a Knudsen cell source of the surface ionization type, focusing means to focus, on a crystal substrate, an ion beam emitted from the ion source, deflection means to deflect the ion beam, detection means to detect the quantity of the ion beam, an electron gun to generate an electron beam which neutralizes ions focused on the crystal substrate, and a quadrupole mass filter to evaluate the ratio of partial pressure of the neutralized molecules within a vacuum specimen chamber in which the crystal substrate exists, the ion beam being scanned on the substrate by the deflection means.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a crystal growing apparatus, and more particularly to an apparatus which can control crystal growth two-dimensionally and precisely by scanning the surface of a crystal with an ion beam.

DESCRIPTION OF THE PRIOR ART

Prior-art crystal growing apparatus conduct crystal growth by the use of a molecular beam. However, where the surface of a crystal substrate to be grown is irradiated during crystal growth by the molecular beam, the following disadvantages are involved. The quantity of molecules to be supplied to the crystal is controlled only by the control of the temperature of the crystal substrate. Moreover, growth cannot be so controlled as to form a two-dimensional molecular concentration distribution in the crystal.

SUMMARY OF THE INVENTION

An object of the present invention is to provide crystal growing apparatus which can provide a two-dimensional concentration distribution in a crystal.

Another object of the present invention is to provide a crystal growing apparatus which can perform a precise crystal growth.

In order to accomplish such objects, the present invention employs an ion beam, and deflects it to scan the surface of a crystal substrate and to control the quantity of the ion beam itself, thereby forming a two-dimensional concentration distribution in the crystal to grow it.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the construction of a prior-art crystal growing apparatus;

FIGS. 2a and 2b are schematic views for explaining the subject matter of the present invention, respectively;

FIG. 3 is a schematic view showing the construction of an embodiment of crystal growing apparatus according to the present invention; and

FIG. 4 is a block diagram showing a control unit suitable for the crystal growing appratus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the accompanying drawings, the present invention will be described hereunder in comparison with an example of prior art.

FIG. 1 is a view shows prior-art crystal growing apparatus using neutral molecular beams. In the figure, numeral 1 designates a vacuum chamber in which crystal growth is carried out, 2 Knudsen cells (in the illustrated case, two cells) used as a molecular beam source, 3 a crystal substrate to be grown, 4 mass analyzing means (for example, a quadrupole mass filter) for evaluating the ratio of partial pressures within the vacuum chamber 1, and 5 an electron gun. Neutral molecular beams emitted from the Knudsen cells 2 are caused to fall on the crystal substrate 3, to grow a single crystal on the substrate 3. It is a common practice in this case that, using an electron beam e generated from the electron gun 5, electron-diffraction patterns of the growing crystal are sequentially observed on a fluorescent screen 6. The control of the crystal growth is effected by a temperature control for the substrate 3. The ratio of partial pressures of the molecules within the vacuum chamber 1 is therefore evaluated by the mass analyzing means 4. In order that the ratio of partial pressure and the crystal growth may hold a predetermined relation, the temperature of the substrate 3 is suitably controlled in response to the magnitude of the ratio of partial pressure. Heating means is omitted from the illustration.

In accordance with such prior-art apparatus, although the crystal growth depends severely on the temperature of the substrate, the quantity of the molecular beams, etc., an extremely thin (for example, in the order of 100 A) multi-layer film can be produced.

Since, however, the prior-art crystal growing apparatus uses neutral molecular beams, it has disadvantages as stated below. The use of the neutral molecular beams makes it impossible to form a two-dimensional molecular concentration distribution on the substrate. Further, the control of the crystal growth is performed only with the substrate temperature. More specificially, the ratio of partial pressure as evaluated by the mass analyzing means is based on those molecules among the total neutral molecular beams being supplied to the substrate which are not absorbed on the substrate, in other words, the quantity of the molecules absorbed on the substrate. The control of the crystal growth with only changes in the quantity of the molecules does not provide a precise control.

In view of such problems, the present invention employs a Knudsen cell source of the surface ionization type in order to generate ion beams, and effects crystal growth with the ion beams.

Here, the Knudsen cell source of the surface ionization type is an ion source which has hitherto been well known. When the atoms are passed through a tungsten porous material, they are ionized according to the Saha-Langmuir's equation at their departure from the surface of the material.

The Knudsen cell source of the surface ionization type is at a temperture of approximately 1,000.degree.C and can efficiently generate ions by increasing the porosity of the tungsten material.

In conformity with the Saha-Langmuir's equation, one of the most important factors for the probability of surface ionization is the relation between the ionization potential of the specimen atom and the work function of the porous material. Where the work function of the employed material is about 5.5 eV, about 50% of the atoms which come into contact with the material are ionized as for indium (In) whose ionization potential is 5.79 eV. About 1 to 50% of atoms are ionized, as for an element whose ionization potential is approximately 10 eV. In accordance with these quantitative calculations, ion beams of 10.sup.10 atoms/sec-cm.sup.2, as required on the substrate for the crystal growth, can be sufficiently obtained.

FIGS. 2a and 2b are views for explaining the subject matter of the present invention which makes use of the Knudsen cell source of the surface ionization type. Referring to FIG. 2a, ion beams from the Knudsen cell 7 source of the surface ionization type pass through focusing electrostatic lenses 8, and are focused on a substrate 3. While, in the illustration, the Knudsen cell source of the surface ionization type consists of two cells, it may be similarly composed of a single cell. The case of using a plurality of cells, however, is advantageous in that the control of the crystal growth is more precise.

The magnitude of the ion beams is detected by means of mesh electrodes and ion detectors 9 which are arranged between the focusing electrostatic lines 8 and the substrate 3. On the other hand, the ion beams focused on the substrate 3 are neutralized by the interaction with low energy-electron rays emanating from an electron gun 11.

As a result, the crystal based on the neutral molecular beams is grown on the substrate 3. Further, if necessary, the growing process is successively observed in such way that electron-diffraction patterns of the crystal by high energy-electron rays emanating from an electron gun 5 are formed on a fluorescent screen 6.

FIG. 2b is a view for explaining the state in which the ion beam is scanned on the crystal by a deflecting electrode. The ion beam ejected from the Knudsen cell source 7 of the surface ionization type is focused by the focusing electrostatic lens 8, to reach the surface of the substrate 3. The ion beam having reached the substrate 3 scans the surface of the substrate 3 as, for example, shown by dotted lines in the figure by virtue of the deflecting electrode 10. In this case, the scanning speed is freely changed, whereby the distributed state of a multi-layer film gradually growing on the substrate 3 can be varied as desired.

FIG. 3 is a schematic view showing the construction of an embodiment of the present invention. The same symbols as in FIGS. 2a and 2b represent the same or equivalent parts. The Knudsen cell source of the surface ionization type 7 consists of three cells. Reference numerals 1 and 4 designate the same vacuum chamber and mass analyzing means as illustrated in FIG. 1, respectively. Numeral 12 indicates a total ion beam detector by which, in case of utilizing a plurality of ion source cells, an ion beam with the respective ion beams focused into a single one is detected.

With such a construction, specimen atom ions ejected from the Knudsen cell source of the surface ionization type 7 are focused on the substrate 3 by the focusing electrostatic lens 8. Simultaneously therewith, ion beams generated from the cells of the Knudsen ion source of the surface ionization type are respectively detected by the ion detectors 9-1, 9-2 and 9-3. The total quantity of the ion beams is detected by the total ion beam detector 12. A signal from the total ion beam detector 12 is fed back to the electron gun 11, to control the magnitude of an electron beams so as to neutralize the ion beam on the substrate 3 by the electron beam. Thus, the phenomenon in which the surface of the crystal growing on the substrate 3 is charged is perfectly eliminated.

The ion beam can scan the surface of the substrate 3 by varying the drive voltage of the deflecting electrode 10. The scanning can be made by signals, such as saw-tooth waves, introduced from deflection voltage-supplying means (not shown). On the other hand, signals from the ion detectors 9-1, 9-2 and 9-3 are compared with the signal from the total ion beam detector 12 (no comparator being shown) so as to evaluate the proportions occupied by the ions of the individual beams relative to the total ions. Simultaneously therewith, the ratio of partial pressure of molecules within the vacuum vessel or specimen chamber 1 is detected by the mass analyzing means, for example, a quadrupole mass filter 4.

In this way, according to the present invention, a two-dimensional concentration distribution of the molecules can be formed in addition to the three-dimensional growth of the crystal in the process of the crystal growth by the scanning of the ion beam. Furthermore, according to the present invention, both the ratio of partial pressure and the proportions occupied by the quantities of the ions of the individual beams (where the Knudsen cell source of the surface ionization type consists of a single cell, the quantity of the ions from the single ion cell corresponds to the above proportions) are produced as output signals. Using the output signals, a precise control for the crystal growth may be made in such a way that the output signals of both the quantities and the substrate temperature as well as the ionic amount are calibrated beforehand, or that the electron-diffraction patterns are observed.

More specifically, let it now be supposed that the ratio of partial pressure of the molecules within the vacuum chamber 1 has changed. Then, it can be known whether the change is attributable to a change in the coefficient of absorption of the substrate 3 due to a change in the temperature of the substrate 3, or it is attributable to a change in the quantity of the ejected ion beam from the Knudsen cell source of the surface ionization type 7. Therefore, the temperature of the substrate and/or the quantity of the ejected ion beam from the ion source may be appropriately controlled so as to effect the desired crystal growth. In order to control the temperature of the substrate, the heating temperature of a heater may be increased or decreased, while in order to control the quantity of ions ejected from the ion source, the tungsten porosity of the Knudsen cell source of the surface ionization type may be increased or decreased.

FIG. 4 is a block diagram of a control unit which, using both the proportions occupied by the quantities of the ions and the ratio of partial pressure as control signals, controls the quantities of the ions and the temperature of the crystal substrate in the apparatus of the embodiment shown in FIG. 3. In FIG. 4, the same symbols as in FIG. 3 indicate the same or equivalent parts. Reference numeral 13 designates a comparator which compares outputs from the ion detector 9 and the total ion detector 12. A computing device 14 receives, as its inputs, an output from the comparator 13 and an output from the quadrupole mass filter 4 (the ratio of partial pressure as referred to above), and generation a control signal for controlling the quantity of emission of the ions or the temperature of the substrate, namely, for controlling the crystal growth. A deflecting voltage-generator 15 generates a deflection voltage (saw-tooth wave voltage) for the deflecting electrode 10. Numerals 17 and 18 represent an ion beam-controller (for example, a current amplifier) and a substrate temperture-controller (for example, a current amplifier), respectively, each of which receives as its input an output from the computing device 14. Shown at 19 is a memory means. Display means 20 is composed of, for example, a Braun tube and displays the concentration of the ion beam. Numeral 21 indicates a heater for heating the crystal substrate 3.

With such a construction, the ion beams generated from the Knudsen cell source of the surface ionization type 7 are respectively detected by the ion detector 9. Each output of the detector 9 is one of the inputs of the comparator 13. The focused ion beam is detected by the total ion beam detector 12, whose output is the other input of the comparator 13 and a control signal for the electron gun 11. The electron gun 11 has its grid potential controlled by the control signal from the total ion beam detector 12, and generates an electron beam sufficient for the total ion beam to be neutralized on the crystal substrate 3. Since the deflecting electrode 10 has the saw-tooth wave voltage applied thereto from the deflection voltage-generator 15, the ion beam having passed through the focusing electrode means 10 is scanned on the crystal substrate 3. On the one hand, since the outputs from the total ion detector 12 and the ion detector 9 are applied to the comparator 13, the proportions occupied by the ions of the individual beams with respect to the total ions are evaluated therein. One of the output signals indicating the proportion is one of the inputs to the computing device 14, while the other output signal is an input to display means 20. On the other hand, a change in the ratio of partial pressures within the vacuum chamber 1 is evaluated by the mass analyzing means 4. The output of the mass analyzing means 4 is the other input of the computing device 14. The computing device 14 calculates, from the ratio of partial pressures of the molecules wiithin the vacuum chamber 1 and the ratio of the quantity of each ion beam to the quantity of all the ion beams, whether a change in, for example, the ratio of partial pressures is due to a change in the temperature of the substrate or due to a change in the quantity of the ion beam ejected from the Knudsen cell source of the surface ionization type 7. In dependence on the calculated result, the output of the computing device 14 becomes the control signal to the ion beam-controller 17 and/or the substrate temperature-control means 18, and is applied thereto. In consequence, an output signal from the ion beam-controller 17 or from the substrate temperature-control means 18 adjusts the quantity of the emitted ion beam from the Knudsen cell source of the surface ionization type 7 or the output of the heater 21, accordingly, the temperature of the crystal substrate. As stated above, the output of the comparator 13 is applied to display means 20. It is, therefore, possible that the ratio of the quantity of the emitted ions of each beam to the total quantity of the emitted ions, namely, the ion beam concentration is displayed by sweeping the applied output signals with the saw-tooth wave voltage from the deflection voltage-generating means 15. Thus, the ion beam concentration can be observed while the crystal is being grown.

Further, the sequence of radiation of the radiated ion beams effecting the crystal growth, the kinds of ions constituting the beams, etc. are stored in the memory means 19. The crystal growth is carried out with the deflection voltage-generating means 15 and the computing device 14 controlled by outputs from the memory means 19. Then, any desired multi-layer film can be manufactured.

Although, in the above description, the employment of a plurality of Knudsen ion source cells of the surface ionization type has been stated, it is a matter of course that the crystal growth can also be controlled in case where the Knudsen cell source of the surface ionization type consists of a single cell. In this case of the single cell, the total ion beam detector 12 shown in FIG. 4 is unnecessary. In the computing device 14, the quantity of the ion beam from the Knudsen cell source of the surface ionization type and the ratio of partial pressure fed from the quadrupole mass filter 4 are merely compared and operated, to calculate the control output to be applied to the ion beam-controller 17 and the temperature control means 18.

In the foregoing explanation, description has been made of the crystal growing apparatus which employs only the Knudsen cell source of the surface ionization type. Needless to say, however, in case where the scanning by the deflection is not required, for example, where a single crystal or the like is merely grown on the substrate, a Knudsen cell can be used jointly with the Knudsen cell source of the surface ionization type. In this case, the three-dimensional growth of the crystal is promoted by neutral molecular beams.

In the above explanation, the control of the quantity of the emitting ion beams of the Knudsen cell source of the surface ionization type has been stated as being conducted by the use of the analyzed result obtained from the mass filter and the detected results obtained from the ion detector means and the total ion detector means. However, where the requested precision of the control for the ion source is not very severe, the analyzed result need not be used. In this case, the arithmetic unit 14 shown in FIG. 4 performs operations on condition that the temperature of the substrate 3 is invariable.

As described above in detail, according to the present invention, two-dimensional of three-dimensional control of crystal growth as has hitherto been impossible becomes possible by employing ion beams. Simultaneously therewith, the control of crystal growth, more precise than has heretofore been possible, becomes possible by making use of the ratio between the total emitted ion beam and each ion beam and the ratio of partial pressure of molecules floating within a specimen chamber and not absorbed on a crystal substrate, or by making use of the quantity of an emitted ion beam and the ratio of partial pressure of the molecules.

Claims

We claim:

1. A crystal growing apparatus comprising:

a vacuum chamber;

a crystal substrate located within said vacuum chamber to be subjected to crystal growth;

a Knudsen cell source of the surface ionization type for irradiating an ion beam onto said crystal substrate;

ion detector means for detecting the quantity of the ion beam irradiated from said ion source;

focusing means for focusing said ion beam onto said crystal substrate;

beam deflector means for scanning the focused ion beam on said crystal substrate;

means for generating an electron beam for neutralizing said ion beam on said crystal substrate;

detector means for detecting the ratio of the partial pressures of the molecules within said vacuum chamber; and

signal generator means operatively connected to said ion detector means and said detector means for detecting the ratio of partial pressures for generating control signals for controlling the crystal growth, in accordance with the output from said ion detector means and the output from said detector means for detecting the ratio of partial pressures,

whereby said crystal is so grown as to have a two-dimensional distribution of concentrations of the molecules by the scanning of said ion beam on said crystal substrate.

2. An apparatus as defined in claim 1, further comprising molecular beam-generating means to further irradiate a neutral molecular beam onto said crystal substrate, whereby the crystal growth is accelerated.

3. A crystal growing apparatus comprising:

a vacuum chamber;

a crystal substrate located within said vacuum chamber to be subjected to crystal growth;

a Knudsen cell source of the surface ionization type consisting of a plurality of cells and for irradiating a plurality of ion beams onto said crystal substrate;

first ion detector means for detecting the quantity of ions of each of said plurality of ion beams;

focusing means for focusing said ion beams onto said crystal substrate;

beam deflector means for scanning the focused ion beam on said crystal substrate;

second ion detector means for detecting the total quantity of ions of said plurality of ion beams;

means for generating an electron beam for neutralizing said focused ion beam on said crystal substrate;

detector means for detecting the ratio of the partial pressures of the molecules within said vacuum chamber;

means for receiving the outputs from said first and second ion detector means and for evaluating the ratio between said total quantity of ions and said quantity of ions of said each ion beam; and

signal generator means operatively connected to said means for evaluating the ratio between said total quantity of ions and said quantity of ions of said each ion beam and said detector means for detecting the ratio of the partial pressures for generating control signals for controlling the crystal growth, in accordance with the output from said means for evaluating the ratio between the quantities of ions and an output from said detector means for detecting the ratio of partial pressures,

whereby the crystal is so grown as to have a two-dimensional distribution of concentrations of the molecules by the scanning of said focused ion beam.

4. An apparatus as defined in claim 3, further comprising molecular beam-generating means to further irradiate a neutral molecular beam onto said crystal substrate, whereby the crystal growth is accelerated.

5. An apparatus as defined in claim 3, further comprising display means receiving as its input, the output from said means for evaluating the ratio between the quantities of ions, the output of said display means being swept in synchronism with the scanning of said focused ion beam by said beam deflector means.

6. A crystal growing apparatus comprising:

a vacuum chamber within which a crystal substrate to be grown may be disposed;

first means for irradiating a crystal substrate disposed within said chamber with particles of opposite charge by at least one ion beam;

second means, coupled to said first means, for detecting the magnitude of said at least one ion beam;

third means, coupled to said chamber, for detecting the ratio of the partial pressures of molecules within said chamber; and

fourth means, coupled to said second and third means, for controlling the irradiation of said at least one ion beam by said first means in accordance with the outputs of said second and third means, whereby a resulting neutralized particle is grown upon the surface of said substrate.

7. An apparatus according to claim 6, wherein said first means further includes means for focusing said at least one ion beam onto said substrate and for scanning the focused ion beam on said crystal substrate.

8. An apparatus according to claim 6, wherein said first means comprises means for irradiating said substrate with a plurality of ion beams.

9. An apparatus according to claim 8, wherein said second means detect the respective magnitudes of each of said ion beams of said plurality, and said apparatus further comprises fifth means for detecting the total quantity of ions of said plurality of beams, and

sixth means, coupled to the outputs of said second and fifth means, for providing a signal representative of the ratio between said total quantity of ions and the quantity of ions of each ion beam,

wherein said fourth means is coupled to said third and sixth means for controlling the irradiation of said ion beams by said first means in accordance with the outputs of said third and sixth means.

10. An apparatus according to claim 6, wherein said first means for irradiating said crystal substrate commprises a Knudsen cell source of the surface ionization type having a plurality of cells therein for irradiating said substrate with a plurality of ion beams.

11. An apparatus according to claim 7, further comprising display means, responsive to the output of said second means, and being synchronized with said scanning means, for displaying the output of said second means, whereby the ion beam concentration can be observed.

12. An apparatus according to claim 6, further comprising means responsive to the output of said fourth means, for controlling the temperature of said crystal substrate.

13. An apparatus according to claim 6, further comprising seventh means, coupled to said first and fourth means, for controlling the quantity of said at least one ion beam, and

eighth means, coupled to said crystal substrate and said fourth means, for controlling the temperature of said substrate,

wherein said fourth means applies a signal to at least one of said seventh and eighth means as a function of the outputs of said second and third means such that crystal growth on said substrate proceeds in accordance with the crystal growth conditions of quantity of said at least one ion beam and temperature of said substrate.

14. An apparatus according to claim 9, further comprising seventh means, coupled to said first and fourth means, for controlling the quantity of said plurality of ion beams, and

eighth means, coupled to said crystal substrate and said fourth means, for controlling the temperature of said substrate,

wherein said fourth means applies a signal to at least one of said seventh and eighth means as a function of the outputs of said third and sixth means such that crystal growth on said substrate proceeds in accordance with the crystal growth conditions of quantity of said plurality of ion beams and temperature of said substrate.

15. An apparatus according to claim 9, wherein said first means further includes means for focusing said ion beams onto said crystal substrate scanning for acanning said focused beams in said crystal substrate.

16. An apparatus according to claim 6, said first means further includes means for accelerating crystal growth of said substrate by irradiating said substrate with a neutral molecular beam.

17. An apparatus according to claim 8, wherein said first means for irradiating said crystal substrate with a plurality of ion beams comprises a Knudsen cell source of the surface ionization type having a plurality of cells therein.

18. An apparatus according to claim 15, further comprising eighth means, responsive to the output of said sixth means, and being synchronized with said scanning means, for displaying the output of said sixth means, whereby the ion beam concentration can be observed.

19. An apparatus according to claim 9, further comprising means responsive to the output of said sixth means, for controlling the temperature of said crystal substrate.