GB2095704A - Molecular beam deposition on a plurality of substrates - Google Patents

Abstract

In molecular beam deposition carried out in an evacuated growth chamber 30, platen 500 holding a plurality of substrates 550 is rotated by means of a shaft 318 and a motor 320 so as to bring the substrates 550 in turn into the path of a molecular beam whilst the substrates are kept at a suitable temperature by heating means 324. With this arrangement a plurality of substrates can be processed simultaneously and the problem of flakes of material falling into the molecular beam sources is avoided thus enabling vertical sources to be used. An analytical chamber 20 and input/output chambers 10, 40, communicating with each other and the growth chamber 30 via valves 11, 21, 31 and a carrier 230 for moving platens from chamber to chamber enable a number of platens-full of substrates to be processed without bringing the growth chamber 30 to atmospheric pressure. <IMAGE>

Description

SPECIFICATION Molecular beam deposition on a plurality of substrates This invention relates to molecular beam deposition and more particularly to simultaneous deposition of materials on a plurality of substrates by one or more molecular beams.

Molecular beam deposition may be broadly defined as a method of growing films, under ultrahigh vacuum conditions, by directing one or more molecular beams at a substrate. A narrower term, which perhaps more accurately describes most current work, is molecular beam epitaxy (MBE) which refers to epitaxial film growth on single crystal substrates by a process that typically involves either the reaction of one or more molecular beams with the substrate or the deposition on the substrate of the beam particles.

The term "molecular beam" refers to beams of monoatomic species as well as polyatomic species. The term molecular beam deposition thus includes epitaxial growth as well as nonepitaxial growth processes. For example, molecular beam deposition includes the growth of layers of polycrystalline GaAs or amorphous silicon on substrates.

Molecular beam deposition, which may be thought of as a variation af simple vacuum evaporation, offers better control over the species incident on the substrate than does vacuum evaporation. Good control over the incident species, coupled with the slow growth rates that are possible, permits the growth of thin layers having compositions, including dopant concentrations, that are precisely defined.

Compositional control is aided by the fact that growth is generally at relatively low substrate temperatures, as compared to other growth techniques such as liquid phase epitaxy or chemical vapour deposition, and diffusion processes are very slow. Essentially arbitrary layer compositions and doping profiles may be obtained with precisely controlled layer thicknesses. In fact, layers as thin as a monolayer are grown by MBE.

Furthermore, the relatively low growth temperature may permit growth of materials and use of substrate materials that could not be used with higher temperature growth techniques because interdiffusion would degrade desired compositional properties.

Molecular beam epitaxy has been used to fabricate films or layers of numerous semiconductor materials including Group IV, Group Ill--V, Group Il-VI and Group lV-Vl materials. Within the Ill-V materials system, devices such as IMPATT diodes, microwave mixer diodes, double heterostructure junction lasers and superlattice devices have been fabricated.

Molecular beam epitaxy of elemental materials, e.g. Si, has developed more recently and devices such as p-n, p-i-n, varactor diodes and MOSFET structures have been made. Interest in Si MBE has increased recently because of the development of silicon-metal silicide heterostructures and improved silicon on insulator overgrowth.

Successful exploitation of MBE has, of course, required the development of new apparatus. This development has reached the state where commercial MBE apparatus is now available.

However, such apparatus and the research and development apparatus also in use generally have limited processing capabilities. For example, depositing from a large number of molecular beams and processing large numbers of wafers in a relatively short time period are usually not possible. The compositions that may be grown and the output of the apparatus are thus limited.

This limited processing capability arises from many factors. The most important factors include apparatus that permits deposition on only a single substrate at a time and uses relatively cumbersome methods to transport a substrate between chambers, for example between the growth chamber and an analytical chamber that is used to house instruments that are used to characterize the substrate surface, or move a substrate from a growth to an analytical station.

Further, the need for a uniform composition over the substrate typically requires that the flux from each oven be constant over the substrate and this requirement thus limits the number of ovens that can be used simultaneously. Additionally, lack of component standardization between, for example, analytical and growth chambers, limits apparatus flexibility and increases cost.

Recent developments have improved processing capability. For example, U.S. Patent 4,137,865 issued to Alfred Y. Cho on February 6, 1 979 describes a molecular beam apparatus for the sequential deposition of material on a plurality of substrates, i.e. deposition on one substrate commences and terminates before deposition on the following substrate commences. An experimental molecular beam apparatus explicitly designed for silicon is schematically illustrated in Journal of Crystal Growth, 45, pp. 287-291, Proceedings of the Fourth International Conference on Vapor Growth and Epitaxy, Nagoya, Japan, July 9-1 3, 1 978. The apparatus has only a single chamber and can process only a single substrate at a time; that is, only one substrate is ever positioned in the chamber at one time.

Another apparatus explicitly designed for silicon is described in Journal of Applied Physics, 48, pp.

3345-3399, August, 1 977. While this apparatus is well suited for silicon MBE, it also can process only a single substrate at a time.

Further, the use of a vertical deposition geometry may result in flakes of accumulated material falling back into the molecular beam sources. If this occurs, the flake can either contaminate the source or evaporate so rapidly that crystalline defects are created in the growing epitaxial layer. To avoid these undesirable results, Group Ill-V MBE apparatus using Knudsen effusion sources are generally tilted from the vertical to permit near-horizontal deposition. Such tilting is not possible with the electron beam evaporation sources generally used for Si MBE.

In a molecular beam deposition process using the invention as claimed a platen in which a plurality of substrates are mounted is rotated so as to bring the substrates in turn into the path of a molecular beam. In this way the plurality of substrates are processed simultaneously in the sense that the desired deposition is carried out on all the substrates in a single processing operation.

Furthermore we find that it is possible to use vertical molecular beam sources since the platen, together with the substrates, occupies substantially all of the area directly over the beam source. Thus, since the platen can be removed with the substrates and cleaned before being used again, large accumulations of material leading to flanking can be avoided. By using a plurality of chambers including loading/unloading chambers, communicating with the growth chamber via valves, and providing means for transporting the platen from chamber to chamber, deposition can be carried out successively on a plurality of platens-full of substrates without bringing the growth chamber up to atmospheric pressure.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, of which: FIG. 1 is a section of an apparatus according to the invention; FIG. 2 is a sectional view of the growth chamber along line 1-1 of the apparatus of FIG. 1; FIG. 3 is a top view of a platen for the apparatus of FIGS. 1 and 2; FIGS. 4 and 5 are perspective views of two forms of carrier means for transporting the platen in the apparatus of FIGS. 1 and 2; and FIG. 6 plots, vertically, the spacing between the substrate and the oven versus, horizontally, the radial distance of a substrate from the axis of rotation of said means for rotating for different flux arrival rates at the substrate.

In the FIGURES, the same elements depicted in different drawings are indicated by the same reference numeral.

The apparatus of FIG. 1 comprises a first loading/unloading chamber 10, analytical chamber 20, growth chamber 30 and a second loading/unloading chamber 40. The chambers are cylindrical and made of stainless steel. Chambers 10 and 40 are preferably identical and chambers 20 and 30 are preferably identical except as described below. Air locks or valves 11, 21 and 31 are placed intermediate chambers 10 and 20, 20 and 30, and 30 and 40, respectively, and isolate the adjacent chambers from each other or place them in communication with each other as desired. The structure of these and other air locks present in the apparatus is well known to those working in the art, see U.S. Patent 4,137,865, and therefore need not be discussed in detaii.

Chamber 10 further comprises port means 101 which, in the apparatus depicted, is a plate attached to the chamber 10 by a hinge (not shown) which opens and permits a platen 500 holding a plurality of substrates 550 to be placed within chamber 10. A pump 102 exhausts chamber 10 through valve 103 after the chamber has been loaded with the platen holding the substrates. The desired pressure is typically 10-6 torn Carrier means 230 are disposed beneath the platen 500.

Analytical chamber 20 comprises a pump 201 connected to the chamber through a valve 202 and a baffle 203. The chamber further comprises a shroud 205, a heating element 207, a radiation heat shield 209, a shaft 210 with a shoulder 211 adapted to support the platen 500 holding the substrates 550, drive means 212 for the shaft 210, and bellows 213. Carrier means 230 are disposed beneath the platen 500. Preparatory and analytical tools having access to the chamber 20 through a flange 214 generally include a sputter gun 215, an infrared pyrometer 217, and an analytical station 240 which may be equipped for such analyses as infrared pyrometry, Auger electron spectroscopy, secondary ion mass spectroscopy, etc.

The pump 201 is typically a cryopump which is capable of achieving pressures of 10-8 torr or lower. Cryogenic fluids, typically liquid nitrogen (LN2), are pumped through and cool the shroud 205. The heating element 207 is conventional and, in the apparatus depicted, heats all of the platen 500 and thus all substrates. If desired, the heating element 207 may be configured to heat only a portion of the platen 500. The heat shield 209 depicted is annular, although a cylindricai shield could also be used, and encloses the heating elements. If only selected portions of the platen 500 are heated the radiation heat shield 209 will desirably be shaped to shield only the heated portions. The radiation heat shield 209 and the platen 500 are desirably made of a refractory material that is relatively chemically inert with respect to the material being deposited.For example, tantalum is a preferred material when silicon is being deposited. The shoulder 211 supports and is friction coupled to the platen 500 and thus enables the shaft 210 to rotate the platen 500 and with it the substrates 550. The bellows 213 is conveniently made of stainless steel and permits the drive means 212 to raise and lower, as well as rotate, the shaft 210 and thus the platen 500.

The molecular beam deposition of at least one layer of at least one material on a plurality of substrates takes piace within the growth chamber 30. Vacuum conditions are obtained in the growth chamber 30, typically, by a cryopump 301, connected to chamber 30 through a valve 302 and a baffle 303. The pump should be capable of maintaining a pressure of 10-10 torr or lower. The interior of the chamber is surrounded by a shroud 305 which is similar to that described with respect to the analytical chamber and is also cryogenically cooled by a fluid such as liquid nitrogen. The growth chamber 30 further comprises an n-type ion source 306 and a p-type ion source 308.

Although two ion sources are depicted, it is to be understood that either fewer or more ion sources may be used depending upon the devices being fabricated. Shutter drives 309 and 311 control the positions of shutters 31 3 and 315, respectively, which may be used to initiate or terminate the ion flow from the sources 306 and 308 respectively to the substrates 550 located in a growth position.

The shutter drives and ion sources have access to chamber 30 through a flange 316. Preferably, chamber 30 above flange 316 is essentially identical to chamber 20 above flange 214. In an apparatus preferred for deposition of silicon, ion sources 306 and 308 are field emission ion sources because of their compact sizes and because there is no need for differential pumping as there is no gas for a plasma. Although field emission sources are preferred, Knudsen effusion cells can also be used.

Carrier means 230 are disposed beneath the platen 500. The growth chamber 30 further comprises a shaft 31 8 supporting the platen 500 holding the substrates 550 and drive means 320 for the shaft 31 8. Shaft 318 is connected to the drive means 320 through a bellows 322. This permits the drive means 320 to raise and lower the platen 500. A shoulder 319 on the shaft 318 permits friction coupling to the platen 500 and thus allows the platen 500 to be rotated by the drive means 320. In the raised platen position the substrates 550 will either be supported in a growth position or in a position from which they can be rotated to a growth position. The growth chamber 30 also comprises heating elements 324 for the substrates 550. Heating elements 324 are surrounded by a radiation heat shield 326.The heat shield 326 is depicted as being annular although a cylindrical shape could be used as it could be with the analytical chamber 20.

However, heating elements 324 will generally heat all of the platens disposed beneath the heat shield 326 as all substrates will be deposited on simultaneously. An infrared pyrometer 328 is used to monitor the temperature of substrates 550.

Chambers 20 and 30, in the apparatus described, are identical except that different elements, ovens and analytical equipment, have access to the chambers through the large bottom flanges.

A second loading/unloading chamber 40 comprises an access port 101 which permits removal of the platen 500 and substrates 550, and pumping means 402 connected to the chamber through a valve 403. Carrier means 230 are disposed beneath the platen 500. Chamber 40 is preferably, for reasons of manufacturing cost reduction, identical to chamber 10 and need not be described further.

Although an apparatus has been described which has two loading/unloading chambers, one analytical and one growth chamber, other configurations are contemplated. For example, a second analytical chamber may be positioned intermediate the growth and second loading/unloading chambers or the analytical chamber could be omitted or combined with the first loading/unloading chamber. Additionally, two growth chambers might be used to prevent crosscontamination. For example, Si might be deposited in one chamber and GaP in another chamber to prevent phosphorus contamination in the silicon chamber. Finally, access to and from the analytical and growth chambers might be from a single loading/unloading chamber although apparatus efficiency with respect to the number of substrates processed would probably be reduced.

Another sectional view, taken along line 1-1 of FIG. 1, of the growth chamber 30 is depicted in FIG. 2. In addition to the elements shown in and described with respect to FIG. 1, and which need not be described further, chamber 30 comprises ovens 330 and 332, depositing meters 334 and 336 and carrier means 230. The ovens generate molecular beams directed to a growth position.

Although two ovens are depicted, a greater or smaller number may be used if a greater or smaller number of molecular beams is desired. Shutters 338 and 340 initiate or terminate the beam flux from ovens 330 and 332, respectively, to the growth position after the ovens are heated. The positions of the shutters 338 and 340 are controlled by shutter drive means 342 and 344, respectively. It is contemplated, for the apparatus depicted, that at least one oven will be loaded with silicon which is melted by an electron beam (not shown). A shaped piece of silicon is placed in the oven and the electron beam melts only a central portion of the silicon with the unmelted silicon acting as a crucible for the molten silicon. This melting technique is well known in the art. The second oven can be loaded with for example silicon or cobalt, when silicon-metal silicide heterostructures are being made.

While the apparatus described in FIGS. 1 and 2 is referred to as molecular beam deposition apparatus, the ovens depicted are not necessarily the Knudsen effusion ovens typically used in the molecular beam epitaxy apparatus for Group Ill-V compounds. It is also apparent that growth on any one substrate may take place at more than one growth position if the growth chamber has more than one molecular beam source. The apparatus described with respect to FIGS. 1 and 2 can also be used to deposit metals such as Au or Cu with nonepitaxial growth.

FIG. 3 is a top view of the platen. The platen 500 that is adapted to hold the substrates is desirably circular in shape with a hole 560 in the centre adapted to receive the shaft 21 8, 310. The plurality of substrates 550 may comprise a semiconductor, such as silicon, or an insulator, such as sapphire, and fit into a plurality of holes having lips adapted to receive and support the substrates. These recesses will be generally circular in shape, as are the substrates depicted, with their centres spaced a distance rfrom the centre of the platen and preferably spaced 360/n, where n is the number of substrates, degrees apart. The platen depicted is approximately 35.5 cm (14 inches) in diameter and contains 8 7.5 cm (3-inch) substrates.

Carrier means for the platen are shown in FIGS.

4 and 5. The type shown in FIG. 4 transfers only the platen while the type shown in FIG. 5 transfers the platen and a frame holding the platen. In FIG. 4, a platen 500, with substrates 550, is carried directly on a drive 501 which moves over wheeis 505 which are driven by a motor 503. The motor 503 is external to the chamber and is coupled to the belt drive through a vacuum feedthrough. The carrier means comprises the drive 501,wheels 505 and motor 503. The motors for different chambers are synchronized to drive the carrier means in all of the chambers at the same time and speed. Alternatively, a single motor may be used to drive the carrier means in all the chambers by means of appropriate coupling between the carrier means.Each chamber has a carrier means with a gap between adjacent carrier means to permit closing of the valves intermediate adjacent chambers. Although the drive depicted is a belt drive, other drives, such as a chain or worm drive, may also be used. The platen 500 need not be carried directly on the drive 501 as is clear from FIG. 5. In this type of carrier, the platen 500 is carried in a plate 570 which has a recess adapted to support the platen 500. The plate 570 has ledges 572 which are supported by and contact the belt drive 501.

The preferred position for the location of the growth position of the substrates and ovens with respect to the axis of rotation of the shaft may be ascertained by reference to FIG. 6. The substrate rotates about a vertical axis and there are point sources at a radial distance of one unit. Curves of equal flux at the growth position are piotted as a function of substrate to oven spacing, vertically, versus the radial spacing of the substrate from the axis of rotation, horizontally. For reasons of clarity and simplicity, the vertical and horizontal coordinates are plotted in units of oven spacing from the axis of rotation, i.e., the oven is considered to be one unit from the axis of rotation.

In other words, when the centre of the substrate is spaced one unit radially from the axis of rotation of the shaft, it is the same radial distance from the axis of rotation as is the oven. As can be seen from the longer horizontal line (1), which represents a 10 cm (4-inch) substrate, a relatively uniform flux is obtained over the substrate when the vertical spacing of the substrates from the oven is slightly greater than the spacing of the oven from the axis of rotation and when the horizontal spacing of the centre of the substrates from the axis of rotation is about .6 of the horizontal spacing of the oven from the axis of rotation. The shorter horizontal line (2) represents a 7.5 cm (3-inch) substrate. A very uniform flux is also obtained when the substrates are approximately 1.5 units from the plane containing the ovens and centred on the axis of rotation.This position is not the most preferred because it severely limits the number of substrates that may be processed simultaneously. Other spacings may be selected for different substrate diameters. In general, as substrates become smaller than the two just described, the ovensubstrate distance decreases and the radial spacing of the substrate from the axis increases.

In the apparatus described, the substrates are placed horizontally in a flat, relatively thin platen.

The use of a flat platen minimizes the cost of machining a platen fabricated from a refractory material and also reduces the amount of material used. Other than flat platens could be used.

In describing the operation of the apparatus, it will be assumed that the chambers are initially at atmospheric pressure. This need not be true after the apparatus has begun operating. After the growth and analytical chambers 30 and 20 have been brought to the desired vacuum, they may be maintained at vacuum except for times when the ovens have to be reloaded. Interlocks could be used to permit reloading the ovens without breaking vacuum. The substrates 550 are first placed in the first platen 530 which is then placed on the carrier means 230 in the first handling chamber 10. The substrate 550 are first cleaned, in accordance with general practice, to remove any surface contaminants. Valve 11 is closed and the pump, with valve 103 open, reduces the pressure of the first loading/unloading chamber 10 to approximately 10-6 torr.After the pressure has been reduced, valve 11 opens. This valve 11 may be connected to open and close simultaneously with the other valves 21, 31 in the apparatus that permit communication between adjacent chambers. The carrier means 230 transports the substrates 550 to the analytical chamber 20 after which valve 11 closes again. At this time, a second platen 500 holding substrates 550 may be placed in the loading chamber 10.

The pump 201 connected to the analytical chamber 20 will have reduced the pressure in the analytical chamber 20 to approximately 10-8 torr.

After the desired analytical and preparatory processes have been concluded, including cleaning of the substrates with the sputter gun 215, the valves 1 11,21,31 connecting adjacent chambers again open and the carrier means 230 simultaneously transports the first platen 500 to the growth chamber 30 and the second platen 500 to the analytical chamber 20. The chamber and valve arrangement permits apparatus operation without either the growth 30 or analytical 20 chambers ever being raised to atmospheric pressure except when the ovens or ion sources are reloaded or other such work is performed. Interlocks could, if desired, permit oven reloading without the chamber vacuum being broken. The drive means 320 in the growth chamber 30 is activated and the shaft 318 raises the platen 500 from the carrier means and begins to rotate the platen 500 and substrates 550 in the growth position. The heating elements 324 heat both the substrates 550 and the platen 500 to a temperature within the range extending from approximately 400 degrees Celcius to approximately 1000 degrees Celsius when silicon is being deposited. Higher temperatures may be used but the advantages of molecular beam deposition are reduced because diffusion processes become important. Lower temperatures may be used when growth of amorphous materials is being carried out. The temperature is monitored with the infrared pyrometer 328.

Since the platen 500 occupies all of the area above the molecular beam sources 330, 332, large accretions of material are avoided. The platen 500 can be cleaned when it is removed from the loading/unloading chamber 40 with the substrates 550. Well-known mechanical or chemical means can be used to clean a platen made from a chemically resistant material such as tantalum. The absence of accumulated material will reduce, if not completely eliminate, the flaking problem commonly encountered with vertical deposition apparatus. Cross contaminations of molecular beam sources and epitaxial defect densities should both be greatly reduced.

Silicon will have been placed in at least one of the ovens 330, 332 and an electron beam gun is directed, in a well-known manner, by an appropriate magnetic field to the surface of the silicon where it forms the silicon molecular beam.

This type of source is well suited for use with silicon because of the high reactivity of molten silicon. The n-type and p-type sources 306, 308 will also have been loaded with appropriate n-type and p-type dopants and the beams from these sources can be initiated or terminated, as discussed previously, by opening or closing their respective shutters 31 3, 31 5.

After the desired layers of material have been deposited the drive means 320 stops rotating and lowers the platen 500 onto the carrier means 230.

The valves 11,21,31 between adjacent chambers open and the carrier means 230 moves the first platen 500 from the growth chamber 30 to the second handling chamber 40 while at the same time it moves a third platen 500 from the first handling chamber 10 to the analytical chamber 20 and the second platen 500 from the analytical chamber 20 to the growth chamber 30. Of course, each chamber need not have a platen although such a procedure will not yield maximum output from the apparatus.

The angular velocity of the shaft 318 and thus the substrates 500 may be controlled in the growth chamber 30 so that layers of material having the desired thickness and composition are grown. For example one oven 330 may be loaded with silicon and the second oven 332 with cobalt and the angular velocity adjusted so that alternating cobalt-silicon layers are deposited. As another example, the flux rates from the p- and n-type ion sources 309, 311 and the angular velocity of the substrates may be adjusted so that alternating p- and n-type layers of Si are deposited. Typical deposition rates are between 1 and 100 Angstroms per second. At lower rates, there may be undesired deposition of contaminants and higher rates may be difficult to control accurately. The precise rate will depend on the device being fabricated.

The operation of the apparatus has been described with respect to the use of silicon, it is to be understood that the apparatus may be used with other semiconductor materials such as Group Ill-V compounds. When such compounds are grown, the ovens described will typically be replaced by the more conventionally used Knudsen effusion cells. The use of a removable platen above the ovens will eliminate flaking problems and permit deposition of Group Ill-V materials in the more convenient vertical direction.

With this configuration, the Knudsen cells are arranged with their axes parallel in a vertical direction, rather than near horizontal and converging toward a single, central substrate position, and a much larger number of ovens can be used. For example, if a source to chamber axis spacing of 1 5 centimetres (6 inches) is used, at least 20 Knudsen cells can be used instead of the 6-8 present in current commercial designs. This large number of cells can be used to increase the number of semiconductor materials or dopants deposited in a single system. The use of a slow platen rotation frequency and a suitable arrangement of ovens permits the growth of structures having modulated compositions. This permits elimination of the complicated oven shattering arrangements now used.

Although the apparatus has been described in terms of simultaneous deposition on a plurality of substrates, it is to be understood that the word "simultaneous" is not used in the sense that all substrates receive equal fluxes at all times but rather in the sense that the desired layers are deposited on all substrates during a single processing operation.

Claims (9)

1. Apparatus for molecular beam deposition on a plurality of substrates comprising an evacuable growth chamber containing at least one molecular beam source for generating a molecular beam directed to a growth position, a platen for mounting a plurality of substrates, means for rotating the platen to bring the substrates in turn to the growth position and means for heating the plurality of substrates.

2. Apparatus as claimed in claim 1 including an evacuable analytical chamber, a valve between the analytical chamber and the growth chamber and carrier means for transporting the platen between the analytical chamber and the growth chamber.

3. Apparatus as claimed in claim 2 including a first and a second loading/unloading chamber, a valve between the first loading/unloading chamber and the analytical chamber and a valve between the growth chamber and the second loading/unloading chamber, the carrier means being arranged to transport the platen between the first loading/unloading chamber and the analytical chamber and between the growth chamber and the second loading/unloading chamber.

4. Apparatus as claimed in any of the preceding claims wherein the or each molecular beam source is arranged to direct the respective molecular beam in a substantially vertical direction.

5. Apparatus as claimed in claim 4 including a plurality of molecular beam sources in the growth chamber at equal radial distances from the axis of rotation of the platen.

6. A method of molecular beam deposition on a plurality of substrates including producing a molecular beam from a molecular beam source in an evacuated growth chamber and rotating a platen in which the substrates are mounted so as to bring the substrates in turn into the path of the molecular beam.

7. A method as claimed in claim 6 wherein there are a plurality of molecular beam sources in the chamber at equal radial distances from the axis of rotation of the platen arranged to produce molecular beams having different compositions whereby a layered structure is deposited on the substrates.

8. Apparatus for molecular beam deposition substantially as herein described with reference to the accompanying drawings.

9. A method of molecular beam deposition substantially as herein described with reference to the accompanying drawings.