USE OF A CVD REACTOR FOR DEPOSITING TWO-DIMENSIONAL LAYERS

Abstract

A two-dimensional layer is deposited onto a substrate in a CVD reactor, in which a process gas is fed into a process chamber. The process gas in the process chamber is brought to the substrate, and the substrate is heated to a process temperature. After a chemical reaction of the process gas, the layer forms on the surface. During or after the heating of the substrate to the process temperature, the process gas with a first mass flow rate is initially fed into the process chamber and then, while the substrate surface is being observed, the mass flow rate of the process gas is increased to a rate at which the layer growth begins, and subsequently the mass flow rate of the process gas is increased by a predetermined value, during which the layer is deposited. The beginning of the layer growth is identified by observing measurements from a pyrometer.

Claims

1. A method for depositing a two-dimensional layer onto a substrate in a chemical vapor deposition (CVD) reactor (1), the method comprising: feeding process gas into a process chamber (3) via a gas inlet element (2) with gas outlet openings (14, 24); bringing the process gas or its decomposition products into contact with a surface of the substrate (4) in the process chamber (3); and heating the substrate (4) to a process temperature (T.sub.P) so that the two-dimensional layer is deposited onto the surface of the substrate (4) after a chemical reaction of the process gas, wherein feeding the process gas into the process chamber (3) comprises: flowing the process gas with a first mass flow rate (Q.sub.1) into the process chamber (3) while heating or after heating the substrate (4) to the process temperature (T.sub.P), at which no layer growth takes place on the surface of the substrate (4), after the substrate (4) has been heated to the process temperature (T.sub.P), increasing the flow of the process gas to a second mass flow rate (Q.sub.2) at which the layer growth on the surface of the substrate (4) starts to occur, increasing the flow of the process gas to a third mass flow rate (Q.sub.3) corresponding to a sum of the second mass flow rate (Q.sub.2) with a prescribed value, and maintaining the flow of the process gas at the third mass flow rate (Q) during which the two-dimensional layer is deposited.

2. A chemical vapor deposition (CVD) reactor (1) for depositing a two-dimensional layer onto a substrate (4), the CVD reactor (1) comprising: a process chamber (3); a gas inlet element (2) with gas outlet openings (14, 24) that empty into the process chamber (3); a susceptor (5) for supporting the substrate (4); a heating device (6) for heating the substrate (4) to a process temperature (T.sub.P); a feed line (10) for flowing a process gas into the gas inlet element (2) through the gas outlet openings (14, 24) and into the process chamber (3); and a control device (29) configured to control one or more components of the CVD reactor (1) so as to: flow the process gas with a first mass flow rate (Q.sub.1) into the process chamber (3) while heating or after heating the substrate (4) to the process temperature (T.sub.P), at which no layer growth takes place on a surface of the substrate (4), after the substrate (4) has been heated to the process temperature T.sub.P) increase the flow of the process to a second mass flow rate (Q.sub.2) at which the layer growth on the surface of the substrate (4) starts to occur, increase the flow of the process gas to a third mass flow rate (Q.sub.3) corresponding to a sum of the second mass flow rate (Q.sub.2) with a prescribed value, and maintain the flow of the process gas at the third mass flow rate (Q) during which the two-dimensional layer is deposited on the surface of the substrate (4).

3. The CVD reactor (1) of claim 2, further comprising an optical device (19) for observing the surface of the substrate (4).

4. The CVD reactor (1) of claim 3, wherein the optical device (19) is a pyrometer.

5. The method of claim 17, wherein at least one of: a starting time of the layer growth is determined by evaluating a measuring curve (26) recorded by the optical device (19), or the starting time of the layer growth is determined by detecting a change in a gradient of the measuring curve (26) recorded by the optical device (19).

6. (canceled)

7. The method of claim 5, wherein the measuring curve (26) is used to determine a number of deposited layers.

8. The method of claim 1, wherein the prescribed value is at least 5 percent of the second mass flow rate (Q.sub.2).

9. The CVD apparatus (1) of claim 2, further comprising: a cooling chamber (8) through which a coolant flows; and a gas distribution volume (11, 21), wherein the gas inlet element (2) has a gas outlet surface (25), which extends over a support surface (15) of the susceptor (5), wherein the gas outlet openings (14, 24) are uniformly distributed over the gas outlet surface (25) and are fluidly connected with the gas distribution volume (11, 21), wherein the gas outlet surface (25) comprises a gas outlet plate (9) of the gas inlet element (2), and wherein the gas outlet plate (9) is adjoined by the cooling chamber (8).

10. (canceled)

11. The CVD apparatus (1) of claim 9, wherein a beam path (18) of the optical device (19) passes through the gas inlet element (2), and wherein a cover plate (16) of the gas inlet element (2) has (i) a window (17) that is transparent to a wavelength of radiation emitted by the optical device (19), and (ii) a tube (12′) through which the beam path (18) opens into the gas outlet surface (25).

12. The method of claim 1, wherein a distance between a support surface (15) of the susceptor (5) and a gas outlet surface (25) of the gas inlet element (2) is changed during the deposition of the two-dimensional layer.

13. The method of claim 1, wherein the process gas is generated by passing a carrier gas through a bubbler (32, 32′) containing a solid or liquid starting material.

14. The method of claim 13, wherein a gas concentration measuring device (31, 31′) downstream from the bubbler (32, 32′) is used to determine a concentration of a vapor of the starting material in the carrier gas.

15. The method of claim 17, wherein the surface of the substrate (4) is further observed and the measuring curve (26) is further evaluated during layer deposition so as to switch off the process gas when a change in a gradient of a measuring curve (26) recorded by the optical device (19) is detected.

16. (canceled)

17. The method of claim 1, wherein an optical device (19) is used to observe the surface of the substrate (4).

18. The method of claim 17, wherein the optical device (19) is a pyrometer.

19. The method of claim 18, wherein the pyrometer is a two-wavelength pyrometer.

20. The method of claim 7, wherein the number of deposited layers is determined by ascertaining a number of maximums or minimums present in the measuring curve (26).

21. The CVD reactor (1) of claim 4, wherein the pyrometer is a two-wavelength pyrometer.

22. The CVD reactor (1) of claim 2, further comprising a bubbler containing a solid or liquid starting material, wherein the process gas is generated by passing a carrier gas through the bubbler (32, 32′).

23. The CVD reactor (1) of claim 22, further comprising a gas concentration measuring device (31, 31′) disposed downstream from the bubbler (32, 32′), wherein the gas concentration measuring device (31, 31′) is configured to determine a concentration of a vapor of the starting material in the carrier gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Exemplary embodiments of the invention will be described below based upon the attached drawings. Shown on:

[0009] FIG. 1 is a schematic cross section through a CVD reactor of a first exemplary embodiment, and a schematic view of the components of a gas mixing system required for explaining the invention,

[0010] FIG. 2 is a magnified view of the cutout II on FIG. 1,

[0011] FIG. 3 is a plot depicting the time progression of the process gases,

[0012] FIG. 4a is a plot depicting a measuring curve of a two-wave pyrometer during layer deposition,

[0013] FIG. 4b is an illustration according to FIG. 3 of the time progression of the gas flow of the reactive gas in the process chamber,

[0014] FIG. 5 is a plot depicting a measuring curve similar to FIG. 4a, but wherein the reactive gas has been fed into the process chamber over the entire time t,

[0015] FIG. 6 is an illustration according to FIG. 1 of a second exemplary embodiment,

[0016] FIG. 7 is a magnified view of the cutout VII on FIG. 6,

[0017] FIG. 8 is a plot illustrating the influence of a process chamber height h on layer growth at various total pressures.

DETAILED DESCRIPTION

[0018] The device shown on FIGS. 1 and 6, 7 is a CVD reactor 1. The CVD reactor 1 has a housing, which is gastight and can be evacuated with a vacuum pump (not shown). The vacuum pump can be connected to a gas outlet element 7.

[0019] Located inside of the CVD reactor 1 is a gas inlet element 2, which has the shape of a shower head (showerhead). In the exemplary embodiment shown on FIGS. 1 and 2, the gas inlet element 2 has two gas distribution chambers 11, 21, into which a respective feed line 10, 20 empties, through which a gas can be fed into the respective gas distribution chamber 11, 21. The feed lines 10, 20 protrude through the wall of the housing. The gas distribution chambers 11, 21 are arranged vertically over each other. Located below the gas distribution chamber 21 is a cooling chamber 8. A coolant can be fed into the cooling chamber 8 through a feed line 8′. The coolant exits the cooling chamber 8 through a discharge line 8″. The feed line 8′ and discharge line 8″ protrude through a wall of the housing of the CVD reactor 1.

[0020] FIG. 1 further shows a cutout of a gas mixing system for providing the process gases. Two reactive gases are each generated by evaporating liquids or solids. The liquid or a powder is stored in gastight containers (bubblers 32, 32′). A mass flow controller 30, 30′ is used to feed a respective inert gas from an inert gas source 39, 39′ into the respective bubbler 32, 32′. The bubblers 32, 32′ are kept at a constant temperature in temperature baths. A vapor of the reactive gas transported with the inert gas acting as the carrier gas exits the respective bubbler 32, 32′. The concentration of reactive gas in the output flow is measured with a concentration measuring device 31, 31′. A device sold under the brand name “Epison” is here involved.

[0021] The two different gas lines for transporting the reactive gas can each be fed by means of a switching valve 33, 33′ into either a vent line 35 that conducts the gas by the reactor 1, or into a run line 34, 34′ that conducts the gas into the reactor 1.

[0022] Provided is a control device 29, which controls the temperature of the heating baths and mass flow controller 30, 30′. The measuring results of the concentration measuring device 31, 31′ are likewise fed to the control device 29.

[0023] The run line 34 of the branch of the gas supply shown on the right hand side of FIG. 1 empties into the feed line 20. The run line 34′ empties into the feed line 10.

[0024] Instead of the reactive gas, the mass flow controller 37, 37′ and valves 36, 36′ can also feed a carrier gas/inert gas into the gas inlet element 2. Reference numbers 40, 40′ denote sources for reactive gases, for example which are carbon compounds and in particular hydrocarbons, such as methanes, which are used for depositing graphene. These reactive gas sources 40, 40′ are connected in terms of flow with the run lines 34, 34′ via mass flow controllers 41, 41′ and valves 38, 38′.

[0025] As a consequence, the gas mixing system shown on FIG. 1 can optionally be used to feed two different reactive gases into the two separate gas distribution chambers 11, 21 simultaneously. However, it is also possible to sequentially feed methane into the gas distribution chamber 11 and an inert gas into the gas distribution chamber 21, and then feed borazine into the gas distribution chamber 21 and the inert gas into the gas distribution chamber 11, for example to deposit a sequence of layers comprised of graphene and hBN. In this way, heterogenous layer structures can be deposited via periodic switching.

[0026] The exemplary embodiment of a CVD reactor 1 shown on FIGS. 6 and 7 essentially differs from the exemplary embodiment shown on FIGS. 1 and 2 in that only one gas distribution chamber 11 is provided. The latter is connected by tubes 12 with a gas outlet surface 25, so that process gas fed into the gas distribution chamber 11 can flow through the tubes 12 and into a process chamber 3.

[0027] The gas mixing system denoted on FIG. 6 has only one bubbler 32, into which a carrier gas is fed by means of the mass flow controller 30. The concentration of the vapor transported in the carrier gas can be determined with the concentration measuring device 31. The switching valve 33 can be used to feed the mass flow of the reactive gas into either a vent line 35 or into the run line 34. The inert gas can be fed into the run line 34 by means of the mass flow controller 37. To this end, the valve 36 must be opened.

[0028] The exemplary embodiment shown on FIGS. 1 and 2 additionally provides tubes 22 that connect a second gas distribution chamber 21 with the gas outlet surface 25. In the gas outlet surface 25 comprised of a gas outlet plate 9, gas outlet openings 14, 24 each connected with a tube 12, 22 are arranged distributed over the entire gas outlet surface 25. The tubes 22 are connected with an intermediate plate 23 that separates the gas distribution chamber 21 from the cooling chamber 8. The tubes 12 are connected with an intermediate plate 13, which separates the gas distribution chamber 11 from the gas distribution chamber 21.

[0029] A support surface 15 of a susceptor 5 comprised of coated or uncoated graphite extends at a distance h from the gas outlet surface 25. Undepicted lifting elements can be used to lift or lower the susceptor 5 and/or the gas inlet element 2. The lifting elements can be used to vary the distance h. FIG. 8 shows a plot illustrating the influence of varying the process chamber height on the growth rate of the deposited layer at different total pressures in the process chamber 3.

[0030] The susceptor 5 is heated from below by means of a heating device 6. The heating device can be a resistance heater, an IR heater, an RF heater, or some other power source with which thermal energy is fed to the susceptor 5.

[0031] The susceptor 5 is surrounded by a gas outlet element 7, through which gaseous reaction products and a carrier gas are discharged.

[0032] One of the tubes 12′ is used as a passage channel for a beam path 18 of an optical device. The cover plate 16 of the gas inlet element 2 has a window 17, through which the beam path 18 passes. The beam path 18 runs between a pyrometer 19, which is a two-wavelength pyrometer, and the support surface 15 or the surface of the substrate 4 that lies on the support surfaces 15. The pyrometer 19 can be used to measure the temperature of the substrate surface. FIGS. 4a and 5 show measuring curves that were measured over time t, and can be interpreted as measured temperature values. The temperature rises up to a maximum in the heating process. The measuring curve then drops off slightly along a straight line with a roughly constant gradient. FIG. 4a shows a first peak 27. FIG. 5 additionally shows a second peak 27′.

[0033] FIG. 4a shows a measuring curve, in which a flow of a reactive gas (for example, methane) or a mixture of several reactive gases with a mass flow rate of Q.sub.1 is fed into the process chamber at a point in time t.sub.1. The mass flow rate of the process gases is steadily increased up to a time t.sub.2. Time t.sub.2 is characterized in that the gradient of the measuring curve 26 rises. Observations have shown this to be correlated with the event where layer growth starts on the layer. As the peak 27 forms, the gradient of the measuring curve 26 then constantly changes during layer deposition, such that the gradient drops until it once again rises at a point in time t.sub.4. Observations have shown that the rise in the measuring curve is accompanied by an end to the two-dimensional growth.

[0034] While the flow of the process gas was turned off at point in time t.sub.4 in the measuring curve according to FIG. 4a, process gas was fed into the process chamber even after the peak 27 while recording the measuring curve according to FIG. 5. The peak 27′ formed in the process.

[0035] Based on the findings, the method according to the invention is implemented as follows:

[0036] The method according to the invention begins with the provision of a CVD reactor of the kind described above. A substrate 4 to be coated is placed in the CVD reactor. The substrate is located on the support surface 15. The temperature of the substrate 4 is increased by means of the heating device 6 from a point in time denoted with t.sub.1 on FIG. 3. In the exemplary embodiment, a gas flow with a low mass flow rate Q.sub.1 of the process gas (for example, methane during the deposition of graphene) can be fed into the process chamber. The mass flow rate Q.sub.1 is lower than a mass flow rate sufficient to cause layer growth. However, it can also be provided that the substrate 4 only be heated in the presence of a carrier gas, for example argon, and the process gas only be switched on at a later point in time.

[0037] After the substrate surface has reached the process temperature T.sub.P, which can lie above 1000° C., the mass flow rate of the process gas is continuously or incrementally linearly or nonlinearly increased. The surface of the substrate 4 is here observed by means of the pyrometer 9. The measuring curve initially runs along a straight line, until the gradient of the measuring curve changes by rising. At the point in time t.sub.2 where the rise in the measuring curve is detected, the mass flow rate Q.sub.2 of the process gas is stored. A third mass flow rate Q.sub.3 is calculated by adding a prescribed value to the second mass flow rate Q.sub.2. The mass flow rate is then increased up to the third mass flow rate Q.sub.3. This third mass flow rate Q.sub.3 is maintained for the layer growth. The prescribed value by which the mass flow rate is increased beyond the second mass flow rate Q.sub.2 or the difference between the third mass flow rate Q.sub.3 and second mass flow rate Q.sub.2 can measure 20 percent of the second mass flow rate Q.sub.2.

[0038] Layer deposition continues until such time as a second event is determined while observing the measuring curve 26, in which the measuring curve rises again after a preceding drop in the gradient of the measuring curve 26. This event takes place at time t.sub.4, and is taken as a reason for switching off the supply of process gas.

[0039] A silicon carbide-coated susceptor can be used during the deposition of hBN. Among others, NH.sub.3 is used as a reactive gas of the process gas in prior art. This gas acts on uncoated graphite. On the other hand, silicon carbide reacts with hydrogen at substrate temperatures in excess of 1300° C. Borazine (B.sub.3N.sub.3H.sub.6) can be used as the reactive gas. This makes it possible to deposit hBN at temperatures ranging between 1400° C. and 1500° C. A noble gas, for example argon, is used as the carrier gas or inert gas.

[0040] The growth rate with a prescribed speed depending on the increase in mass flow rate from the second to third mass flow rate is increased as growth starts from a very low value to a higher value with the method according to the invention. This makes it possible to control the initial growth, in particular of graphene, and reduces the number of germination sites, thereby raising the quality of the two-dimensional graphene layer.

[0041] The method according to the invention relates to all material pairs mentioned at the outset, and in particular to the deposition of two-dimensional heterostructures.

[0042] The above statements serve to explain the inventions covered by the application as a whole, which each also independently advance the prior art at least by the following feature combinations, wherein two, several or all of these feature combinations can also be combined, specifically

[0043] A method, characterized in that a gas flow with a first mass flow rate Q.sub.1 of the process gas is initially fed into the process chamber 3 while heating or after heating the substrate 4 to the process temperature T.sub.P, wherein no layer growth takes place on the surface of the substrate 4, after which the mass flow rate is increased during observation of the substrate surface until layer growth starts at a second rate Q.sub.2, and the mass flow rate is then increased to a third rate Q.sub.3 corresponding to the sum of the second rate Q.sub.2 with a prescribed value, and the layer is deposited at the third rate Q.sub.3.

[0044] A use, characterized in that a gas flow with a first mass flow rate Q.sub.1 of the process gas is initially fed into the process chamber 3 while heating or after heating the substrate 4 to the process temperature T.sub.P, wherein no layer growth takes place on the surface of the substrate 4, after which the mass flow rate is increased during observation of the substrate surface until layer growth starts at a second rate Q.sub.2, and the mass flow rate is then increased to a third rate Q.sub.3 corresponding to the sum of the second rate Q.sub.2 with a prescribed value, and the layer is deposited at the third rate Q.sub.3.

[0045] A method or use, characterized in that an optical device 19 is used or provided on the CVD reactor 1 for observing the substrate surface.

[0046] A method or use, characterized in that the optical device 19 is a pyrometer and/or a two-wavelength pyrometer.

[0047] A method or use, characterized in that a measuring curve 26 of the optical device 19 recorded while observing the substrate surface is evaluated to determine when layer growth starts and/or that the start of layer growth is determined by detecting a change in the gradient of the measuring curve 26 of the optical device 19, wherein the change in particular is a rise or a drop.

[0048] A method, in which the measuring curve is used to determine the number of deposited layers and/or the number of deposited layers is determined by ascertaining the number of maximums or minimums in the measuring curve.

[0049] A method or use, characterized in that the prescribed value is greater than 0 and/or is at least 5 percent of the second mass flow rate Q.sub.2, or at least 10 percent of the second mass flow rate Q.sub.2, or at least 20 percent of the second mass flow rate Q.sub.2.

[0050] A method or use, characterized in that the gas inlet element 2 has a gas outlet surface 25, which extends over a support surface 15 of the susceptor 5 and has a plurality of uniformly distributed gas outlet openings 14, 24 that are connected with a gas distribution volume 11, 21 in terms of flow.

[0051] A method or use, characterized in that the gas outlet surface 25 is comprised of a gas outlet plate 9 of the gas inlet element 2, which is adjoined by a cooling chamber 8 through which a coolant flows.

[0052] A method or use, characterized in that a beam path 18 of the optical device 19 passes through the gas inlet element 2 and/or that a cover plate 16 of the gas inlet element 2 has a window 17 transparent for the used wavelengths, and a tube 12′ through which the beam path 18 passes empties into the gas outlet surface 25.

[0053] A method or use, characterized in that a distance between a support surface 15 of the susceptor 5 and the gas outlet surface 25 is changed during deposition.

[0054] A method or use, characterized in that the process gas is generated by passing a carrier gas through a bubbler 32, 32′ containing a solid or liquid starting material.

[0055] A method or use, characterized in that a gas concentration measuring device 31, 31′ is used downstream from the bubbler 32, 32′ to determine the concentration of vapor of the starting material in the carrier gas.

[0056] A method or use, characterized in that the surface is further observed and/or the measuring curve 26 is further evaluated during layer deposition, so as to switch off the process gas if an event arises, and/or that the gas flow of the process gas is switched off when a change in the gradient of the measuring curve 26 is detected, wherein the change in particular is a rise or a drop.

[0057] All disclosed features (whether taken separately or in combination with each other) are essential to the invention. The disclosure of the application hereby also incorporates the disclosure content of the accompanying/attached priority documents (copy of the prior application) in its entirety, also for the purpose of including features of these documents in claims of the present application. Even without the features of a referenced claim, the subclaims characterize standalone inventive further developments of prior art with their features, in particular so as to submit partial applications based upon these claims. The invention indicated in each claim can additionally have one or several of the features indicated in the above description, in particular those provided with reference numbers and/or indicated on the reference list. The invention also relates to design forms in which individual features specified in the above description are not realized, in particular if they are recognizably superfluous with regard to the respective intended use, or can be replaced by other technically equivalent means.

TABLE-US-00001 Reference List  1 CVD reactor  2 Gas inlet element  3 Process chamber  4 Substrate  5 Susceptor  6 Heating device  7 Gas outlet element  8 Cooling chamber  8′ Feed line  8″ Discharge line  9 Gas outlet plate 10 Feed line 11 Gas distribution chamber 12 Tube 12′ Tube 13 Intermediate plate 14 Gas outlet opening 15 Support surface 16 Cover plate 17 Window 18 Beam path 19 Optical device, pyrometer 20 Feed line 21 Gas distribution chamber 22 Tube 23 Intermediate plate 24 Gas outlet opening 25 Gas outlet surface 26 Measuring curve 27 Peak 27′ Peak 28 Mass flow rate 29 Control 30 Mass flow controller 30′ Mass flow controller 31 Concentration measuring device 31′ Concentration measuring device 32 Bubbler 32′ Bubbler 33 Switching valve 33′ Switching valve 34 Run line 34′ Run line 35 Vent line 36 Valve 36′ Valve 37 Mass flow controller 37′ Mass flow controller 38 Valve 38′ Valve 39 Inert gas source 39′ Inert gas source 40 Reactive gas source 40′ Reactive gas source 41 Mass flow controller 41′ Mass flow controller Q.sub.1 Mass flow rate Q.sub.2 Mass flow rate Q.sub.3 Mass flow rate T.sub.P Process temperature h Process chamber height, distance t.sub.1 Point in time t.sub.2 Point in time t.sub.3 Point in time t.sub.4 Point in time