Method of providing a process gas for a CVD reactor and corresponding apparatus

Abstract

The invention relates to an arrangement for provision of a process gas for a CVD reactor, having a first carrier gas feed with a first mass flow regulator having a small flow range and a second carrier gas feed with a second mass flow regulator having a large flow range, each of which is fed by a carrier gas source, wherein the carrier gas feed opens out in an evaporation device for evaporation of a solid or liquid starting material, from which there originates a process gas feed to the CVD reactor, wherein, disposed downstream of the second mass flow regulator in the second carrier gas feed with the second mass flow regulator, there is a valve arrangement that has an inlet and a first outlet having flow connection to the evaporation device and having a second outlet having flow connection to a bypass conduit that does not open into the evaporation device, wherein, by switching of the valve arrangement, the mass flow of carrier gas regulated by the mass flow regulator electively flows through or past the evaporation device, and to a method of operating such an arrangement.

Description

Description Method for providing a process gas for a CVD reactor and related device field of technology

[0001] The invention relates to a method for providing a process gas for a CVD reactor, comprising a first carrier gas supply line, each fed by a carrier gas source, with a first mass flow regulator having a small flow range, and a second carrier gas supply line with a second mass flow regulator having a large flow range. The carrier gas supply line opens into an evaporation device for evaporating a solid or liquid starting material originating from a process gas supply line to the CVD reactor. A valve is arranged in the second carrier gas supply line, which has the second mass flow regulator and has an inlet and a first outlet fluidically connected to the evaporation device. The invention further relates to a device suitable for this purpose. State of the art

[0002] DE 10 2021 117457 A1 describes an arrangement for providing a process gas for use in one or more CVD reactors. A vaporization device is provided, which has a supply line through which a carrier gas can flow from a carrier gas source into a container of the vaporization device. The carrier gas supply line is connected to two mass flow controllers arranged in parallel, which can have different flow ranges. A shutoff valve is arranged upstream of the mass flow controller in the supply line to the mass flow controller, which has a larger value range. The mass flow controller can be selectively activated by means of this shutoff valve.

[0003] DE 10 2019117543 A1 discloses a gas mixing system of a substrate treatment device, in particular an MOCVD reactor. The carrier gas supply line is connected to a mass flow controller. Downstream of the mass flow controller, a valve is arranged with which the supply line to the evaporation device can be closed. With a further valve, the mass flow controller can be brought into flow connection with a bypass line for calibration, which opens into the gas outlet of the reactor.

[0004] US 10,109,483 B2 discloses an MOCVD reactor system with an evaporation vessel of an evaporation device containing a liquid precursor. A carrier gas mass flow, controlled by a mass flow controller, flows from a gas source into the evaporation vessel through a carrier gas supply line. A valve is arranged downstream of the mass flow controller. When this valve is closed, the carrier gas can flow through a bypass line connected to the process gas supply line via a three-way valve.

[0005] US 2014 / 0083512 A1 describes an evaporation source with two supply lines, each of which can feed nitrogen into an evaporation chamber. A process gas line emerges from the evaporation chamber, which can transport steam from the evaporation chamber to a process chamber of a CVD reactor. The two supply lines are each connected to the process gas line via a bypass line.

[0006] US 2017 / 0362701 A1 describes an evaporation source with several evaporation chambers that are part of a central gas supply. A supply line leads into each of the evaporation chambers, which is connected via a bypass line can be connected to a drain if one of the evaporation chambers needs to be changed.

[0007] DE 10 2020 103822 A1 describes a device for vaporizing a powder, wherein two supply lines lead into a vaporization chamber. One of the two supply lines has a valve arrangement with which a carrier gas flow fed into the vaporization chamber in a first operating position can be guided around the vaporization chamber in a second operating position. Summary of the invention

[0008] In order to realize multiple flow ranges for a carrier gas mass flow flowing through an evaporation vessel of an evaporation device of a CVD reactor, the prior art involves connecting a second mass flow controller covering a larger flow range in parallel with a first mass flow controller covering a small flow range. The second mass flow controller can be connected to the first mass flow controller via an upstream valve. To reduce the carrier gas mass flow and limit it back to the flow range of the first mass flow controller, the second mass flow controller is deactivated with the valve. For this purpose, the second mass flow controller is regulated to a minimum flow. The volume between the mass flow controller and the shut-off valve is maintained at overpressure.Due to the resulting unavoidable dead volume, a carrier gas mass flow, which diminishes over time, continues to flow into the evaporation vessel of an evaporation device even after inactivation, in addition to the carrier gas mass flow flowing through the first mass flow controller. This leads to a significant distortion of the gas feed into the evaporation vessel, especially at low carrier gas mass flows.

[0009] The invention is based on the object of implementing measures that enable a rapid and precise switch from a high carrier gas mass flow to a low carrier gas mass flow. In particular, the invention is based on the object of providing a method by which a precisely controlled process gas flow is supplied to one or more CVD reactors.

[0010] The problem is solved by the invention specified in the claims, whereby the subclaims not only represent advantageous developments of the technical solutions specified in the main claim, but are also independent solutions to the problem.

[0011] First and foremost, the invention proposes that a valve arrangement, preferably a switching valve, be arranged downstream of the second mass flow controller. With the switching valve, the carrier gas mass flow controlled by the second mass flow controller can optionally flow through the evaporation vessel in addition to the carrier gas mass flow controlled by the first mass flow controller or be guided past the evaporation device. The switching valve has an inlet through which the carrier gas mass flow controlled by the second mass flow controller flows in. The carrier gas mass flow can either flow out of the switching valve through a first outlet and into the evaporation device or flow out of the switching valve through a second outlet and past the evaporation device. The second outlet can be fluidly connected to a bypass line.The bypass line may be connected to the gas outlet of an evaporation vessel of the evaporation device or directly connected to one or more process gas supply lines leading into one or more CVD reactors.

[0012] The valve arrangement can comprise two valves. A first valve can be arranged downstream of the second mass flow controller, and a second valve can be arranged in the bypass line. The bypass line can branch off from a section of the second carrier gas supply line located downstream of the second mass flow controller and upstream of the first valve. The valves can be selectively opened or closed. In a first method step, the first valve is open and the second valve is closed. In a second method step, the first valve is closed and the second valve is open. It is provided that the carrier gas mass flow controlled by the second mass flow controller cannot flow through the evaporation vessel and the bypass line simultaneously.

[0013] To prevent the first and second valves from being opened simultaneously, the first and second valves can be switched with a time delay. When switching from the first to the second process step, the first valve can be closed first, followed by a time delay when the second valve opens. When switching from the second to the first process step, the second valve can be closed first, followed by a time delay when the first valve opens.

[0014] To prevent the carrier gas mass flow flowing through the first mass flow controller from briefly bypassing the evaporation device, for example, into the bypass line, during switching between the two process steps, the valves can have throttle openings. The throttle openings can be designed in such a way that a certain pressure difference is maintained between the inlet pressure and the outlet pressure of a valve when switching between the first and second process steps. This prevents the carrier gas mass flow controlled by the first mass flow controller from bypassing the evaporation device. device through the bypass line. The cross-sectional constriction of the throttle openings can be variable. The cross-sectional constriction can be achieved by means of a throttle device, such as a flap. The throttle valve can be an electric throttle valve. The cross-sectional constriction can be regulated by a control device. However, the throttle valve can also be controlled manually. Furthermore, the throttle valve can be pneumatically operated. The throttle valve can, for example, be a screw-in throttle valve or a butterfly valve.

[0015] With a preferably electronic pressure regulator, the total pressure within the gas mixing system can be kept constant during switching between the first and second process steps of the switching valve. The use of a switching valve makes it possible to keep the carrier gas mass flow permanently active through the second mass flow regulator, thus avoiding deactivation of the second mass flow regulator. As a result, there are no significant deviations between the actual value and the setpoint value of the carrier gas mass flow through the evaporation vessel during the switching process. As a result of the arrangement according to the invention, the pressure regulator only has to compensate for minor fluctuations if the total carrier gas mass flow through the evaporation vessel changes during the switching process, since the total flow in the gas outlet does not change due to the refeeding of the carrier gas mass flow bypassing the evaporation device.

[0016] The mass flow controllers have a control loop that regulates a mass flow according to a specified setpoint. The mass flow controllers can be operated with different setpoints. For example, the setpoint can be gradually decreased or increased. This is preferably controlled by a control device. The setpoints can, for example, be determined by a Recipe provided. The switching valve can also be controlled by a control device.

[0017] By connecting several mass flow controllers with different flow ranges connected in parallel to each other, a large flow range can be covered by selectively switching them into the carrier gas mass flow through the evaporation vessel.

[0018] The one or more mass flow controllers connected in parallel form a mass flow controller device. The mass flow controller device is arranged upstream of an evaporation vessel of the evaporation device, which contains a liquid or solid starting material, for example, an organometallic compound for the deposition of layers consisting of GaN, GaAs, InP, and / or SiC. This compound is converted into a vapor by applying heat, which is transported into a CVD reactor with the carrier gas mass flow. The evaporation vessel can be temperature-controlled. It can be provided that the evaporation vessel is continuously refilled from a storage container.

[0019] In one variant of the invention, the carrier gas mass flow provided by a carrier gas source is distributed among several mass flow controllers, each of which is fluidly connected to an evaporation device. The carrier gas mass flow is directed from each of the mass flow controllers into an evaporation vessel. However, the carrier gas mass flow can also be partially guided past the evaporation vessel. The carrier gas mass flow conveys the vapor generated in the evaporation vessel from a gas outlet of the evaporation device into a process gas supply line. The process gas supply line opens into one or more process chambers of one or more CVD reactors.

[0020] In a further variant of the invention, a carrier gas mass flow from several different carrier gas sources flows into one or more different mass flow controllers, each of which can be connected to an evaporation device. Thus, different carrier gases can be used. The carrier gas can be an inert gas, for example, nitrogen or hydrogen.

[0021] With the device described above, the carrier gas mass flow from a carrier gas source through an evaporation device can be switched from a high carrier gas mass flow to a low carrier gas mass flow. The carrier gas mass flow from the carrier gas source flows through the mass flow regulator device. In the mass flow regulator device, the carrier gas mass flow is divided between at least two mass flow regulators, with the carrier gas mass flow continuously flowing through both mass flow regulators. By closing the first valve and opening the second valve of the switching valve, the carrier gas mass flow flowing through the second mass flow regulator can flow into the bypass line and thus bypass the evaporation device arranged downstream of the mass flow regulator device.As a result, the carrier gas mass flow flowing through the evaporation vessel of the evaporation device is controlled only by the first mass flow controller, which can have a smaller flow range than the second mass flow controller. The carrier gas mass flow is conveyed from the bypass line into the process gas supply line. During the switchover process, the opening / closing of the first valve can be delayed relative to the opening / closing of the second valve to ensure that both valves are not open simultaneously. This prevents the carrier gas mass flow flowing through the first mass flow controller from flowing directly into the process gas supply line via the bypass line, rather than first flowing through the evaporation vessel as intended.

[0022] With the method according to the invention, switching between high and low carrier fluxes can occur during operation of the CVD reactor, for example, during a layer growth process. For this purpose, the switching time is minimized such that the layer composition does not change uncontrollably during the switching process. Short description of the drawings

[0023] The invention is explained in detail below with reference to the accompanying drawings. They show: Fig. 1 is a circuit diagram of a first embodiment in which a CVD reactor 1 is fed by a process gas supply system 19, Fig. 2 is a circuit diagram of a second embodiment of the invention, in which a process gas supply system 19 feeds several CVD reactors, Fig. 3 is a circuit diagram of a third embodiment in which a CVD reactor 1 is fed by a plurality of process gas supply systems 19, 19', 19", Fig. 4 is a circuit diagram of a fourth embodiment of a process gas supply system 19, in which a central carrier gas supply line is connected to several process gas supply systems, each feeding a CVD reactor 1. Description of the embodiments

[0024] In the process gas supply systems 19, 19', 19" shown in Figures 1 to 4, a carrier gas is fed into an evaporation vessel 17 of an evaporation device 6. The carrier gas, for example nitrogen, hydrogen or a noble gas, is saturated with the vapor of the solid or liquid starting material 24 stored in the evaporation vessel 17 and leaves the evaporation vessel 17 through a discharge line.

[0025] The supply and discharge lines of the evaporation vessel 17 are connected to a switching device 18 which has a plurality of valves which can be switched such that a carrier gas mass flow supplied by a mass flow controller arrangement comprising one or more mass flow controllers 3, 5, 5' flows either through the evaporation vessel or past the evaporation vessel.

[0026] In the embodiments shown in Figures 1 to 4, one or more process gas supply lines 7, into which the vapor of a starting material 24 conveyed by the carrier gas mass flow from the evaporation device 6 is fed, open into one or more CVD reactors 1, 1', 1".

[0027] The process gas supply systems 19, 19', 19" are used to provide a metal-organic starting material 24 for a CVD reactor 1, 1', 1", in which one or more substrates 20 rest on a heated susceptor 21, which forms the floor of a process chamber 22.

[0028] The embodiments show a pressure regulator 16. The pressure regulator keeps the total pressure in the process gas supply system 19, 19', 19" constant.

[0029] The arrangement shown in Figure 1 for providing a process gas comprises two mass flow controllers 3, 5 connected in parallel, which have different flow ranges from one another, for example, different by at least a factor of 2, 5 or 10. The mass flow controllers 3, 5, 5' regulate the carrier gas mass flow into the evaporation device 6. The first mass flow controller 3 can have a smaller flow range, for example 100 standard cubic centimeters, than the second mass flow controller 5, which can have a flow range of 1000 standard cubic centimeters, for example. The first mass flow controller 3 is arranged in a first carrier gas supply line 2 and the second mass flow controller 5 in a second carrier gas supply line 4. The carrier gas supply lines 2, 4 are connected to a central carrier gas source (not shown).From the carrier gas source, a first carrier gas mass flow flows through the first carrier gas supply line 2 and a second carrier gas mass flow flows through the second carrier gas supply line 4 into the evaporation device 6. The first carrier gas mass flow provided by the first mass flow controller 3 is continuously fed into the evaporation device 6. Using a valve arrangement 8 arranged downstream of the second mass flow controller 5, which is in particular a changeover valve, the second carrier gas mass flow provided by the second mass flow controller 5 can optionally also be fed into the evaporation device 6 or into a bypass line 12. For this purpose, the valve arrangement 8 comprises a first valve 13 and a second valve 14. The first valve 13 is arranged in the second carrier gas supply line 4. The second valve 14 is arranged in the bypass line 12.The bypass line 12 branches off upstream of the first valve 13 and flows into the process gas supply line 7 downstream of the evaporation vessel 17. A mass flow controller 15 is located in the process gas supply line 7, with which the mass flow of the process gas fed into the CVD reactor 1 through the process gas supply line 7 can be regulated. With such a device, the carrier gas mass flow fed into the evaporation device can be controlled quickly and precisely, in particular. The system can be switched from a high carrier gas mass flow to a low carrier gas mass flow, since the total mass flow of the gas flowing through the process gas supply line remains constant. As a result, the total pressure within the process gas supply system 19, measured by a pressure regulator 16, remains constant.

[0030] In a further variant of the invention, a switching valve can also be arranged in the first carrier gas supply line 2.

[0031] Figure 2 shows a further embodiment as a variant of the embodiment shown in Figure 1. Here, a further second mass flow controller 5' is arranged parallel to the first mass flow controller 3 and the second mass flow controller 5'. This has a flow range that is larger than the flow range of the first mass flow controller 3, but different from the flow range of the second mass flow controller 5. For example, the mass flow controller 5 can have a flow range that is at least a factor of 2 larger than the mass flow controller 3, and the mass flow controller 5' can have a flow range that is at least a factor of 2 larger than the flow range of the mass flow controller 5. This expands the flow range of the carrier gas mass flow flowing into the evaporation device. In addition, the mass flow of the process gas is split into several partial flows, each of which is directed to different CVD reactors 1, T, l zz, each leading to a CVD reactor 1, T, l zz leading process gas supply line 7, 7 Z , 7 ZZ an individual mass flow controller 15, 15 z , 15".

[0032] The embodiment shown in Figure 3 shows a variant of the embodiment shown in Figure 1. Here, a CVD reactor with several different process gas supply systems 19, 19 z , 19". The process gas supply systems 19, 19', 19" can be supplied by various carriers gas sources or a central carrier gas source. The mass flow of the process gas flowing through the process gas supply lines 7, 7', 7" can be controlled with a mass flow controller 15, 15', 15". The process gas mass flows flow into a gas inlet element of a CVD reactor 1, whereby the process gases can exit from various gas inlet openings of the gas inlet element.

[0033] In the embodiment shown in Figure 4, three process gas supply lines 7, 7', 7" connected to different process gas supply systems 19, 19', 19" each open into a gas inlet opening 23 of a CVD reactor 1, 1', 1". The process gas supply systems 19, 19', 19" are fed from a central carrier gas source.

[0034] The above statements serve to explain the inventions covered by the application as a whole, which each independently develop the state of the art by at least the following combinations of features, whereby two, several or all of these combinations of features can also be combined, namely:

[0035] An arrangement which is characterized in that the valve arrangement 8, 8' is arranged downstream of the second mass flow controller 5, 5' and has a second outlet 11, 11' in flow connection with a bypass line 12, 12' which does not open into the evaporation device 6, wherein by switching the valve arrangement 8, 8' the carrier gas mass flow regulated by the second mass flow controller 5, 5' optionally flows through the evaporation device 6 or is guided past the evaporation device 6.

[0036] An arrangement characterized in that the valve arrangement 8, 8' is a switching valve comprising a first valve 13, 13' arranged in the second carrier gas supply line 4, 4' and a second valve 14, 14' arranged in the bypass line 12, 12'.

[0037] An arrangement characterized in that the bypass line 12 opens into the process gas supply line 7.

[0038] An arrangement characterized in that the valves 13, 13', 14, 14' of the switching valve 8, 8' have throttle openings designed such that when the switching valve 8, 8' is switched, a positive pressure difference is obtained between an inlet pressure and an outlet pressure of the first valve 13, 13', 14, 14', so that a carrier gas mass flow controlled by the first mass flow controller 3 is not bypassed by the evaporation device 6, wherein the cross section of the throttle openings can be variably controlled.

[0039] An arrangement characterized in that a control device is provided which controls the changeover valve 8, 8' and the mass flow controllers 3, 5, 5'.

[0040] A CVD reactor arrangement characterized by an arrangement for providing a process gas according to any one of the preceding claims.

[0041] A method for providing a process gas, wherein a first mass flow controller 3 provides a first carrier gas mass flow and a second mass flow controller 5, 5' provides a second carrier gas mass flow, wherein in a first method step at least the second carrier gas mass flow flows through the evaporation device 6 and in a second process step the first carrier gas mass flow flows through the evaporation device 6 and the second carrier gas mass flow flows past the evaporation device 6, wherein the total carrier gas mass flow flowing through the evaporation device 6 is greater in the first process position than in the second process position.

[0042] A method which is characterized in that the first carrier gas flow flows through a first carrier gas supply line 2 and the second carrier gas flow through a second carrier gas supply line 4, 4', wherein by closing a first valve 13, 13' arranged in the second carrier gas supply line 4, 4' and opening a second valve 14, 14' arranged in a bypass line 12, 12' branching off from the second carrier gas supply line 4, 4', the carrier gas mass flow controlled by the second mass flow controller 5, 5' flows through the bypass line 12, 12' and by opening the first valve 13, 13' and closing the second valve 14, 14', the carrier gas mass flow flows via the second carrier gas supply line 4, 4' into the evaporation device 6.

[0043] A method which is characterized in that the first valve 13, 13' and the second valve 14, 14' are switched with a time delay, wherein when switching from the first method step to the second method step the first valve 13, 13' is closed and subsequently the second valve 14, 14' is opened and when switching from the second method step to the first method step the second valve 14, 14' is closed and subsequently the first valve 13, 13' is opened.

[0044] All disclosed features are (individually, but also in combination with each other) essential to the invention. The disclosure of the application hereby also includes the disclosure content of the associated / attached priority documents. documents (copy of the prior application) are included in full, also for the purpose of incorporating features of these documents into claims of the present application. The subclaims characterize, even without the features of a referenced claim, with their features independent inventive developments of the prior art, in particular in order to make divisional applications based on these claims. The invention specified in each claim may additionally have one or more of the features provided in the above description, in particular with reference numbers and / or specified in the list of reference numbers. The invention also relates to designs in which individual features mentioned in the above description are not implemented, in particular insofar as they are clearly dispensable for the respective intended use or can be replaced by other technically equivalent means. List of reference symbols 1 CVD reactor 14' second valve 1' CVD reactor 15 mass flow controller 1" CVD reactor 15' mass flow controller 2 carrier gas supply lines 15" mass flow controller 3 first mass flow controller 16 pressure controller 4 Carrier gas supply line 17 Evaporation vessel 4' Carrier gas supply line 19 Process gas supply system 5 second mass flow controller 19' process gas supply system 5' second mass flow controller 19" process gas supply system 6 Evaporation device 20 Substrate 7 Process gas supply line 21 Susceptor 7' Process gas supply line 22 Process chamber 7' Process gas supply line 23 Gas inlet opening 7" process gas supply line 24 liquid feedstock 8 Valve arrangement 8' valve arrangement 9 Entrance 9' entrance 10 first exit 10' first exit 11 second exit 11' second exit 12 bypass line 12' bypass line 13 first valve 13' first valve 14 second valve

Claims

Claims 1. A method for providing a process gas for a CVD reactor (1, 1', 1"), wherein a first carrier gas supply line (2) and a second carrier gas supply line (4) open into an evaporation device (6) for evaporating a solid or liquid starting material, wherein a process gas supply line (7) to the CVD reactor (1, 1', 1") originates from the evaporation device (6), wherein a first mass flow controller (3) with a small flow range is arranged in the first carrier gas supply line (2) and a second mass flow controller (5, 5') with a large flow range is arranged in the second carrier gas supply line (4, 4'), wherein a solid or liquid starting material is evaporated in the evaporation device (6), wherein in a first method step, a first mass flow of a carrier gas fed from a carrier gas source into the first carrier gas supply line (2) and a second mass flow controller (5,5') controlled second mass flow of the carrier gas fed from the carrier gas source into the second carrier gas supply line (4, 4') flows through the evaporation device (6) into the process gas supply line (7), wherein in a second process step only the first mass flow controlled by the first mass flow (3) flows into the evaporation device (6) and the second mass flow is guided past the evaporation device (6) through a bypass line (12, 12') and conveyed into the process gas supply line (7), wherein the total carrier mass flow flowing through the evaporation device (6) in the first process step is greater than in the second process step.

2. Method according to claim 1, characterized in that when changing from the first method step to the second method step, a first valve (13, 13') arranged in the second carrier gas supply line (4, 4') is closed and a valve (13, 13') arranged in the second carrier gas supply line (4, 4') is closed. a second valve (14, 14') arranged in the branched bypass line (12, 12') is closed.

3. Method according to one of the preceding claims, characterized in that the first valve (13, 13') and the second valve (14, 14') are switched with a time delay, wherein first the first valve (13, 13') is closed and subsequently the second valve (14, 14') is opened.

4. Method according to one of the preceding claims, characterized in that when switching from the second method step to the first method step, the second valve is first closed and then the first valve (13, 13') is opened.

5. Method according to one of the preceding claims, characterized in that a variable cross-sectional constriction of a throttle opening in the valves (13, 13', 14, 14') prevents the first mass flow regulated by the first mass flow controller (3) from flowing into the bypass line (12, 12') when switching between the first and the second method step.

6. Device for carrying out a method according to one of the preceding claims, with a first carrier gas supply line (2) with a first mass flow controller (3, 3') having a small flow range and with a second carrier gas supply line (4, 4') with a second mass flow controller (5, 5') having a large flow range, each of which is fed by a carrier gas source and controlled by a control device, wherein the carrier gas supply lines (2, 4, 4') are connected to an evaporation device (6) for evaporating a solid or liquid a process gas feed line (7) to a CVD reactor (1, 1', 1") originating from the evaporation device (6), a valve arrangement (8, 8') controlled by the control device is arranged in the second carrier gas feed line (4, 4') downstream of the second mass flow controller (5, 5'), which valve arrangement has an inlet (9, 9'), a first outlet (10, 10') fluidly connected to the evaporation device (6), and a second outlet (11, 11'), characterized in that the second outlet (11, 11') is fluidly connected to a bypass line (12, 12') which is guided past the evaporation device (6) and opens into the process gas feed line (7).

7. Device according to claim 6, characterized in that the control device is arranged to carry out a method according to one of claims 1 to 5.

8. Device according to claim 6 or 7, characterized in that the valve arrangement (8, 8') is a switching valve that comprises a first valve (13, 13') arranged in the second carrier gas supply line (4, 4') and a second valve (14, 14') arranged in the bypass line (12, 12').

9. Device according to claim 6, 7 or 8, characterized in that the valves (13, 13', 14, 14') of the switching valve (8, 8') have throttle openings which are designed such that when the switching valve (8, 8') is switched over, a positive pressure difference is obtained between an inlet pressure and an outlet pressure at the first valve (13, 13') or at the second valve (14, 14'), so that a carrier gas mass flow controlled by the first mass flow controller (3) is not guided past the evaporation device (6).

10. Apparatus for carrying out a method according to one of claims 1 to 5, comprising a first carrier gas supply line (2) with a first mass flow controller (3, 3') having a small flow range and a second carrier gas supply line (4, 4') with a second mass flow controller (5, 5') having a large flow range, wherein the carrier gas supply lines (2, 4, 4') open into an evaporation device (6) for evaporating a solid or liquid starting material, wherein a process gas supply line (7) to a CVD reactor (1, 1', 1") originates from the evaporation device (6), wherein in the second supply line (4, 4') downstream of the second mass flow controller (5, 5') a valve arrangement (8, 8') controlled by a control device is arranged, which has an inlet (9, 9'), a first outlet fluidically connected to the evaporation device (6) (10, 10') and a second output (11, 11'), wherein the second output (11, 11') is connected to a bypass line (12,12') which is guided past the evaporation device (6) and opens into the process gas supply line (7), characterized in that the valves (13, 13', 14, 14') of the changeover valve (8, 8') have variably controllable throttle openings.

11. Method or device characterized by one or more of the characterizing features of one of the preceding claims.

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