A method for using catalyst in growth of semiconductors comprising n-and p- atoms and device for the method

Abstract

A semiconductor material of nitride-type in a gas phase epitaxial process in a growth chamber is grown on a wafer (1 ) on a susceptor whereby process gases (2) flows over the wafer, where one of the process gases consists of ammonia and one of phosphine (PH3) or arsine (AsH3), where the ammonia gas is forced to flow through a metallic catalyst (4, 7) before reaching the wafer (1 ).

Description

A METHOD FOR USING A CATALYST IN GROWTH OF SEMICONDUCTORS COMPRISING N- AND P-ATOMS AND A DEVICE FOR THE METHODTechnical field

[0001] The present invention relates generally to a method for using ammonia (NH3) or phosphine (PH3) in growth of semiconductors of for example nitride-type, where a catalyst is used to force the gases, ammonia or phosphine, closer to thermal equilibrium and make them more reactive and thereby increase the growth rate of a grown epitactic layer of nitride semiconductors as well as decrease the consumption of ammonia or phosphine at a gas phase epitaxy.

[0002] When manufacturing for example a semiconductor material with nitrogen or phosphorus as an element, for example various nitrides (GaN, GaPN, AIPN), via gas phase epitaxy, ammonia (NH3) is used as a nitrogen source. In a process used in such manufacturing, ammonia is split up into its elements whereby the hydrogen atoms are released from the nitrogen atom in an ammonia molecule.The problem is that a great amount of ammonia must be used in such process. It is only a few percent of added ammonia that actually take part in the gas phase epitaxial process. A major part of the ammonia, i.e. the ammonia molecules added to the process goes straight through the process without reacting with other substances or split up. This leads to high consumption costs with high consumption of ammonia in previous known techniques. Further, costs arise when taking care of unused ammonia in the process at an outlet of the equipment that is used in the process.

[0003] The process of manufacturing nitride semiconductors is typically a batch process, meaning that one or more wafers are loaded in a growth chamber. Said growth chamber is heated up to just over 1000 °C at reduced pressure. Thereafter, process gases are streamed over the surface of a wafer to achieve growth of an epitactic layer of desired material, in this case nitride semiconductors, such as AIGaN, InAIGaN, InAIGaPN and InGaN.

[0004] The use of catalysts in the production of semiconductors is described in the document: CATALYST ROLE IN CHEMICAL VAPOR DEPOSITION (CVD) PROCESS: A REVIEW by H.U. Rashid, K.Yu, M.N. Umar, M.N. Anjum, K.Khan, N. Ahmad and M.T.Jan; Journal: Rev. Adv, Mater. Sci. 40, pages 235 - 248, Year 2015. In said document, the use of catalysts at low temperatures are pointed out. However, nothing is mentioned about investigating the possibility of using catalysts when manufacturing a Wide Band Gap semiconductor in HTCVD-processes where the temperature during growth reaches 800° C or higher. Nor is mentioned the possibility or need of using catalysts for the purpose of decreasing the consumption of ammonia during growth of nitride semiconductors. “Rashid” describes for instance how a catalyst is used to be able to lower the temperature in the process chamber when the produced material is temperature sensitive, i.e., far from the conditions that applies in HTCVD-processes.

[0005] Prior art is also disclosed in for example CN107740189. In CN107740189 no use of a catalyst is mentioned in order to decrease the consumption of ammonia during growth of a nitride semiconductor.Summary of invention

[0006] Epitactic layers of semiconductor nitrides such as Gallium Nitride (GaN) and Aluminium Nitride (AIN) are manufactured by growth at an elevated temperature in a growth chamber arranged in a reactor housing by means of a process which in this technical area is denoted as HTCVD (High Temperature Chemical Vapor Deposition). The epitactic layer is grown on a wafer that usually is arranged on a rotating susceptor in the growth chamber. Process gases for the growth are arranged to flow over the wafer. In growth of semiconductor nitrides, ammonia in gas form is used as one of the process gases to provide the necessary nitrogen-atoms for building up the semiconductor. The temperature in the HTCVD-reactor reaches over 800° C, but temperatures down to 600° C can occur.

[0007] Many so-called MO-CVD reactors have high temperature gradients in the gas phase. For example the temperature of incoming gas to a growth chamber is +25° C, and on a short distance with high gas-flow rate, equilibrium in the chemical balance don’t have time to appear due to the slower course of the gas kinetics. One way of increasing the rate of the gas on its way towards kinetic equilibrium as a function of the temperature in the area of the incoming gases, is to use a catalyst. Another desired result is that the rate of the epitaxial growth process can be increased and thereby less amount of origin gas is needed and in such way the consumption of other active gases (for example TMGa) can be reduced. In such way the cost is decreased for every epitaxy layer.

[0008] According to one aspect of the invention, the invention is constituted by a method of growing a semiconductor material of nitride-type in a gas phase epitaxial process in a growth chamber, in which a semiconductor is grown on a wafer on a susceptor whereby the process gases flows over the wafer, where one of the gases consists of ammonia and where the ammonia gas is forced to flow through a metallic catalyst before reaching the wafer.

[0009] The purpose of the invention is to use a catalyst that split up and makes the ammonia more reactive, whereby a greater utilization of the ammonia is achieved. This also leads to less amount of ammonia that needs to be added to the process, but also that the growth rate of the epitactic layer increases with maintained quality on the grown layer.

[0010] When growing for example GaN and AIN according to prior art, the different process gases, including ammonia gas that are added to the process, are preheated separately to avoid that the gases react with each other in a disadvantageous way. However, this means that the gases, relatively close to the wafer to be coated with an epitactic layer, are partially separated, i.e., the ammonia gas and for example process gas to grow GaN and AIN. It is advantageous if the gases can be mixed. Depending on the geometry of a reactor enclosing the growth chamber, it is sometimes advantageously to locate the catalyst in such way that it partially catalyses the ammonia, but also contribute to mix the gases more efficiently, i.e., that the ammonia gas and other process gas are mixed effectively close to the wafer, i.e., at the injector outlet.

[0011] The method can also be used for gas phase epitaxy for other semiconductor materials of nitride-type where ammonia is used as a part in the process, for example in the growth of semiconductors such as AIGaPN and InAIGaPN.

[0012] The catalyst that is used in the method according to the invention, will eventually be coated with different depositions which leads to a decrease in the effectivity of the process. However, the catalyst may be regenerated by an etching gas that etches away such arising depositions. Another way of removing depositions is to heat them up to a high temperature. Such regenerations are suitably performed between two batches at said growth of epitactic layers of nitride semiconductors.

[0013] If the reactor utilized is designed in such way that the incoming gases are mixed effectively by another way than by the catalyst, it is advantageous to place the catalyst in the part of the gas inlet of the growth chamber where the gases still remain separated. In this way it is prevented that the catalyst is coated, which results in a decrease of efficiency of the catalyst in the growth process.

[0014] The catalyst can be geometrically designed according to various alternatives. According to a first alternative, in which the catalyst is intended to contribute to a more efficient mixing of the process gases, the elements of the catalyst are designed to create a local turbulence at these elements. To achieve this, a plate of graphite or other suitable material that does not react with any of the process gases is arranged inside the injector along the bottom of the injector. On the plate, several rods of the catalyst material are arranged in the flow path of the process gases, where these rods reach up from the plate at the bottom of the injector up to the roof of the injector.

[0015] As such, the rods are distributed over the whole width of the injector and divided in one or more rows. By dividing them in multiple rows, a larger space is achieved between the rods, and the gas can pass through easier. The location of the rods is so decided that the gas is affected in a desired extent, which is dependent on the remaining geometries of the reactor. When the rods arearranged in multiple rows, they will be given different temperatures. Since a certain temperature is optimal, it is advantageous to keep together the formation and let the distance between two rows be approximately 1 - 2 times the diameter of the rods.

[0016] The rods are made of a material that acts as a catalyst in the method mentioned. The catalyst is hereby constituted by for example palladium or platina. Moreover, the material must be chosen not to be affected by the gases intended to be used when cleaning the wafer, the catalyst or the walls and roof of the growth chamber.Brief description of drawingsThe invention is now described, by way of example, with reference to the accompanying drawings, in which:Fig. 1 schematically shows a principle sketch of the gas flow in a reactor during growth;Fig. 2 illustrates the changes in temperature in a chart showing the heating of the gas,Fig. 3 shows the changing of gas temperature in the same way as in figure 2, where the injector and the wafer are displayed from above,Fig. 4 displays an alternative embodiment of the gas flow with separated gases, where the temperature changes of the gas is illustrated as in figures 2 and 3,Fig. 5 shows a catalyst designed as a honeycomb where gas can flow along the channels attached to each other according to the cross section that is shown in the honeycomb pattern,Fig, 6 indicates an embodiment for assembly of catalysts where rods of catalyst material are arranged in rows on a plate which is placed inside the injector (only a few of the rods occurring on the plate are shown in the figure),Fig. 7 shows the embodiment from figure 6 in a view from the side, where the rods are arranged in rows,Fig. 8 shows the rows of rods according to figure 7, where it appears that the rods of catalyst material are arranged so close that the flow of ammonia gas cannot pass by unaffected (only a fraction of occurring rods are shown on the plate in the figure).Description of embodiments

[0017] In the following, a detailed description of several embodiments of the invention is given, with support of the attached figures. The figures only show the schematical principles of the device and is not made to scale showing any proportions between different elements of the device.

[0018] Flere is disclosed an embodiment of a device according to the invention. Figure 1 shows an example of the principle for growth of an epitaxial layer on a wafer 1 in a reactor of FITCVD type (the complete reactor is not shown). Flere, process gas 2 is introduced from the left in the figure. The process gas 2 flows thereafter in through an injector represented by reference number 3. In the injector, the gas is heated up to the process temperature for growth and the gas flow is formed. When the process gas flows out of the injector, it continues to flow over the surface of the wafer 1 on which the epitaxial layer is built up by crystallization of atoms in the process gases used in the process.

[0019] One embodiment according to a first alternative embodiment shown in figure 2. In figure 2 the same set of components is shown as in figure 1. Figure 2 shows that a catalyst 4 according to the invention is installed inside the injector 3.It is also shown in the figure how the temperature of the process gas is changed as they flow through the injector towards the wafer. The temperature T is indicated along the vertical axis in the diagram, while the position of the gas along the device is indicated with x. The temperature of the gases at different positions along its way towards the wafer is shown in the curve in the diagram. The temperature of the gas mixture increases during its passage through the injector,but when the gas mixture exits the injector, it has reached the process temperature which is shown by the dashed line in the diagrams of figure 2 and 3. The catalyst is situated in a place where the temperature is high enough for the catalyst to split up the gas. The location is depending on the geometry of the injector but there is always present a position where the temperature is high enough.

[0020] Figure 3 shows the prevailing conditions of the process in the reactor, in which the gas flow is shown from the top of the injector 3 and the wafer 1.

[0021] According to a second alternative embodiment, the gas flow is formed according to figure 4. In figure 4, the process gases are brought through the injector 3 in separated channels 5, 6, where ammonia gas 2a flow through one of the channels 5, in which the catalyst, denoted with 7 in this case, is situated. Remaining process gases 2b brought to the wafer 1 via a second channel 6 in the injector 3. The catalyst 7 may in this case be formed as a honeycomb according to figure 5 to achieve the larger surface at the catalyst as mentioned earlier, at which the gas can react. When the catalyst consists of a honeycomb, the channels have a hexagonal cross section and are adjacent to each other. The channels are located in the direction of the gas flow.

[0022] Depending on the geometric design in general, the catalyst is designed in different ways. For the catalyst 3, 7 to contribute to a more efficient mixing of the incoming gases, it is designed to create local turbulence. This is achieved in one embodiment by designing the catalyst according to figure 5. A plate 8 of graphite or other material that does not react with the gases in the process gas, is placed inside the injector 3 on its bottom. In the plate 8 are several rods 9 assembled, and the rods are given a height for reaching from the plate 8 up to the roof 10 of the injector (see figure 2). Across the flow direction of the gases, is a field 11 with holes for the rods in the plate as shown. Only a few assembled rods 9 of those arranged in the field 11 are shown in the figure.

[0023] The rods 9 are distributed over the width of the injector 3 and distributed into one or more rows. By distributing the rods on multiple rows, a larger space isachieved between the rods, and the gas that actively will be affected by the catalyst can pass through the distributed rods in an easier way. The placement of the rods is made in a way that the gas is affected in a desired extent, which is dependent on the remaining geometry of the reactor. When the rods are arranged in multiple rows, they will have different temperatures. Since a certain temperature is optimal for splitting up the ammonia (optimized for the growth rate) it is preferred to keep together the formation of rods 9 and have the distance in the direction of flow between two rows of rods be approximately 1 - 2 the diameter of the rods.

[0024] The rows in one formation of catalyst rods 9 is shown on the plate 8 in a side view in figure 7.

[0025] The rods 9 are made of a material acting as a catalyst for the gas that is used in the growth of nitride-semiconductors according to the inventive aspect, for example palladium or platinum. The catalyst material must also be chosen so it is not affected by the gases intended to be used when cleaning the catalyst, the wafer or the ceilings and roof of the growth chamber.

[0026] In figure 8 is shown a view of the rods 9 of the catalyst, in a formation on the plate 8 seen upstream in the flow direction of the gases. Here it is apparent that the rods 9 are arranged so narrow that the gas stream cannot pass through the catalyst anywhere unaffected. This is accomplished by that the rods 9 of respective row of catalyst rods in a formation are offset in relation to each other seen sideways in such way that no longitudinal free ways for the gas appears. In the figure only a fraction of rods 9 along the width of the plate 8 are shown to correspond to the rods shown in figure 6, 7 and 8.

Claims

CLAIMS1. A method of growing a semiconductor of nitride-type by a gas phase epitaxial process in a growth chamber wherein a semiconductor is grown on a wafer (1) on a susceptor, whereby process gases (2) flows over the wafer, where one of the process gases consists of ammonia and one of phosphine (PH3) or arsine (AsH3), characterized by the step of:- forcing the ammonia gas to flow through a metallic catalyst (4, 7) before reaching the wafer (1).

2. Method according to claim 1 , further comprising the step of:- heating the ammonia gas together with other process gases to a temperature higher than 600 °C in a HTCVD process, alternatively higher than 800 °C in a HTCVD process.

3. Method according to claim 1 , further comprising the step of placing the catalyst (4, 7) in an injector (3) through which the ammonia gas is brought forward on the way to the wafer (1 ).

4. Method according to claim 3, further comprising the step of placing the catalyst (4, 7) adjacent the outlet of the injector (3).

5. Method according to claim 4, further comprising the step of bringing the ammonia gas forward via a first channel (5) inside the injector (3) to the wafer (1 ) while at least the process gases (2b) reacting with ammonia (2b) is channelled to the wafer via a second channel (6) inside the injector (3).

6. A device for performing the method according to claim 3, characterized in that the catalyst (4, 7) is located inside the injector (3) across the flow path of the process gas.

7. Device for performing the method according to claim 6, characterized in that the catalyst (4, 7) has the shape of a number of rods (9) placed in rows on a field (11) arranged on a plate (8) located at the bottom of the injector (3), wherein the rows of rods (9) are arranged across the flow path of the gases and where the rods (9) extends from the plate (8) up to the roof (10) of the injector (3).

8. Device according to claim 6, characterized in that the injector (3) comprises a first channel (5) and a second channel (6), wherein the catalyst (7) is located in the first channel (5) in which the ammonia gas flows, while the process gases that reacts with the ammonia flows in the other channel (6).

9. Device according to claim 6, wherein the catalyst (4, 7) has the shape of a honeycomb in which channels with hexagonal cross section are adjacent to each other and located in the direction of the gas flow inside the injector (3).

10. Device according to any of the previous claims, where the catalyst consists of any of the metals Palladium or Platinum, or any other metal having the property that it is acts as catalyst for the decomposition of ammonia.

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