Semiconductor device and method of manufacturing the same

Abstract

Disclosed are a semiconductor device and a method of manufacturing the same. The invention relates to a high electron mobility transistor (HEMT) based on nitrogen-polar GaN. The invention also relates to a method of manufacturing such a HEMT. According to the invention, the HEMT based on nitrogen-polar GaN comprises an AlGaN back barrier arranged on a buffer layer, a channel layer arranged on the buffer layer, an AlGaN cap layer arranged on the channel layer, and a second GaN layer arranged on the AlGaN cap layer. According to the invention, the channel layer comprises a first GaN layer and an AlGaN channel layer, wherein the AlGaN channel layer is arranged between the first GaN layer and the AlGaN back barrier. Furthermore, a Schottky contact is arranged on the first GaN layer.

Description

Technical Field

[0001] This invention relates to a high electron mobility transistor (HEMT) based on nitrogen-polarized GaN (gallium nitride). The invention further relates to a method for manufacturing such a HEMT. Background Technology

[0002] GaN-based HEMT devices are increasingly used in high-frequency, high-power applications, such as active components in base station amplifiers for mobile wireless communications. Recently, nitrogen-polar GaN-based HEMT devices have gained attention. In nitrogen-polar GaN-based HEMT devices, the polarization within the material is reversed compared to more conventional gallium-polar GaN-based HEMT devices.

[0003] The nitrogen-polar GaN-based HEMT device described in the preamble of claim 1 is known from the paper "Very Low Ohmic Contact Resistance through an AlGaN Etch-Stop in Nitrogen-Polar GaN-Based High Electron Mobility Transistors" published by Nidhi Brown et al. in the Japanese Journal of Applied Physics, Volume 49, Issue 2R, 2010. This known device has an epitaxial structure comprising a substrate, a buffer layer disposed on the substrate, and an AlGaN (galvanium aluminum nitride) back barrier disposed on the buffer layer. The epitaxial structure further comprises a GaN channel layer disposed on the buffer layer. In known nitrogen-polar GaN-based HEMTs, a two-dimensional electron gas (2DEG) or a three-dimensional electron gas (3DEG) is formed in the GaN channel layer.

[0004] To avoid relatively large gate leakage current, the channel layer of known HEMTs is covered by an AlGaN cap layer, on which Schottky contacts are formed. Furthermore, to obtain low-ohmic-resistance drain and source contacts, a second GaN layer is formed on the AlGaN cap layer, and these contacts are formed on this second GaN layer.

[0005] To establish the Schottky contact, the second GaN layer needs to be etched down to the AlGaN cap layer. This requires either a dry etching step at elevated temperatures or a KOH-based etching step. A problem associated with dry etching of the second GaN layer is the damage introduced into the epitaxial structure (i.e., the underlying AlGaN cap layer) during etching. This damage degrades the gate leakage performance of the Schottky contact. On the other hand, a problem with KOH-based etching of the second GaN layer is the lack of selectivity towards the underlying AlGaN cap layer, leading to difficulties in depth control. Summary of the Invention

[0006] The object of this invention is to provide a nitrogen-polar GaN-based HEMT with improved depth control and at least acceptable gate leakage current. This object is achieved by a nitrogen-polar GaN-based HEMT as defined in claim 1, characterized in that the channel layer further comprises a first GaN layer and an Al layer. a Ga (1-a) Al between N back barrier layers b Ga (1 -b) An N-channel layer is provided, and a Schottky contact is disposed on the first GaN layer.

[0007] According to the present invention, the AlGaN cap layer is not used to form the Schottky contact. Instead, this layer serves as an etch stop layer during the etching of the second GaN layer. For example, the second GaN layer can be etched using a fluorine-based dry etching process, while the underlying AlGaN cap layer is difficult to etch. Alternatively, the AlGaN cap layer itself can be oxidized first and then etched using a wet chemical etching process. Unlike dry etching, wet chemical etching does not etch or damage the first GaN layer. By additionally using an AlGaN channel layer, a relatively smooth transition between the AlGaN back barrier and the first GaN layer can be obtained, thereby improving the crystal quality of the epitaxial layer, i.e., fewer defects, and therefore less defect-related gate leakage current in the Schottky contact. Furthermore, the high selectivity provided by wet chemical etching enables precise depth control.

[0008] Al a Ga (1-a) N-back barrier, Al b Ga (1-b) Channel layer and Al c Ga (1-b) The Al content of the cap layer is correlated with each other. More specifically, 1>a>b≥c>0. In a particular embodiment, at least in Al... a Ga (1-a) N-back barrier and Al b Ga (1-b) At the junction between N-channel layers, ab ≥ 0.05, more preferably, ab ≥ 0.1.

[0009] Al a Ga (1-a) The Al content in the N-back barrier can be constant in the direction from the substrate to the second GaN layer. Optionally, Al a Ga (1-a) The Al content in the N-back barrier can be reduced in the direction from the substrate to the second GaN layer. This can help reduce the number of defects in the epitaxial structure by enabling a smooth transition between different band gaps within the structure.

[0010] Similarly, Al b Ga (1-b) The Al content in the N-channel can be constant in the direction from the substrate to the second GaN layer. Optionally, Al b Ga (1-b) The Al content in the N-channel layer can be reduced in the direction from the substrate to the second GaN layer. Furthermore, Al... c Ga (1-c) The Al content in the N-cap layer can be constant in the direction from the substrate to the second GaN layer, or Al c Ga (1-c) The Al content in the N-cap layer can be reduced in the direction from the substrate to the second GaN layer.

[0011] In a particular embodiment, the buffer layer comprises an undoped or unintentionally doped GaN layer.

[0012] The thickness of each layer in the epitaxial structure can be selected so that Al a Ga (1-a) The thickness of the N-back barrier ranges from 10 nm to 200 nm, Al b Ga (1-a) The thickness of the N-channel layer ranges from 5 nm to 30 nm, and the thickness of the first GaN layer ranges from 1 nm to 10 nm. Al c Ga (1-c) The thickness of the N-cap layer is in the range of 5 nm to 50 nm, and / or the thickness of the second GaN layer is in the range of 0 nm to 50 nm.

[0013] The concentration of n-type doping in each layer of the epitaxial structure can be selected to make Al a Ga (1-a) The concentration of n-type doping in the N-back barrier is between 0.1 and 1 times 10. 19 # / cm 3 Within the range, Al b Ga (1-b) The concentration of n-type doping in the N-channel layer is between 0 and 10 times. 19 # / cm 3 Within the range, Al cGa (1-c) The concentration of n-type doping in the N-channel layer is between 0 and 10 times. 19 # / cm 3 Within the range, the concentration of n-type doping in the first GaN layer is between 0 and 1 times 10. 19 # / cm 3 Within this range, the concentration of n-type doping in the second GaN layer is between 0.1 and 1 times 10. 19 # / cm 3 Within the range.

[0014] The substrate can be a silicon substrate, a gallium nitride substrate, a silicon carbide substrate, or a sapphire substrate. The buffer layer may include a GaN layer. Alternatively, AlN or AlGaN may be used. The thickness of the buffer layer is typically in the range of 0 to 10 micrometers, and the concentration of Si / Fe / C doping in the buffer layer is preferably less than 1:10. 19 # / cm 3 .

[0015] Ohmic contacts may comprise annealed metal layer stacks, such as Ti / Al / Ni / Au metal stacks. Alternatively, Ti / Al / Ti / TiN, Ti / Cu / Ti / TiN, or Ti / AlCu / Ti / TiN may be used. Similarly, Schottky contacts may comprise metal layer stacks, such as Ni / Au, Ni / Pt / Au, or TiN / AlCu / TiN.

[0016] According to a second aspect, the present invention provides a method for manufacturing a HEMT as defined above. The method includes the steps of: providing an epitaxial structure as described above; providing a first mask layer on a second GaN layer; and patterning the first mask layer, for example, using photolithography, to define a Schottky region. Here, the Schottky region is the region that will define a Schottky contact in a later stage.

[0017] The method according to the present invention further includes the following steps: opening the Schottky region of the first mask layer; and dry etching the second GaN layer through the opened first mask layer to expose Al. c Ga (1-c) N-cap layer. Next, the exposed Al c Ga (1-c) The N-cap layer is oxidized. This allows for the oxidation of the exposed Al in the next step. c Ga (1-c) The N-cap layer is etched. This etching includes wet chemical etching to expose the first GaN layer. Once exposed, Schottky contacts can be formed on the exposed first GaN layer.

[0018] Typically, ohmic contacts are formed by depositing or evaporating a metal layer stack on a second GaN layer. A high-temperature annealing process is then performed to obtain the desired ohmic contact behavior. This invention enables this step to be performed without exposing the first GaN layer, i.e., while the first GaN layer is still covered by Al. c Ga (1-c) The N-cap layer protection is implemented at specific times. Therefore, the surface of the first GaN layer is less likely to react with any material present on its surface. Such a reaction would degrade the quality of the Schottky contact; for example, it could increase gate leakage current.

[0019] According to the present invention, when etching through Al c Ga (1-c) Following the N-cap layer, a Schottky contact is formed on the first GaN layer. Since the Al content in the subsequent layer is not zero, Al contacts can be formed. c Ga (1-c) After the N-cap layer is oxidized, a wet chemical etching step is used. Compared with dry etching, less damage is introduced into the underlying first GaN layer. Furthermore, the use of a wet chemical etching step allows for high selectivity. More specifically, using a wet chemical etching step, Al... c Ga (1-c) The etch rate ratio of the N-cap layer to the first GaN layer can exceed 10:1. Therefore, the position of the Schottky contact relative to the two-dimensional electron gas in Al can be precisely controlled. b Ga (1-b) Location within the N-channel layer.

[0020] Therefore, since etching stops at the first GaN layer, and the gate leakage current remains low due to minimal damage introduced into the first GaN layer, the combination of these two effects enables a high degree of depth control. By using Al... b Ga (1-b) To further enhance this effect, an N-channel layer is used instead of a GaN layer. Al b Ga (1-b) The N-channel layer exhibits a transition from Al a Ga (1-a) A smoother transition from the N-back barrier to the first GaN layer introduces crystal defects.

[0021] For the oxidized exposed Al c Ga (1-c) Etching the N-cap layer may include an initial dry etching step prior to the wet chemical etching, wherein the initial dry etching step preferably includes the use of a chemical substance based on Cl2, BCl3, or CF4. Performing an initial dry etching step can reduce the etching through the oxidized exposed Al. c Ga (1-c)The total etching time of the N-cap layer can be reduced, and lateral etching can be suppressed. Alternatively, wet chemical etching may include the use of chemicals based on KOH, NaOH, or H3PO4 at elevated temperatures.

[0022] For the exposed Al c Ga (1-c) Oxidation of the N-cap layer may optionally include, at elevated temperatures, causing Al to... c Ga (1-c) The N-cap layer is exposed to O2, O3, or H2O. This is achieved by using a relatively thin Al layer. c Ga (1-c) The N-hat layer can ensure Al c Ga (1-c) Complete oxidation of the N-cap layer.

[0023] Dry etching of the second GaN layer can include using a fluorine-based etching process. The fluorine material will adhere to the Al... c Ga (1-c) The N-cap layer is placed on Al, thus significantly reducing the etching rate. For example, for SF6-based etching processes, the etching rate of the second GaN layer is significantly lower than that of Al. c Ga (1-c) The etching rate of the N-cap layer can be 20:1 or greater. Different dry etching processes can be used, such as reactive ion etching (RIE) or inductively coupled plasma (ICP) etching.

[0024] The method may further include: depositing metal on a first GaN layer to form a Schottky contact; and depositing metal on a second GaN layer to form a drain ohmic contact and a source ohmic contact. As described above, it is preferable to form the drain and source contacts before depositing metal to form the Schottky contact.

[0025] The method may further include: forming an insulating layer on the exposed first GaN layer before forming the Schottky contact. In this manner, a metal-insulator-semiconductor HEMT (MIS-HEMT) is formed. The insulating layer may include a silicon nitride layer, Al₂O₃, or AlON. Attached Figure Description

[0026] The invention will now be described with reference to the accompanying drawings, in which:

[0027] Figure 1 A known example of a HEMT based on nitrogen polar GaN is shown;

[0028] Figure 2 An embodiment of a nitrogen-polar GaN-based HEMT according to the present invention is shown; and

[0029] Figures 3A to 3D Manufacturing process is shown Figure 2A method for HEMT based on nitrogen polar GaN. Detailed Implementation

[0030] Figure 1 A known example of a nitrogen-polar GaN-based HEMT is shown. This HEMT includes an epitaxial structure. The epitaxial structure includes a substrate 101, such as a sapphire substrate, SiC substrate, Si substrate, or GaN substrate, on which multiple epitaxial layers are grown, for example, by metal-organic vapor phase epitaxy (MOVPE). Following the substrate 101 is a buffer layer 102. The buffer layer 102 typically includes a nucleation layer that forms a bridge between the substrate 101 and the subsequent epitaxial layers. This nucleation layer is typically very thin, approximately 1 nm to 200 nm. The buffer layer 102 further includes an unintentionally doped GaN layer. Typically, the thickness of the unintentionally doped GaN layer is in the range of 0.1 μm to 5 μm.

[0031] Al is grown on buffer layer 102 a Ga (1-a) N-back barrier 103. A GaN channel layer 104 is grown on this layer. During operation, due to Al... a Ga (1-a) The bandgap difference between the back barrier 103 and the GaN channel layer 104 will generate a two-dimensional electron gas (2DEG). The location of the 2DEG is... Figure 1 The middle part is represented by arrow A.

[0032] Al is grown on top of the GaN channel layer 104. b Ga (1-b) Cap layer 106. In Al b Ga (1-b) A Schottky gate contact 108 will be disposed on the cap layer 106. More specifically, the Schottky gate contact 108 will be formed in the Schottky region 108A defined by an etching process. Finally, in Al b Ga (1-b) On top of the cap layer 106, a highly n-type doped GaN cap layer 107 is grown. Drain contact 109 and source contact 110 will be formed on the GaN cap layer 107.

[0033] Figure 2 An embodiment of a nitrogen-polar GaN-based HEMT according to the present invention is shown. (Compared to...) Figure 1 In comparison, the following differences can be observed. First, the channel layer no longer passes through the GaN layer (e.g., Figure 1 The GaN layer (104) is formed in Al. b Ga (1-b)The channel layer 5 is disposed between the GaN layer 4 (hereinafter referred to as the first GaN layer 4) and the AlGaN back barrier 3. Furthermore, the Schottky contact 8 is disposed on the first GaN layer 4, rather than on the AlGaN layer.

[0034] about Figure 2 The following characteristics apply to the epitaxial structure in the specific embodiments shown.

[0035] layer thickness Aluminum (Al) content silicon carbide (SiC) substrate 500 micrometers none Buffer layer 1.5 micrometers none AlGaN back barrier 50nm 0.35 AlGaN channel layer 10nm 0.25 First GaN layer 3nm none AlGaN cap layer 20nm 0.15 Second GaN layer 2nm none

[0036] It should be noted that the present invention is related to Figure 2 The specific embodiment shown is not relevant. Instead, various modifications can be made to the thickness, doping concentration, and Al content without departing from the spirit of the invention.

[0037] Figures 3A to 3D Manufacturing process is shown Figure 2 A method for HEMT based on nitrogen polar GaN.

[0038] As a first step, such as Figure 3A As shown, a mask layer 11 is provided on the second GaN layer 7. The mask layer 11 may be, for example, a silicon nitride layer. As a next step, the mask layer 11 is patterned using photolithography and subsequent etching steps. As a result, the mask layer 11 is opened in the Schottky region 8A, i.e., the region where the Schottky contact 8 will be set in a later stage.

[0039] Then, a dry etching step is used to etch through the second GaN layer 7. For example, reactive ion etching using a fluorine-containing material can be employed. For example, a Cl2 / BCl3 gas composition can be used. Figure 3B As shown, due to the presence of fluorine-containing substances, the etching step will occur in Al c Ga (1-c) Stop at 6 points on the cap layer.

[0040] As a next step, the exposed Al c Ga (1-c) The cap layer 6 is oxidized. This can be achieved, for example, at elevated temperatures in an oxygen atmosphere. The oxidized exposed Al is then subjected to oxidation. c Ga (1-c) The cap layer 6 is etched. Initially, a dry etching technique, such as chlorine-based etching, is used. This etching step is timed to such an extent that the exposed Al oxide layer is oxidized. c Ga (1-c) Cap layer 6 was not completely etched. A wet chemical etching process will be used to etch the Al. c Ga (1-c) The remaining portion of the cap layer 6 is etched, for example, at elevated temperatures using a KOH-based etching step on Al. c Ga (1-c)The remaining portion of the cap layer 6 is etched. This etching step is highly selective relative to the first GaN layer 4. More specifically, the etching step stops at the surface of the first GaN layer 4.

[0041] Preferably, the drain contact 9 and source contact 10 are formed before etching through the second GaN layer 7. For example, even before applying the mask layer 11, contacts 9 and 10 can be disposed on top of the second GaN layer 7. For example, a Ti / Al / Ni / Au metal layer stack is disposed using an evaporation technique. A high-temperature (>800°C) annealing step is performed to obtain ohmic behavior. Preferably, the step of disposing of the mask layer 11 is performed after the annealing step is completed.

[0042] Etching through Al c Ga (1-c) After the cap layer 6, Schottky contacts 8 can be applied. Optionally, a cleaning step can be performed to clean the surface before applying the metal layer stack. For example, evaporation can be used to set the Ni / Au metal layer stack.

[0043] This invention does not exclude embodiments in which an insulating layer is disposed between the first GaN layer 4 and the Schottky contact 8. This structure is called a metal-insulator-semiconductor field-effect transistor (MISFET). Examples of suitable insulating layers are silicon nitride, SiO2, Al2O3, or SiON.

[0044] The invention has been explained above using detailed embodiments. However, the invention is not limited to these embodiments. Instead, various modifications can be made without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

1. A method of fabricating a Nitrogen-polarity GaN based High Electron Mobility Transistor (HEMT), the method comprising: providing an epitaxial structure, wherein the epitaxial structure comprises: a substrate; a buffer layer disposed on the substrate; Al disposed on the buffer layer a Ga (1-a) N back barrier a channel layer disposed on the Al a Ga (1-a) N back barrier, the channel layer comprising a first GaN layer, the channel layer further comprising an Al a Ga (1-a) N channel layer disposed between the first GaN layer and the Al b Ga (1-b) N back barrier; Al disposed on the channel layer c Ga (1-c) N cap layer a second GaN layer disposed on the Al c Ga (1-c) N cap layer; a first mask layer provided on the second GaN layer; patterning the first mask layer using photolithography to define a Schottky region; opening the Schottky region of the first mask layer; The second GaN layer is dry etched by the opened first mask layer to expose the Al c Ga (1-c) N cap layer; oxidizing the exposed Al c Ga (1-c) N cap layer; For the oxidized exposed Al c Ga (1-c) The N-cap layer is etched, the etching including wet chemical etching, to expose the first GaN layer; disposing a Schottky contact on the exposed first GaN layer.

2. The method of claim 1, wherein, said oxidized exposed Al c Ga (1-c) N cap layer includes an initial dry etch step prior to said wet chemical etch, said initial dry etch step including the use of a Cl2, BCl3 or CF4 based chemistry.

3. The method of any one of claims 1 and 2, wherein, the wet chemical etching comprises using a chemical based on KOH, NaOH or H3PO4 at elevated temperature.

4. The method of any one of claims 1 and 2, wherein, said exposed Al c Ga (1-c) N cap layer is oxidized by subjecting said Al c Ga (1-c) N cap layer to O2, O3, or H2O at an elevated temperature.

5. The method of any one of claims 1 and 2, wherein, the dry etching of the second GaN layer comprises using a fluorine based etching process.

6. The method of claim 5, wherein, the fluorine based etching process is a Reactive Ion Etching (RIE) or an Inductively Coupled Plasma (ICP) etching.

7. The method of any one of claims 1 and 2, further comprising: disposing a metal on the first GaN layer to form the Schottky contact; and disposing a metal on the second GaN layer to form a drain contact and a source contact.

8. The method of claim 7, wherein, forming the drain contact and the source contact before disposing the metal to form the Schottky contact.

9. The method of any one of claims 1 and 2, further comprising: disposing an insulating layer on the exposed first GaN layer before disposing the Schottky contact.

10. A Nitrogen-polarity GaN based High Electron Mobility Transistor (HEMT) fabricated according to the method of any one of the preceding claims, comprising: an epitaxial structure, the epitaxial structure comprising: a substrate; a buffer layer disposed on the substrate; Al disposed on the buffer layer a Ga (1-a) N back barrier a channel layer disposed on the buffer layer, the channel layer comprising a first GaN layer; Al disposed on the channel layer c Ga (1-c) N cap layer a second GaN layer disposed on the Al c Ga (1-c) N cap layer; and a drain contact and a source contact disposed on the second GaN layer; and a Schottky contact; characterized in that: The channel layer further includes an Al a Ga (1-a) N back barrier disposed between the first GaN layer and the Al b Ga (1-b) N channel layer, and the Schottky contact is disposed on the first GaN layer.

11. The HEMT of claim 10, wherein, 1 > a > b > c > 0.

12. The HEMT of claim 11, wherein: at least at the interface between the Al a Ga (1-a) N back barrier and the Al b Ga (1-b) N channel layer, a-b > 0.

05.

13. The HEMT of claim 12, wherein, a - b > 0.

1.

14. The HEMT of any one of claims 11 to 13, wherein, The Al a Ga (1-a) The Al content in the AlN back barrier is constant in the direction from the substrate to the second GaN layer.

15. The HEMT of any one of claims 11 to 13, wherein, The Al a Ga (1-a) The Al content in the AlGaN back barrier decreases in a direction from the substrate to the second GaN layer.

16. The HEMT of any one of claims 11 to 13, wherein, The Al b Ga (1-b) The Al content in the N-channel layer is constant in a direction from the substrate to the second GaN layer.

17. The HEMT of any one of claims 11 to 13, wherein, The Al b Ga (1-b) The Al content in the N-channel layer decreases in a direction from the substrate to the second GaN layer.

18. The HEMT of any one of claims 11 to 13, wherein, The Al c Ga (1-c) The Al content in the GaN cap layer is constant in a direction from the substrate to the second GaN layer.

19. The HEMT of any one of claims 11 to 13, wherein, The Al c Ga (1-c) The Al content in the GaN cap layer decreases in a direction from the substrate to the second GaN layer.

20. The HEMT of any one of claims 11 to 13, wherein: The Al a Ga (1-a) thickness of the N-back barrier is in the range of 10 nm to 200 nm; and / or The Al b Ga (1-b) The thickness of the N-channel layer is in the range of 5 nm to 30 nm; and / or the first GaN layer has a thickness in the range of 1 nm to 10 nm; and / or the Al c Ga (1-c) the thickness of the GaN cap layer is in the range of 5 nm to 50 nm; and / or the second GaN layer has a thickness in the range of 0 nm to 50 nm.

21. The HEMT of any one of claims 11 to 13, wherein: The Al a Ga (1-a) The concentration of n-type doping in the N back barrier is in the range of 0.1 to 1 times 10 19 # / cm 3 ; and / or The Al b Ga (1-b) The concentration of n-type doping in the channel layer is in the range of 0 to 1 times 10 19 # / cm 3 and / or The Al c Ga (1-c) The concentration of n-type doping in the cap layer is in the range of 0 to 1 times 10 19 # / cm 3 ; and / or The concentration of n-type doping in the first GaN layer is in the range of 0 to 1 times 10 19 / cm 3 and / or The concentration of n-type doping in the second GaN layer is in the range of 0.1 to 1 times 10 19 / cm 3 The concentration of n-type doping in the second GaN layer is in the range of 0.1 to 1 times 10 19 / cm 3 22. The HEMT of any of claims 11 to 13, wherein, the substrate is a silicon substrate, a gallium nitride substrate, a silicon carbide substrate or a sapphire substrate.

23. The HEMT of any one of claims 11 to 13, wherein, the buffer layer comprises a GaN layer.

24. The HEMT of any one of claims 11 to 13, wherein, the drain contact and the source contact comprise Ti / Al / Ti / TiN, Ti / Cu / Ti / TiN or Ti / AlCu / Ti / TiN.

25. The HEMT of any one of claims 11 to 13, wherein, the Schottky contact comprises Ni / Au, Ni / Pt / Au or TiN / AlCu / TiN.

26. The HEMT of any one of claims 11 to 13, further comprising an insulating layer disposed between the Schottky contact and the first GaN layer.

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