JP2015181190A - Method of manufacturing semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a semiconductor device capable of improving reliability.SOLUTION: A method of manufacturing a semiconductor device comprises the steps of: forming a source electrode 20 and a drain electrode 22 on a surface of a nitride semiconductor layer containing gallium; applying oxygen plasma treatment having power density of 0.2 to 0.3 W/cmto a surface of a nitride semiconductor layer 18 (cap layer) between the source electrode 20 and the drain electrode 22, after forming the source electrode 20 and the drain electrode 22, and thereby forming a conductive layer 26 having a gallium composition larger than that of the nitride semiconductor layer 18 before the oxygen plasma treatment; forming an insulating film on the upper surface of the nitride semiconductor layer after the conductive layer 26 is formed; exposing the nitride semiconductor layer 18 by removing a part of the insulating film on the conductive layer 26; and forming a gate electrode 24 on the exposed surface of the nitride semiconductor layer 18.

Description

  The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for manufacturing a semiconductor device having a nitride semiconductor layer.

  A semiconductor device using a nitride semiconductor, for example, a semiconductor device such as a field effect transistor (FET) may be used as a high-frequency output amplification element. Patent Document 1 discloses an invention in which a SiN (silicon nitride) film with a controlled hydrogen content is provided on a nitride semiconductor layer.

JP-A-2005-286135

  In the conventional technology, there is a possibility that the reliability of the semiconductor device is lowered due to the ingress of moisture. In view of the above problems, an object of the present invention is to provide a semiconductor device manufacturing method capable of improving reliability.

The present invention is a method for manufacturing a semiconductor device including a step of performing oxygen plasma treatment with a power density of 0.2 to 0.3 W / cm ² on the surface of a nitride semiconductor layer. According to the present invention, it is possible to improve the reliability of a semiconductor device.

  In the above structure, the gas supplied in the step of performing the oxygen plasma treatment may be configured to include only oxygen gas. According to this configuration, a good conductive layer can be formed.

  In the above structure, the nitride semiconductor layer may be a nitride semiconductor layer containing gallium. According to this configuration, a good conductive layer can be formed.

  In the above structure, the nitride semiconductor layer may be a nitride semiconductor layer containing any of gallium nitride, aluminum gallium nitride, aluminum nitride, indium nitride, indium aluminum nitride, or aluminum indium gallium nitride. According to this configuration, a good conductive layer can be formed.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the semiconductor device which can improve reliability can be provided.

FIG. 1A to FIG. 1C are cross-sectional views illustrating a method for manufacturing a semiconductor device according to the first embodiment. FIG. 2A to FIG. 2C are cross-sectional views illustrating the method for manufacturing the semiconductor device according to the first embodiment. FIG. 3A and FIG. 3B are cross-sectional views illustrating the method for manufacturing the semiconductor device according to the first embodiment. FIG. 4A and FIG. 4B are diagrams showing the results of the experiment.

  Prior to the description of the embodiments, first, the cause of the decrease in the reliability of the semiconductor device will be described. Among semiconductor devices, in a FET, a source electrode, a drain electrode, and a gate electrode are formed on a channel layer made of a nitride semiconductor such as i-GaN (gallium nitride). Each electrode is made of a metal such as Au.

  When moisture enters the semiconductor device, Au forming the electrode may dissolve into the moisture and be ionized. In this case, when a voltage is applied to the electrode, a so-called ion migration phenomenon may occur in which Au ions dissolved from one electrode move and are reduced and precipitated by another electrode. In order to verify the ion migration phenomenon, an acceleration test was performed in which a pinch-off semiconductor device having a drain voltage Vd = 50 V and a gate voltage Vg = −3 to −5 V was placed in an environment of a temperature of 130 ° C. and a humidity of 85%. As a result, Au dissolved from the drain electrode was deposited at the source electrode and the gate electrode. When the ion migration phenomenon occurs, the reliability of the semiconductor device decreases, for example, the semiconductor device is destroyed. In particular, in the case of a semiconductor device using a nitride semiconductor, since a high voltage is applied, the influence of the ion migration phenomenon becomes large.

  A protective film with low water permeability may be used to suppress moisture intrusion. However, in this case, it is required to manage the quality of the protective film, the film thickness, the adhesion with the electrodes and the like. Therefore, the configuration and manufacturing process of the semiconductor device may be complicated. In addition, when the quality of the protective film varies, the moisture resistance may vary among many semiconductor devices.

  The inventor of the present invention shows that the moisture resistance of the semiconductor device is greatly improved by flowing a small leak current between the source and the drain or between the source and the gate as compared with the drain current flowing during the operation of the semiconductor device. I found it. The present invention is based on this finding.

  Next, embodiments of the present invention will be described with reference to the drawings.

  Example 1 is an example in which oxygen plasma treatment is performed. FIG. 1A to FIG. 3B are cross-sectional views illustrating the semiconductor device according to the first embodiment. FIGS. 1A to 3B are schematic views, and the thickness of each layer is shown in a simplified manner.

  As shown in FIG. 1A, a semiconductor substrate is prepared by laminating a substrate 10, a barrier layer 12, a channel layer 14, an electron supply layer 16, and a cap layer 18 from below. The substrate 10 is made of, for example, SiC (silicon carbide). The barrier layer 12 is made of, for example, AlN (aluminum nitride) having a thickness of 300 nm. The channel layer 14 is made of, for example, i-GaN having a thickness of 1000 nm. The electron supply layer 16 is made of, for example, AlGaN (aluminum gallium nitride) having a thickness of 20 nm. The cap layer 18 is made of, for example, n-GaN having a thickness of 5 nm. The barrier layer 12, the channel layer 14, the electron supply layer 16 and the cap layer 18 form the nitride semiconductor layer 11. The nitride semiconductor layer 11 is formed by epitaxial growth, for example, by MOCVD (Metal Organic Chemical Vapor Deposition). Note that the nitride semiconductor layer 11 may be formed of the barrier layer 12, the channel layer 14, and the electron supply layer 16 without the cap layer 18.

  A resist 13 is formed on a part of the cap layer 18, and the cap layer 18 is etched. The portion of the cap layer 18 exposed from the resist 13 is removed by etching, and a recess 28 is formed. The electron supply layer 16 is exposed from the recess 28.

  As shown in FIG. 1B, the source electrode 20 and the drain electrode 22 are formed in the recess 28 by vapor deposition and lift-off. More specifically, a resist 15 is formed and a metal is deposited. The resist 15 is removed. The source electrode 20 and the drain electrode 22 are ohmic electrodes formed by stacking, for example, metals such as Ti / Al and Ta / Al in the order closer to the electron supply layer 16. In addition, heat treatment is performed to obtain a good ohmic electrode.

As shown in FIG. 1C, oxygen plasma treatment is performed on the surface of the cap layer 18 using an asher. The conditions for the oxygen plasma treatment are as follows. The power density is the power per unit area of the electrode provided in the asher.
Usher electrode area: 4000 cm ²
Plasma power: 800 W (corresponding to a power density of 0.2 W / cm ² )
RF frequency: 13.56 MHz
Processing time: 1 minute

  By the oxygen plasma treatment, N (nitrogen) contained in the cap layer 18 is removed in combination with O (oxygen). As a result, the region of the cap layer 18 where the oxygen plasma treatment is performed has a higher Ga (gallium) composition ratio than the region where the oxygen plasma treatment is not performed. Thereby, the conductive layer 26 is formed on the upper surface of the cap layer 18.

  As shown in FIG. 2A, a SiN (silicon nitride) layer 30 is formed on the cap layer 18, the source electrode 20 and the drain electrode 22. The thickness of the SiN layer 30 is 20 nm, for example. The SiN layer 30 is etched to form an opening 31 in the SiN layer 30 in a partial region between the source electrode 20 and the drain electrode 22. The cap layer 18 is exposed from the opening 31. At this time, the conductive layer 26 in the opening 31 is also etched.

  As shown in FIG. 2B, the gate electrode 24 is formed on the cap layer 18 by, for example, a vapor deposition method and a lift-off method. The gate electrode 24 is formed by laminating a metal such as Ni / Au, for example, in the order closer to the cap layer 18. As shown in FIG. 2C, the SiN layer 32 is formed on the gate electrode 24 and the SiN layer 30. The thickness of the SiN layer 32 is 40 nm, for example.

  As shown in FIG. 3A, openings are formed in the SiN layer 30 and the SiN layer 32 to expose the source electrode 20 and the drain electrode 22. Thereafter, two wirings 36 that are in contact with each of the source electrode 20 and the drain electrode 22 are formed. The wiring 36 is made of a metal such as Au. As shown in FIG. 3B, a SiN layer 34 is formed on the SiN layer 32 and the wiring 36, and passivation is performed. The SiN layers 30, 32 and 34 function as moisture-resistant protective layers. This completes the method for manufacturing the semiconductor device according to the first embodiment.

  Here, an experiment for verifying the effect of the oxygen plasma treatment will be described. In the experiment, an XPS (X-ray Photoelectron Spectroscopy: X-ray photoelectron spectroscopy) analysis and a gate-- were performed on a sample subjected to oxygen plasma treatment (Example 1) and a sample not subjected to oxygen plasma treatment (Comparative Example). The drain-to-drain current was measured.

表１に示すように、比較例ではＮ／Ｇａ比が０．８２であった。これに対して、実施例１ではＮ／Ｇａ比が０．４８であった。このことから、酸素プラズマ処理により、キャップ層１８のＧａの組成比が大きくなったことが分かった。 First, XPS analysis will be described. In this experiment, the N / Ga ratio (nitrogen / gallium ratio) of the cap layer 18 was measured by XPS analysis. The results are shown in Table 1.

As shown in Table 1, in the comparative example, the N / Ga ratio was 0.82. In contrast, in Example 1, the N / Ga ratio was 0.48. From this, it was found that the Ga composition ratio of the cap layer 18 was increased by the oxygen plasma treatment.

  Next, current measurement will be described. In this experiment, a gate-drain current Igd was measured when a 3-inch wafer was used and a gate-drain voltage Vgd was applied. Current measurements were taken at five measurement points: top, bottom, left, right, and center with the wafer facet down. The measurement points on the top, bottom, left, and right were points located at a distance of about 10 mm from the outer periphery of the wafer.

  FIG. 4A and FIG. 4B are diagrams showing the results of the experiment. FIG. 4A shows a comparative example, and FIG. 4B shows a measurement result of Example 1. The horizontal axis represents the gate-drain voltage, and the vertical axis represents the gate-drain current. The solid line is the top, the dotted line is the center, the broken line is the bottom, the one-dot chain line is the left, the three-dot chain line is the right, and the results at each measurement point are shown.

  As shown in FIG. 4A, in the comparative example in which the oxygen plasma treatment was not performed, Igd was several μA even when Vgd was increased. On the other hand, as shown in FIG. 4B, in Example 1 where the oxygen plasma treatment was performed, Igd was several tens of μA. For example, in the comparative example, when Vgd = 40 V, the measurement result at each measurement point was about 1 μA. In Example 1, when Vgd = 40V, the measurement result at each measurement point was about 10 μA. That is, by performing the oxygen plasma treatment, the Igd was about 10 times that in the case of not performing the oxygen plasma treatment.

  It is considered that when an electric current flows through the conductive layer 26, Au ions moving between the electrodes are reduced, and the ion migration phenomenon is suppressed. Alternatively, the conductive layer 26 generates heat when a current flows. It is believed that the heat generated by the conductive layer 26 prevents the invaded water from evaporating and Au from melting out.

  According to Example 1, since the conductive layer 26 is formed on the cap layer 18, the ion migration phenomenon can be suppressed. That is, the moisture resistance of the semiconductor device is improved and the reliability can be improved. Further, the conductive layer 26 can be formed by oxygen plasma treatment to improve moisture resistance. For this reason, the structure and manufacturing process of a semiconductor device can be simplified.

The power density of the oxygen plasma treatment may be set to such a size that the conductive layer 26 having a high Ga composition ratio can be formed. However, if the power is too high, damage to the nitride semiconductor layer 11 increases. For this reason, it is preferable that a power density is 0.2-0.3 W / cm < ² >. The power density, greater than 0.2 W / cm ^2, may be less than 0.3 W / cm ^2. Furthermore, the power density may be 0.22 to 028 W / cm ² . As described above, N contained in the cap layer 18 is removed by oxygen plasma treatment to form the conductive layer 26 having a large Ga composition ratio. If a gas other than oxygen gas is supplied in the oxygen plasma treatment, the good conductive layer 26 may not be formed. In order to form a good conductive layer 26, it is preferable that the gas supplied by the oxygen plasma treatment is only oxygen gas.

  A nitride semiconductor other than the above may be used for the nitride semiconductor layer 11. The nitride semiconductor is a semiconductor containing nitrogen, and examples thereof include InN (indium nitride), InGaN (indium gallium nitride), InAlN (indium aluminum nitride), and AlInGaN (aluminum indium gallium nitride). That is, the nitride semiconductor layer 11 may be made of a nitride semiconductor other than n-GaN. However, in order to satisfactorily form the conductive layer 26, the nitride semiconductor layer 11 is preferably made of a nitride semiconductor containing Ga, and is preferably made of GaN or AlGaN.

  The current flowing through the conductive layer 26 is several orders of magnitude smaller than the drain current flowing during operation of the semiconductor device. For this reason, the fluctuation of the characteristics of the semiconductor device due to the current flowing through the conductive layer 26 is extremely small. Thereby, the deterioration of the characteristics of the semiconductor device can be suppressed and the moisture resistance can be improved.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

Board 10
Barrier layer 12
Channel layer 14
Electron supply layer 16
Cap layer 18
Source electrode 20
Drain electrode 22
Gate electrode 24
Conductive layer 26
SiN layer 30, 32, 34

Claims (3)

Forming a source electrode and a drain electrode on the surface of the nitride semiconductor layer containing gallium;
After forming the source electrode and the drain electrode, an oxygen plasma having a power density of 0.2 to 0.3 W / cm ² on the surface of the nitride semiconductor layer between the source electrode and the drain electrode. Forming a conductive layer having a gallium composition larger than that of the nitride semiconductor layer before the oxygen plasma treatment by performing a treatment;
Forming an insulating film on the top surface of the nitride semiconductor layer after the formation of the conductive layer;
Removing a portion of the insulating film on the conductive layer to expose the nitride semiconductor layer;
And a step of forming a gate electrode on the surface of the exposed nitride semiconductor layer.   2. The method of manufacturing a semiconductor device according to claim 1, wherein the gas supplied in the step of performing the oxygen plasma treatment is only oxygen gas.   3. The method of manufacturing a semiconductor device according to claim 1, wherein the nitride semiconductor layer is a nitride semiconductor layer containing any one of gallium nitride and aluminum gallium nitride.

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