JP3630068B2 - Manufacturing method of semiconductor device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is generally _{(In X Al 1-X)} Y Ga 1 - Y N (0 ≦ X ≦ 1,0 ≦ Y ≦ 1) with gallium nitride represented (hereinafter, referred to as "GaN-based") The present invention relates to a method for manufacturing a semiconductor device using a semiconductor layer.
[0002]
[Prior art]
GaN-based semiconductors such as GaN, AlGaN, InGaN, AlInN, and InAlGaN are not only important materials in the field of short-wavelength optical devices such as semiconductor lasers that emit blue light, but also recently their high Attention has been focused on dielectric breakdown electric field strength, high thermal conductivity, and high electron saturation speed, and it is also considered promising as a material used for high-frequency power electronic devices. In particular, in the AlGaN / GaN heterojunction structure, high-concentration electrons are accumulated near the heterojunction interface between AlGaN and GaN, and so-called two-dimensional electron gas is formed. Since this two-dimensional electron gas exists spatially separated from the donor impurity added to AlGaN, it exhibits high mobility and contributes to reducing the source resistance component when this heterostructure is used in a field effect transistor. To do. Further, since the distance d from the gate electrode to the two-dimensional electron gas is usually as short as several tens of nm, the ratio Lg / d to the gate length Lg called the aspect ratio is 5 to 10 even if Lg is as short as about 100 nm. Since it can be made large, it has an excellent feature that it is easy to produce a field effect transistor having good saturation characteristics with a short channel effect. Furthermore, the two-dimensional electrons in the AlGaN / GaN heterostructure are in a high electric field region of about 1 × 10 ⁵ V / cm, and the electron velocity is more than twice as high as that in the AlGaAs / InGaAs system that is currently popular as a high frequency transistor. In addition, the concentration of electrons accumulated at the heterointerface is about 1 × 10 ¹³ / cm ² when the Al composition of AlGaN is 0.2 to 0.3, which is about three times that of GaAs-based devices. From such a fact, the GaN-based heterostructure FET is very promising as a power electronic device.
[0003]
However, the GaN heterostructure FET has many problems to be improved. One problem with GaN-based semiconductor devices is that processing or surface treatment of GaN-based semiconductors is very difficult. GaN is a chemically very stable material and is difficult to wet etch. This problem is particularly serious when the surface is a c-plane of a group 3 atom, and the etching rate is extremely low even with hot phosphoric acid. Even if etching can be performed, selective etching of a part of the surface using hot phosphoric acid at a temperature of 170 ° C. or higher cannot be used to find an appropriate mask material that can withstand it. It is almost impossible.
[0004]
[Problems to be solved by the invention]
Due to the nature of such a GaN-based semiconductor, there is a technical problem that an appropriate process for removing contaminants on the surface of the GaN-based semiconductor and obtaining a clean surface cannot be found. This problem is specifically enumerated as to what kind of problem occurs in the characteristics and manufacturing process of the semiconductor device.
[0005]
1) The adhesion strength of the insulating film deposited on the GaN-based semiconductor layer with the GaN-based semiconductor layer is weak, and when wire bonding is performed on the pad electrode formed on the insulating film, the gap between the GaN-based semiconductor layer and the insulating film during bonding As a result, peeling occurs and a defect occurs when the semiconductor device is assembled into a package.
[0006]
2) The current-voltage characteristics of the Schottky diode formed on the GaN-based semiconductor layer are generally poor (ideal factor n becomes larger than 1.5), and therefore the leakage current at a voltage near 0 V is large.
[0007]
3) When performing plasma etching mainly for chemical reaction, etching hardly progresses in the initial stage, and physical etching is required in the initial stage. Therefore, it is difficult to control the etching amount when a small amount of etching is required. Further, an increase in leakage current of the device caused by damage to the semiconductor surface due to physical etching is also a problem depending on the process.
[0008]
The above problems are all related to the surface state of the GaN-based semiconductor. This is thought to be caused by the oxide formed on the surface of the GaN-based semiconductor layer, but the details are unknown. In any case, development of a process technique for obtaining a clean surface of the GaN-based semiconductor layer is required.
[0009]
The present invention has been made in view of the problems relating to the method of manufacturing a GaN-based semiconductor device described above, and its first object is to increase the adhesion strength of the insulating film formed on the GaN-based semiconductor layer. In other words, the insulating film is prevented from being peeled off during bonding, and the assembly failure of the semiconductor device into the package does not occur.
[0010]
The second object of the present invention is to improve the characteristics of the Schottky diode formed in the GaN-based semiconductor and to reduce the leakage current at the same time by applying an appropriate surface treatment to the GaN-based semiconductor layer.
[0011]
A third object of the present invention is to provide means for reducing the contact resistance of an ohmic electrode by performing an appropriate surface treatment when the surface of a GaN-based semiconductor is contaminated by oxidation or the like.
[0012]
The fourth object of the present invention is to perform surface treatment without causing damage to the semiconductor surface in the initial stage of etching failure that occurs when performing plasma etching mainly on chemical reaction on GaN-based semiconductors. And providing means for improving the controllability of etching.
[0013]
[Means for Solving the Problems]
In the present invention, a plasma etching process using ammonia gas is used as a method for cleaning the surface of the GaN-based semiconductor. Even if it is called a plasma etching process, the GaN-based semiconductor surface itself is hardly etched by this process. However, it is considered that contaminants attached to or combined with the GaN-based semiconductor surface are effectively removed.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
(Embodiment 1)
Embodiment 1 of the present invention relates to improvement in adhesion between a GaN-based semiconductor layer and an insulating film formed thereon, and a method for manufacturing a semiconductor device according to this will be described with reference to FIG.
[0015]
FIG. 1 is a process cross-sectional view for illustrating the method for manufacturing a semiconductor device in the first embodiment. First, as shown in FIG. 1A, a GaN layer 21 having a film thickness of about 3 mm is formed on a sapphire substrate 11 whose main surface is a c-plane by MOCVD (metal organic chemical vapor deposition). A sample was used.
[0016]
Next, as shown in FIG. 1B, the surface 3 of the GaN layer 21 is subjected to ammonia plasma treatment.
[0017]
Further, as shown in FIG. 1C, an SiO ₂ film 4 as an insulating film is deposited on the GaN layer 21 to a thickness of 100 nm by plasma CVD. Here, although the insulating film is the SiO ₂ film 4, other than this, a SiN film or the like may be used.
[0018]
Here, a commercially available plasma etching apparatus (not shown) was used for the ammonia plasma treatment, a sample was placed in the apparatus, and ammonia gas was introduced into the apparatus at a flow rate of 100 sccm. The other conditions in the plasma etching were as follows: the degree of vacuum was 1 torr, the power was 30 W, the plasma etching apparatus had an electrode-to-electrode spacing of 20 mm, and the etching time was 5 minutes. The conditions for this ammonia plasma treatment are the same as the conditions for ammonia plasma treatment in all the embodiments of the present invention, and in order to reduce damage to the sample by the ammonia plasma treatment, it is 1 in comparison with the conditions for normal plasma etching. This is done with a low power of / 3 to 1/10.
[0019]
In order to evaluate the adhesion strength of the SiO ₂ film 4 to the GaN layer 21, a tensile strength test was performed. As a result, when the ammonia plasma treatment was performed, the tensile strength showed a value of 764 kg weight / cm ² or more, and no peeling of the SiO ₂ film 4 was observed in the tensile strength test. On the other hand, without performing the ammonia plasma treatment, after washing the samples in the usual way, in case of forming the SiO ₂ film is not only obtained the value of 100~350kg heavy / cm ² as a tensile strength, SiO ₂ film 4 Peeling occurred.
[0020]
From this result, it became clear that the adhesion strength between the SiO ₂ film 4 and the GaN layer 21 is drastically improved by at least 2-3 times by performing the ammonia plasma treatment. This is because the surface of the GaN layer 21 is cleaned by ammonia plasma treatment.
[0021]
Note that the ammonia plasma treatment of the present invention is performed on the surface 3 of the GaN layer 21, and thereafter, although not shown, an insulating film is deposited or oxidized on the surface 3, and a metal layer is further formed thereon. Thus, it is considered that a MOS (metal-oxide-semiconductor) structure having a lower interface state density than that without ammonia plasma treatment can be realized.
[0022]
In the present embodiment, the GaN layer 21 has been described as an example of the GaN-based semiconductor, but it has been confirmed that the same effect can be obtained with other AlGaN, InGaN, AlInN, AlInGaN, and the like.
[0023]
(Embodiment 2)
Next, a method for manufacturing a semiconductor device according to the second embodiment of the present invention will be described.
[0024]
The second embodiment relates to the formation of a Schottky electrode on a GaN-based semiconductor layer.
[0025]
From the experimental results in the first embodiment, it is considered that when the surface of the GaN-based semiconductor layer (GaN layer 21) is subjected to ammonia plasma treatment, some change occurs on the surface. This surface state is considered to be a clean state by removing substances that contaminated the surface of the GaN-based semiconductor. If this is the case, a conventional Schottky diode formed on a GaN-based semiconductor layer, which had a high ideal factor of about 1.5 or more, should be an ideal factor closer to 1. is there. An experiment was tried to prove this. In order to perform this experiment, a semiconductor device was manufactured by the following method.
[0026]
FIG. 2 is a process sectional view for illustrating the method for manufacturing the semiconductor device according to the second embodiment. First, as shown in FIG. 2A, a GaN-based semiconductor layer 2 was formed on a SiC substrate 1 having a c-plane main surface by MOCVD (metal organic chemical vapor deposition), and this was used as a sample. The GaN-based semiconductor layer 2 includes an AlN layer on a SiC substrate 1, a GaN layer with a thickness of 3 mm, an undoped Al _0.25 Ga _0.75 N layer with a thickness of 2 nm, and 2 × Si which is an n-type impurity. A heterostructure formed by epitaxially growing an n-type Al _0.25 Ga _0.75 N layer having a thickness of 20 nm added to a concentration of 10 ¹⁸ cm ⁻³ and an undoped Al _0.25 Ga _0.75 N layer having a thickness of 3 nm sequentially. It is a structure.
[0027]
Next, as shown in FIG. 2B, the surface 3 of the GaN-based semiconductor layer 2 is subjected to ammonia plasma treatment. The conditions for the ammonia plasma treatment are the same as those described in the first embodiment.
[0028]
Next, as shown in FIG. 2C, after forming an ohmic electrode 5 made of Ti / Al on the GaN-based semiconductor layer 2, a PdSi shot containing 10 wt% Si at a predetermined position using a lift-off method. A key electrode 6 is formed. Incidentally, Ti / Al ohmic electrodes are formed by sequentially vacuum-depositing Ti having a thickness of 25 nm and Al having a thickness of 200 nm and performing heat treatment in a hydrogen atmosphere at 550 ° C. for 1 minute. The current-voltage characteristics of the Schottky diode, which is a semiconductor device manufactured in this way, and the Schottky diode prepared without the ammonia plasma treatment were measured. The Schottky barrier height was 1.59 eV. On the other hand, in the sample subjected to the ammonia plasma treatment, the ideal factor is 1.27 and the Schottky barrier height is 0.74 eV, and the Schottky barrier height is improved by the ammonia plasma treatment, and the ideal factor is closer to 1. It became clear.
[0029]
(Embodiment 3)
It was shown as an experimental result that the effect of surface cleaning on the ammonia plasma-treated GaN-based semiconductor is considerably large in the first and second embodiments. Embodiment 3 of the present invention relates to improvement of contact resistance of an ohmic electrode.
[0030]
FIG. 3 is a process sectional view for illustrating the method for manufacturing the semiconductor device in the third embodiment. First, as shown in FIG. 3A, a GaN-based semiconductor layer 2 is formed by MOCVD (metal organic chemical vapor deposition) on an SiC substrate 1 whose main surface is a c-plane, and is the same as in the second embodiment. This was used as a sample. In order to evaluate the contact resistance of the ohmic electrode to be formed later, the surface of the GaN-based semiconductor layer 2 is selectively removed by etching to form a rectangular island region 7.
[0031]
Next, as shown in FIG. 3B, ammonia plasma treatment was performed on the surface 3 of the GaN-based semiconductor layer 2 under the same conditions as in the second embodiment.
[0032]
Next, as shown in FIG. 3C, an ohmic electrode 5 made of Ti / Al was formed as a TLM pattern using a lift-off method, and sheet resistance and contact resistance were evaluated. At the same time, a sample in which the surface 3 of the GaN-based semiconductor layer 2 was not subjected to the ammonia plasma treatment was prepared, and the sheet resistance and contact resistance were similarly evaluated.
[0033]
As a result of the evaluation, the sheet resistance and the contact resistivity of the sample that was not subjected to the ammonia plasma treatment were 620Ω / □ and 1.2 to 0.7 × 10 ⁻³ Ωcm ² , respectively. On the other hand, in the sample subjected to the ammonia plasma treatment, the sheet resistance was hardly changed to 625Ω / □, indicating that the sample surface was not damaged by the ammonia plasma treatment. More importantly, the contact resistance value of the sample subjected to the ammonia plasma treatment is 3 × 10 ⁻⁴ Ωcm ² , which is about 1/2 to ¼ compared with the sample not subjected to the ammonia plasma treatment. A small value was obtained. The above experimental results show that even if there is any damage given to the sample by the ammonia plasma treatment, the surface treatment by the ammonia plasma of the present invention is effective for obtaining a low ohmic resistance. The reason why this low ohmic resistance is obtained is thought to be that the oxide etc. formed on the surface 3 of the GaN-based semiconductor layer 2 were removed by the ammonia plasma treatment, and the surface was cleaned.
[0034]
(Embodiment 4)
Embodiment 4 of the present invention relates to improvement of controllability of dry etching. In general, reactive ion etching (RIE) is used for etching for the purpose of processing the formed GaN-based semiconductor layer 2. The gas used for etching is mainly composed of chlorine. As a phenomenon often observed when etching the GaN-based semiconductor layer 2 by RIE, there is a case where etching hardly proceeds at the initial stage of etching and normal etching is started after several minutes. Since the time during which etching is hardly performed varies from sample to sample, it is difficult to precisely control the etching depth with time. For example, when etching Al _0.2 Ga _0.8 N with chlorine gas pressure of 3 Pa and power of 75 W by RIE using an ECR (Electron Cyctron Resonance) plasma source, this non-etching time is around 2 minutes, and the etching rate is about 2 minutes. Since it is 30 nm / min, when the non-etching time is shifted by 30 seconds, the etching depth varies about 15 nm. This non-etching time variation is considered to be related to the surface state of the GaN-based semiconductor layer 2 and can be expected to be improved by performing surface cleaning by the ammonia plasma treatment of the present invention.
[0035]
As an experiment for verifying this, ammonia plasma treatment was performed on the GaN-based semiconductor layer 2 used in Embodiments 2 and 3 under the same conditions as before (treatment time 5 minutes), and then ECR plasma etching with chlorine gas was performed. went. As a result, the time not etched by ECR plasma etching was significantly reduced to within 30 seconds. Furthermore, when a similar etching experiment was performed by increasing the processing time of the ammonia plasma processing from 5 minutes to 15 minutes, the time not etched by the ECR plasma etching was almost zero, and the etching progressed simultaneously with the start of the ECR plasma etching. It became so. Thus, it became clear that the controllability of the etching depth of dry etching is greatly improved by performing ammonia plasma treatment.
[0036]
【The invention's effect】
As described above, according to the present invention, the surface of the GaN-based semiconductor is modified, the adhesion of the insulating film formed thereon to the GaN-based semiconductor is improved, and the GaN-based semiconductor device can be packaged. It is possible to significantly reduce mounting defects. In addition, the ideal factor of the Schottky electrode formed on the GaN-based semiconductor by ammonia plasma treatment approaches 1, the contact resistance of the ohmic electrode is improved, and the controllability of the etching depth of dry etching is improved. The manufacturing yield of the semiconductor device and the performance of the semiconductor device can be improved.
[Brief description of the drawings]
FIG. 1 is a process cross-sectional view illustrating a method for manufacturing a semiconductor device according to a first embodiment of the present invention. FIG. 2 is a process cross-sectional view illustrating a method for manufacturing a semiconductor device according to a second embodiment of the present invention. Process sectional drawing explaining the manufacturing method of the semiconductor device in Embodiment 3 of this invention
1 SiC substrate 2 GaN-based semiconductor layer 3 Surface 4 SiO ₂ film 5 Ohmic electrode 6 Schottky electrode 7 Island region 11 Sapphire substrate 21 GaN layer

Claims (4)

Ammonia gas was introduced into the plasma etching apparatus, by allowed to rise to ammonia plasma meets the plasma etching apparatus with the ammonia _{gas, (In x Al 1-x} ) in the plasma etching apparatus _y Ga _1-y N layer After the step of cleaning the surface of (0 ≦ x ≦ 1, 0 ≦ y <1 ) and the step of cleaning the surface, by introducing a chlorine-based gas, the (In _x Al _1-x ) _y A method of manufacturing a semiconductor device, comprising: etching a Ga _1-y N layer. Ammonia gas was introduced into the plasma etching apparatus, by allowed to rise to ammonia plasma meets the plasma etching apparatus with the ammonia _{gas, (In x Al 1-x} ) in the plasma etching apparatus _y Ga _1-y N layer After the step of cleaning the surface of (0 ≦ x ≦ 1, 0 ≦ y <1 ) and the step of cleaning the surface, on the (In _x Al _1-x ) _y Ga _1-y N layer And a step of forming an ohmic electrode. Ammonia gas was introduced into the plasma etching apparatus, by allowed to rise to ammonia plasma meets the plasma etching apparatus with the ammonia _{gas, (In x Al 1-x} ) in the plasma etching apparatus _y Ga _1-y N layer After the step of cleaning the surface of (0 ≦ x ≦ 1, 0 ≦ y <1 ) and the step of cleaning the surface, on the (In _x Al _1-x ) _y Ga _1-y N layer And a step of forming a Schottky electrode. The step of cleaning the surface, the step of etching the surface of the ammonia in the plasma atmosphere _{(In x Al 1-x)} y Ga 1-y N layer (0 ≦ x ≦ 1, 0 ≦ y <1) 4. The method for manufacturing a semiconductor device according to claim 1, wherein the method is a semiconductor device.

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