METHOD FOR FABRICATING SEMICONDUCTOR DEVICE
A method for fabricating a semiconductor device includes: forming a first film on a nitride semiconductor layer so as to contact the nitride semiconductor layer and have a thickness equal to or larger than 1 nm and equal to or smaller than 5 nm, the first film being made of silicon nitride having a composition ratio of silicon to nitrogen larger than 0.75, silicon oxide having a composition ratio of silicon to oxygen larger than 0.5, or aluminum; and forming a source electrode, a gate electrode and a drain electrode on the nitride semiconductor layer.
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This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-081797 filed on Mar. 30, 2012, the entire contents of which are incorporated herein by reference.
BACKGROUND(i) Technical Field
The present invention relates to a method for fabricating a semiconductor device.
(ii) Related Art
Nitride semiconductors are used in semiconductor devices such as an FET (Field Effect Transistor). In order to protect a nitride semiconductor layer, an insulating layer that covers the nitride semiconductor layer may be provided. For example, Japanese Patent Application Publication No. 2010-166040 discloses an arrangement in which a protection film made of silicon oxide is provided on a nitride semiconductor layer.
Conventionally, the capacitance of the semiconductor device, which may include the intrinsic capacitance and the parasitic capacitance, may change due to an oxide layer formed on the surface of the nitride semiconductor layer. Variation in the capacitance may drift the gain. Further, electrons are captured in electron traps in the insulating film, so that the current of the semiconductor device may change. Conventionally, it is difficult to suppress both variation in the capacitance and that in the current.
SUMMARYAccording to an aspect of the present invention, it is possible to suppress both variation in the capacitance and that in the current.
According to another aspect of the present invention, there is provided a method for fabricating a semiconductor device including: forming a first film that contact a surface of the nitride semiconductor layer and have a thickness equal to or larger than 1 nm and equal to or smaller than 5 nm, the first film being made of silicon nitride having a composition ratio of silicon to nitrogen larger than 0.75, silicon oxide having a composition ratio of silicon to oxygen larger than 0.5, or aluminum; and forming a source electrode, a gate electrode and a drain electrode on the nitride semiconductor layer.
First, a comparative example is described.
Referring to
The barrier layer 12 contacts the upper surface of the substrate 10, and the channel layer 14 contacts the upper surface of the barrier layer 12. The electron supply layer 16 contacts the upper layer of the channel layer 14, and the cap layer 18 contacts the upper surface of the electron supply layer 16. The insulating film 20 contacts the upper surface of the cap layer 18. The source electrode 22, the drain electrode 24 and the gate electrode 26 are formed in openings in the insulating film 20, and contact the upper surface of the cap layer 18. The interlayer insulating film 30 contacts the upper layers of the insulating film 20 and the gate electrode 26. Interconnection lines 32 are formed in openings in the interlayer insulating film 30, and contact the upper surface of the source electrode 22 or that of the drain electrode 24.
The substrate 10 is formed of, for example, silicon carbide (SiC) or sapphire. The semiconductor layers (the barrier layer 12, channel layer 14, electron supply layer 16 and cap layer 18) are nitride semiconductor layers. The barrier layer 12 is made of, for example, aluminum nitride (AlN). The channel layer 14 and the cap layer 18 are made of, for example, gallium nitride (GaN). The electron supply layer 16 is made of, for example, aluminum gallium nitride (AlGaN). The insulating film 20 and the interlayer insulating film 30 are made of, for example, silicon nitride (SiN). The source electrode 22 and the drain electrode 24 are made of a metal, each of which may be composed of, for example, a titanium (Ti) layer and an aluminum (Al) layer stacked in this order from the cap layer 18. The gate electrode 26 is made of a metal and may be composed of a nickel (Ni) layer 26a and a gold (Au) layer 26b stacked in this order from the cap layer 18. When the drain electrode 24 is set at a positive potential and the gate electrode 26 is set at a negative potential with the source electrode 22 being grounded, a two-dimensional electron gas (2DEG) is generated in the channel layer 14. And electrons in 2DEG move between the source and the drain.
A description is now given of a problem that occurs in the comparative example.
Referring to
Next, a description is given of an exemplary structure in which the insulating film 20 has a stoichiometric composition (Si3N4).
A first embodiment has an exemplary structure having a film having a comparatively high composition ratio of Si to N, and a stoichiometric film provided on the large composition film.
Referring to
Table 1 shows the standard Gibbs energies of formation of some substances (hereinafter simply referred to as Gibbs energy).
As shown in Table 1, the Gibbs energy decreases in the order of indium oxide (In2O3), gallium oxide (Ga2O3), aluminum oxide (Al2O3) and silicon oxide (SiO2). As the Gibbs energy is lower, the substance is more stable. The Gibbs energy of Ga2O3 formed in the denatured layer is −499 kJ/mol. As illustrated in
A description is now given of a fabrication method of the semiconductor device 100.
Referring to
Referring to
A description is now given of conditions for growing the first film 34 and the second films 36. The ALD method forms films as follows. A first source gas is supplied in an ALD apparatus, and a single-atom-thick layer of Si is formed. Then, the first source gas is exhausted from the ALD apparatus. A second source gas is supplied to the ALD apparatus, and the Si layer is converted into nitride. Then, the second source gas is exhausted from the ALD apparatus. A supply time is defined as the time during which the first source gas is supplied. A first exhaust time is defined as the time during which the first source gas is exhausted. The nitridization time is defined as the time during which the second source gas is supplied. A second exhaust time is defined as the time during which the second source gas is exhausted. One cycle is defined as the time from the initiation of supply of the first source gas to the completion of exhaust of the second source gas.
By changing the first supply time and the second supply time, it is possible to grow, from the same source gas, the first film 34 and the second film 36 having different compositions. The number of cycles may be changed in accordance with the thickness of the first film 34 and that of the second film 36. The film growing temperature (the temperature in the ALD apparatus) may be equal to or higher than 200° C. and may be equal to or lower than 400° C., for example. Any of the film growing conditions illustrated in
A description is now given of the reason why the first film 34 is formed by the ALD method. When the first film 34 is too thick, many electron traps are formed since a large number of Si atoms having anti-bonding orbitals exists in the first film 34. Thus, electros in 2DEG may be captured in the electron traps. In contrast, when the first film 34 is too thin, the removal of the denatured layer has a difficulty because a small number of Si atoms having anti-bonding orbitals exists. In order to suppress the trapping of electrons and remove the denatured layer, the thickness of the first film 34 has a thickness equal to or larger than 1 nm and equal to or smaller than 5 nm, and may be not smaller than 1.5 nm or 2 nm and may be not larger than 4.5 nm or 4 nm. As described above, the first film 34 is required to be reliably formed so that the first film 34 is thin and is even or almost even in thickness. This requirement may be preferably achieved by the ALD method. Although the first film 34 may be formed by plasma CVD, there is a difficulty in reliably forming a film thickness of not larger than 5 nm, which corresponds to a three- or four-atom-thick layer and in removing the denatured layer efficiently, as compared with the ALD method. Another method besides the ALD method may be used as long as the first film 34 is formed so as to be thin and even or almost even in thickness. The composition ratio of Si to N in the first film 34 is larger than 0.75, and may be 0.78, 0.8, 0.85 or 0.9.
The second film 36 may be formed to have the substantive stoichiometric composition. The substantive stoichiometric composition includes not only the strict stoichiometric composition but also a composition including an impurity having a difficulty in removal in the fabrication process. For example, the second film 36 may be formed by sputtering. However, in order to make the composition of the second film 36 closer to the stoichiometric value, the second film 36 is preferably formed by the ALD method. The second film 36 is preferably equal to or larger than 20 nm and is equal to or smaller than 100 nm in order to make it difficult for the electron traps to be formed and to protect the semiconductor layer from moisture. For example, the thickness of the second film 36 may be not smaller than 25 nm or 30 nm and not larger than 95 nm or 90 nm.
The second film 36 has the following effects other than those described above. In a case where the gate electrode 26 is formed in an opening in the first film on the cap layer 18, a parasitic capacitance between the gate electrode 26 and the cap layer 18 is concerned because the first film 34 is thin. With the above in mind, the gate electrode 26 is formed in the opening in the first film 34 and the second film 36 on the cap layer 18 in order to increase the distance between the gate electrode 26 and the cap layer 18 and to reduce the parasitic capacitance. However, it is not essential to form the second film 36 on the first film 34, but the second film 36 may be omitted.
Second EmbodimentA second embodiment has an exemplary structure in which the first film 34 is made of Al. A cross-sectional view of a semiconductor device 200 in accordance with the second embodiment is the same as that of the semiconductor device 100.
Referring to
As illustrated in
Al is likely to be oxidized. When the substrate 10 on which the first film 34 has been form is removed from the ALD apparatus and is exposed to atmosphere, an oxide film is formed on the surface of the first film 34. In order to prevent the oxide film from being formed, it is preferable that the step of forming the first film 34 and that of forming the second film 36 are successively carried out. Since the first film 34 is formed by the ALD apparatus, the first film 34 is thin and even in thickness. Thus, the denatured layer is effectively removed.
The Al atoms and O ions are bonded to generate Al2O3. As previously shown in Table 1, the Gibbs energy of Al2O3 is −791 kJ/mol, and is lower than that of Ga2O3. Thus, the reaction of the Al atoms and the O ions proceeds, and the denatured layer is effectively removed. Since the Gibbs energy of Al2O3 is lower than that of In2O3, the denatured layer is effectively removed even in an exemplary case where the gap layer 18 includes In. Since the Gibbs energy of Al2O3 is higher than that of SiO2, Al is hard to be bonded to the ions as compared with Si. Therefore, Al is solely arranged to contact the cap layer 18 like the first film 34, so that the bonding of the Al atoms and the O ions can proceed and the denatured layer can be removed.
In the first and second embodiments, the second film 36 may be formed of an insulative substance having a stoichiometric composition other than Si3N4. The second film 36 may include at least one of SiO2, Al2O3 and AlN, for example. The first film 34 may be formed of a substance other than SiN and Al, and the second film 36 may be formed of a substance other than Si3N4.
An exemplary case using silicon oxide is now described. The composition of the second film 36 is a stoichiometric composition (for example, SiO2). The composition ratio of Si to O in the second film 36 is 0.5. The Si/O ratio in the first film 34 is larger than 0.5. As shown in Table 1, the Gibbs energy of SiO2 is low. Thus, the arrangement of the first film 34 and the cap layer 18 which contact each other facilitates the bonding of the Si atoms and the O ions in the first film 34. Therefore, the denatured layer is removal.
Another exemplary case using aluminum oxide is now described. The composition of the second film 36 has the stoichiometric composition (Al2O3). The composition ratio of Al to O in the second film 36 is 0.67.
Yet another exemplary case is now described. The second film 36 has the stoichiometric composition (AlN). The composition ratio of Al to N in the second film 36 is 1.
The barrier layer 12, the channel layer 14, the electron supply layer 16 and the cap layer 18 may be made of nitride semiconductors other than the above-described semiconductors. The nitride semiconductors are semiconductors including N, and may be indium gallium nitride (InGaN), indium nitride (InN) and aluminum indium gallium nitride (AlInGaN) other than the above-described semiconductors.
The present invention is not limited to the specifically described embodiments, but may include other embodiments and variations without departing from the scope of the present invention.
Claims
1. A method for fabricating a semiconductor device comprising:
- forming a first film that contact a surface of the nitride semiconductor layer and have a thickness equal to or larger than 1 nm and equal to or smaller than 5 nm, the first film being made of silicon nitride having a composition ratio of silicon to nitrogen larger than 0.75, silicon oxide having a composition ratio of silicon to oxygen larger than 0.5, or aluminum; and
- forming a source electrode, a gate electrode and a drain electrode on the nitride semiconductor layer.
2. The method according to claim 1, further comprising forming a second film that contacts a surface of the first film, wherein the second film is substantially a stoichiometric composition and is made of one of silicon nitride, silicon oxide, aluminum oxide and aluminum nitride.
3. The method according to claim 1, wherein the first film is silicon nitride, and the method further comprises forming of a second film composed of silicon nitride having a silicon composition smaller than the first film.
4. The method according to claim 2, wherein a formation method of the first film is an atomic layer deposition method, and a formation method of the second film is an atomic layer deposition method.
5. The method according to claim 3, wherein a formation method of the first film is an atomic layer deposition method, and a formation method of the second film is an atomic layer deposition method.
6. The method according to claim 2, wherein the second film has a thickness that is equal to or larger than 20 nm and is equal to or smaller than 100 nm.
7. The method according to claim 3, wherein the second film has a thickness that is equal to or larger than 20 nm and is equal to or smaller than 100 nm.
8. The method according to claim 1, wherein the first film is composed of aluminum, and the method comprises forming a second film on the first film, a formation of the second film is performed in-situ after a formation of the first film.
9. The method according to claim 8, wherein the formation of the first and second film is performed under an atomic layer deposition apparatus.
10. The method according to claim 1, wherein the first film is formed between the source electrode and the gate electrode, and is between the gate electrode and the drain electrode.
11. The method according to claim 1, wherein the first film is composed of silicon nitride, and the composition ratio of silicon to nitrogen of first film is larger than 0.8.
12. The method according to claim 1, wherein the first film is composed of silicon nitride, and the composition ratio of silicon to nitrogen of first film is larger than 0.85.
13. The method according to claim 1, wherein the first film is composed of silicon nitride, and the composition ratio of silicon to nitrogen of first film is larger than 0.9.
Type: Application
Filed: Mar 29, 2013
Publication Date: Oct 3, 2013
Applicant: SUMITOMO ELECTRIC DEVICE INNOVATIONS, INC. (Yokohama-shi)
Inventor: Tsutomu Komatani (Yokohama-shi)
Application Number: 13/853,742
International Classification: H01L 29/66 (20060101);