In recent years, an electronic device (compound semiconductor device), in which a GaN layer and an AlGaN layer are disposed sequentially above a substrate and the GaN layer is used as an electron transit layer, has been developed actively. As for an example of such compound semiconductor devices, a GaN based high electron mobility transistor (HEMT) is mentioned. In the GaN based HEMT, a high-concentration two-dimensional electron gas (2DEG) generated at a heterojunction interface between AlGaN and GaN is utilized.
The band gap of GaN is 3.4 eV and is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV). That is, GaN has a high breakdown field strength. Furthermore, GaN has also a large saturation electron velocity. Therefore, GaN is a very promising material for a compound semiconductor device capable of high-voltage operation and production of high output, for example, a material for a semiconductor device for a power supply. Consequently, the compound semiconductor device by using the GaN based compound semiconductor device is expected as a high-breakdown voltage power device for a high-efficiency switching element, an electric car, and the like.
Regarding the GaN based HEMT, the material for a gate electrode is different from the material for the source electrode and the drain electrode. Therefore, the gate electrode is formed by a process different from that of the source electrode and the drain electrode. The gate electrode, the source electrode, and the drain electrode are formed by, for example, a lift-off method. That is, in formation of the electrode, formation of a resist pattern, formation of an electrode material, and removal of the resist pattern are performed. Meanwhile, in production of a GaN based HEMT, recesses or opening portions may be formed in regions to be provided with the gate electrode, the source electrode, and the drain electrode of the compound semiconductor layer. In this case, a compound semiconductor layer is etched by using a resist pattern, so as to form a recess or an opening portion, and thereafter, the resist is removed. Furthermore, after the source electrode and the drain electrode are formed, the source electrode and the drain electrode may be covered with a passivation film, and before formation of the gate electrode, the passivation film may be dry-etched by using a resist pattern. In this case, the resist is removed after the dry etching.
In these methods, although the resist is removed, residues of the resist may remain. Moreover, after the dry etching of the passivation film, etching residues may remain. In this regard, these residues may not be removed sufficiently even by cleaning with an organic material. This is because of effects of alteration and the like during post baking after development performed in formation of the resist pattern and dry etching.
Meanwhile, when the residues are removed by an acid treatment through the use of, for example, a mixture of sulfuric acid and aqueous hydrogen peroxide, an electrode exposed at that time is damaged. For example, in the case where a recess for the gate electrode is formed after the source electrode and the drain electrode are formed, the source electrode and the drain electrode are exposed during formation of the recess. Therefore, if the above-described mixture is used, the source electrode and the drain electrode are damaged. Alternatively, even when the electrodes are not damaged, the exposed surface of the compound semiconductor layer may undergo oxidation, surface roughening, and the like, so that the characteristics may be changed.
Then, in the case where the above-described residues are present, an increase in leakage current, fluctuations in threshold voltage due to trap of charge, and the like occur, so that the yield is reduced significantly.
The followings are reference documents:
[Document 1] Japanese Laid-open Patent publication No. 2003-167360 and
[Document 2] Japanese Laid-open Patent publication No. 2007-128038.