T 0225/94 (Ordered state of Dielectric layer/AT&T) 09-11-1999

Reasons for the Decision

1. The appeal complies with Articles 106 to 108 and Rule 64 EPC and is therefore admissible.

2. Amendments (Article 123(2) EPC)

With respect to the claims, the present claims have been amended in that "semiconductor material" is replaced everywhere by "Group IV semiconductor material", and that present claim 1 is now directed towards "a Group IV semiconductor device" instead of "a semiconductor device."

The basis for the above restriction can be found on page 4, lines 16 to 21 and 28 to 33, as well as in Examples 1 to 5 described in the application as filed.

3. Clarity and support of the claims (Article 84 EPC)

In the decision under appeal, the only ground for the refusal of the application was that under Article 54(1) and 54(2) EPC. In the decision, however, it was also observed that independent claim 14 relating to a method did not comply with Article 84 EPC, since the statements in the claim that the first step "results in the production of said strain" and that the second step "results in the formation of a dielectric compound ... whereby at least a monolayer of said dielectric is in an ordered state" defined the method steps in terms of desired results to be achieved without specifying details of the method steps.

In a number of decisions, the boards of appeal have held that functional features defining a technical result to be achieved are permissible in a claim, if such features cannot otherwise be defined more precisely without restricting the scope of the invention, and if these features provide sufficient clear instructions to reduce them to practice (see, e.g. T 68/85, OJ EPO 1987, 228; T 418/89, OJ EPO 1993, 20; T 104/93, unpublished).

In the present case, it is evident from the prior art document "Physical Review Letters, vol. 55, No. 10, pages 1106 - 1109" referred to in the application in suit that techniques for producing strained semiconductor films by epitaxial growth on a substrate of dissimilar lattice constant are well-known in the art. Moreover, it is also disclosed in the application in suit (see page 3, line 33 to page 4, line 10) that the degree of lattice mismatch and thereby the resulting strain, can be adjusted by changing the composition of the Group IV semiconductor material and its thickness. Furthermore, it follows from the application in suit (see page 4, lines 11 to 27) that techniques such as molecular beam epitaxy or chemical vapour deposition which are employed in the application in suit for the formation of a dielectric layer on a substrate are per se well-known in the art. In the Board's view, therefore, the skilled person in the art would be in a position to carry out these method steps without any undue burden. Moreover, he would also be in a position to verify by the known diffraction techniques such as LEED, RHEED or transmission electron diffraction mentioned in the application in suit (see page 3, lines 23 to 30) whether or not the desired results, i.e., a strained region and an ordered monolayer of the dielectric, were achieved. In the Board's opinion, therefore, claim 14 clearly defines the subject matter for which protection is sought.

4. Novelty (Articles 52(1) and 54 EPC)

Document D1, which the Board considers to be the closest prior art, discloses a semiconductor device comprising a substrate 1 made of a Group III-V semiconductor material, such as GaAs, a layer 2 made of Si formed on the substrate 1, an insulating layer 3 made of SiO2 (cf. D1, abstract). As also pointed out in D1, due to the difference in lattice constants between the substrate 1 and the Si layer 2, the Si layer will be strained.

The purpose of the Si layer is to provide a defect-free interface between the semiconductor and the insulating layer in a device with an MIS structure (cf. D1, abstract, "Purpose"), since the interface between a Group III-V semiconductor and an oxide of the Group III-V compound contains many defects, thereby deteriorating the properties of the semiconductor device. The thickness of the Si layer 2 is limited to 1 to 5 atom layers in order to firstly prevent defect formation in the strained Si layer, and secondly, to prevent the conduction channel of the MOS to be formed in the Si layer. Otherwise, a Si-MOS on a III-V substrate would be produced instead of a MIS device made of III-V semiconductor (cf. D1a, page 3, lines 17 to 27).

Thus, in contrast to the Group IV semiconductor device according to claim 1, the device of document D1 is to be regarded as a Group III-V semiconductor device, despite the fact that it comprises a layer made of silicon.

The subject matter of claim 1 is thus new within the meaning of Article 54 EPC.

5. Inventive step (Articles 52(1) and 56 EPC)

5.1. In relation to document D1, the problem which the present application seeks to solve can be regarded as reducing the number of defects at a semiconductor/insulator interface of a Group IV semiconductor device. The number of defects at the semiconductor/insulator interface is known in the art to be crucial for the performance of MOS devices, and therefore, the technical problem is as such well-known in the art.

5.2. Document D1 also describes a method for reducing defects at an semiconductor/insulator interface, but only in the context of Group III-V semiconductor materials which are known to have a very large density of interfacial surface defects (of the order of 1011 to 1012 cm-2) at the semiconductor/insulator interface. The high number of interfacial surface defects is believed to be caused by the non-stoichiometry of the III-V compound at the interface and the non-stoichiometry of the oxide film itself at the interface (cf. D1, the paragraph bridging pages 1 and 2). In the detailed description of the invention in document D1a (see page 2, last paragraph of page 3), it is stated "Also, between the Si film 2 and SiO2 film 3, a good interface is obtained and the low interfacial level density of less than 1011 cm-2 is realized." It thus follows from the teaching of document D1a that the insertion of a silicon layer between a III-V compound substrate and a SiO2 layer reduces such a high level of interfacial defect density which is known to exist at the interface of a III-V compound semiconductor and an insulator. The interface between a Group IV semiconductor such as Si and an insulator such as SiO2 is known to have a considerably lower defect density than the interface between a III-V compound and SiO2. Consequently, it cannot be derived from the teaching of document D1 that the insertion of a layer of Si would reduce further the interfacial defect density in the case of a Group IV semiconductor substrate, which is known to be considerably low in comparison with that in a Group III-V compound semiconductor. Moreover, it cannot be derived that the measure taught in document D1a would lead to the formation of an ordered monolayer of SiO2.

For the foregoing reasons, in the Board's opinion, it was not obvious to extend the teaching of document D1 to a Group IV semiconductor device.

5.3. The remaining documents cited in the search report, do not provide a hint leading to the claimed invention. Although it was known in the art that a 7x7 reconstruction is preserved at a buried amorphous-Si/Si(111) interface (cf. the application, page 2, lines 13 to 17), this reconstruction is not preserved at a Si/SiO2 interface (cf. Example 3 of the present application).

For the foregoing reasons, in the Board's judgement, the subject matter of claim 1 involves an inventive step within the meaning of Article 56 EPC and therefore meets the requirements of Article 52(1) EPC. Claims 2 to 16 meet the requirements of Article 52(1) EPC as well, since they all contain the features of claim 1.