T 0411/97 12-09-2000

Reasons for the Decision

1. The appeal meets the requirements of Articles 106 to 108 and Rule 64 EPC, and is therefore admissible.

2.1. Late filed documents - admissibility

As stated in paragraphs III and V above, documents D8 to D10, D13 and D14 were cited by the appellant for first time in the appeal proceedings, so that they are outside the opposition period as laid down in Article 99(1) EPC in conjunction with Rule 55(c) EPC. In the Board's view, there was no proper justification for the late-filing of these documents, since the claims forming the basis of the decision were not amended during the opposition proceedings, so that the new documents were not in response to amendments to claims, and the new documents do not purport to rebut the findings in the decision under appeal, for example, by way of filling-in gaps in the appellant's case before the opposition division. Therefore, the question, which is to be decided at the outset, is whether the Board, in the exercise of its discretion under Article 114(2) EPC, should disregard these late-filed documents as not filed in due time. According to decision T 1002/92 (OJ EPO 1995, 605) (cf. also T 389/95, not published in the Official Journal), which considered the impact of the opinion of the Enlarged Board of Appeal in G 10/91 (OJ EPO 1993, 420) on the admissibility of late-filed evidence, a decisive criterion for the admissibility of the late-filed evidence at the appeal stage is whether the late-filed evidence is prima-facie highly relevant in the sense that it is highly likely to prejudice the maintenance of the patent.

Applying the above criterion in the present case, only documents D9, D10, and D13 are prima-facie highly relevant, since they would have prejudiced the maintenance of the patent in the unamended form. These documents are accordingly introduced into the proceedings, and the remaining late-filed documents are disregarded in accordance with Article 114(2) EPC.

2.2. Late filed request - admissibility

As to the admissibility of the new claims forming the respondent's request submitted at the beginning of the oral proceedings, the Board notes that this was in fact caused by the late-filing of the new evidence by the appellant during the appeal proceedings. Moreover, as will be apparent from the following, the amendments to the claims are such that they clearly meet the formal requirements, i.e. the requirements of Article 84 and Article 123(2) and (3) of the EPC. The Board therefore sees no reason to disregard the late-filed request.

3. Amendments

Claim 1 contains the features of claims 1 and 2 as filed with the additional specification that the gate electrode is abutted against the opposite face of the superconducting channel to that where the source and drain electrodes are formed, and that the thickness of the oxide superconductor thin film determines the gate length. These features are disclosed in Figures 1I and 2I and in page 6, lines 20 to 27 of the application as filed, respectively. Claims 2 to 13 correspond to claims 3 to 14 of the application as filed.

Therefore, the claims meet the requirements of Article 123(2) EPC.

Since claims 1 to 14 as granted correspond to claims 1 to 14 of the application as filed, it follows from the above that the requirements of Article 123(3) EPC are also met.

4. Inventive step

The only issue in the present appeal is that of inventive step.

4.1. At the oral proceedings, only documents D1, D3, D4, D7, D9, D10, and D13 were relied upon by the parties in the consideration of inventive step. In the following therefore, only these documents are taken into consideration.

4.2. The patent in suit concerns a superconducting device having a channel layer formed of an oxide superconductor layer and a gate electrode made of an oxide superconductor separated from the channel layer by a gate insulator layer, i.e. the current in the channel is controlled by subjecting the channel to an electric field induced by applying a voltage to the gate electrode in a manner analogous to a semiconductor MOSFET device. As described in the patent specification, an extremely short channel length is desired in order to obtain high switching speed of the superconducting device (cf. column 2, lines 48 to 53). In the claimed device and method, the channel layer is at an angle with respect to the substrate surface and the gate length is determined by the thickness of the gate electrode layer. As the thickness of the gate electrode layer can be controlled precisely during its formation, e.g. by deposition, the claimed structure of the device enables gate lengths of the order of 0.1 m without having to resort to fine-processing techniques (cf. column 4, lines 24 to 34).

4.3. It was not in dispute that document D1 represents the closest prior art. Document D1 discloses a superconducting device operating according to the principle of quasi particle injection, i.e. a tunnel current injected into a superconducting channel from a superconducting gate electrode via a tunnel barrier causes a superconducting to normal-conducting phase transition in the channel layer. The device of document D1 comprises a superconducting channel layer made of a NbN thin film formed to have an angle relative to a deposition surface of a substrate (cf. D1, Figure 1). Source and drain electrodes made of Cu are formed at opposite ends on the upper face of the channel layer and a gate electrode (called "injector" in document D1) is formed of Nb and abuts against a tunnel barrier layer at the lower face of the channel layer.

4.3.1. The device of claim 1 differs from that of document D1 firstly in that the claimed device works through electric field effect, i.e. the number of charged carriers is depleted by subjecting the channel layer to an electric field, whereas the device of document D1 works through the principle of inducing a superconducting to normal phase transition in the channel by injecting a tunnel current into the channel. Secondly, the claimed device specifies oxide superconductors whereas in document D1 "classic" superconducting materials are used. Finally, a portion of the Nb gate electrode at one end overlaps the source/drain copper electrode and the gate electrode does not extend the entire length of the NbN channel layer. Thus, it is apparent that the thickness of the Nb gate electrode does not determine the gate length (cf. Figure 1).

4.4. In view of these differences, the technical problem addressed by the invention as claimed can be seen to provide a FET type superconducting device having a structure which enables to provide a short superconducting channel made of an oxide superconducting layer (cf. also column 2, line 57 to column 3, line 16 of the patent in suit).

4.5. Documents D4, D7, D10, and D13 all disclose a superconducting device of the same type as that of the patent in suit, i.e. a device having a channel made of an oxide superconductor thin film and an insulated gate electrode overlying the channel.(cf. D4, Section "High-Tc SFETs" on pages 315 to 316; D7, Section "Field-Induced Effects on Superconductors" on page 711; D10, abstract; D13, page 281, last paragraph, Figure 2).

4.5.1. In document D4, it was estimated that the electric field strength had to be in the range of 107 and 108 V/cm in order to induce a noticeable change of the carrier concentration in the superconducting channel layer. Since such high electric fields were considered to be outside the range of the technology at the publication date of document D4 (cf. D4, page 316, second paragraph), document D4 concentrates on describing other types of superconducting devices (cf. D4, Figures 1 and 8; Sections "FET-like Devices" and "High-Tc SFETs" on pages 313 to 314 and 315 to 316, respectively).

4.5.2. In document D10, it is taught that the superconducting channel has to be very thin (about 2.5 nm) in order to allow proper switching at high speed (about 30 ns) (cf. D7, column 3, lines 23 to 30; column 4, lines 16 to 25; column 5, lines 13 to 15; column 7, lines 21 to 27 and 48. to 56). Oxide superconductor materials, such as YBaCuO, are considered suitable for such thin films (cf. D7, column 8, lines 11 to 17).

4.5.3. Document D13 similarly discusses the importance of keeping the channel layer thin due to the screening effects (cf. D13, page 282, first paragraph). Due to the low density of states of carriers in oxide superconductors, these materials are considered suitable for the channel layer. In addition, it is noted that the projected performance of field-effect devices improves greatly as the gate length is reduced to the nanometer scale (cf. D13, page 283, second paragraph). As a means to form a gate electrode with such dimensions, it is suggested to use electron beam lithography and to form the superconducting device on a thin membrane (cf. D13, Figure 3).

4.5.4. In summary therefore, none of the documents D4, D7, D10 and D13 discloses a channel at an angle with respect to the substrate deposition surface, as the case is in the claimed device.

4.6. In this connection, the appellant submitted that a skilled person faced with the above technical problem would consider combining the teaching of document D1 with that of document D10. The Board however, does not agree with the appellant since the devices of documents D1 and D10 operate according to entirely different principles, the skilled person would not consider such a combination. Thus, the device of document D1 operates by injecting quasi particles through a tunnel barrier from the gate electrode into the superconducting channel to trigger a phase transition in the channel layer, the device described in document D10, as well as those in documents D4, D7, and D13, relate to field effect devices where no significant tunneling of carriers takes place across the gate insulating layer.

4.6.1. Moreover, it appears from document D1 that the purpose of using a construction where the channel layer has an angle with respect to the substrate surface is to form the channel (called "link" in document D1) as short as possible (cf. page 394, last paragraph), since the small dimensions of the channel layer were believed to improve the switching speed of the device. The particular choice of NbN (not an oxide superconductor) for the channel layer is also described as an important factor for enhancing the switching speed, since NbN combines the properties of having a high critical superconducting current and a high resistivity in its normal state. The effective length of the gate electrode ("injector"), however, does not appear to be given any particular importance in document D1.

In documents D4, D7, D10, and D13, on the other hand, the main focus is on the difficulties associated with producing a superconducting channel layer that can be controlled by field effect. These problems are, in particular, related to finding superconducting materials with sufficiently low density of states to be controllable by applying an electric field, and growing sufficiently thin superconducting layers having such low density of states, and finally finding materials for the gate insulating layer which can withstand very high electric fields required in these type of devices (cf. points 4.5 to 4.5.3 above).

From the above, it is apparent that the technical considerations underlying the design of the device structure disclosed in document D1 are different from those disclosed in documents D4, D7, D10, and D13.

4.6.2. Finally, the gate electrode of the device of document D1 partially overlaps one source/drain electrode ("bank") which has the consequence that the thickness of the gate electrode does not determine the gate length. Thus, even if one would contemplate combining the features of the device of document D1 with those of the device disclosed in any of the documents D4, D7, D10, and D13, such a combination would still fail to have all the features of claim 1.

Thus, for the above reasons, the Board finds that the skilled person seeking to improve the device of document D1 would not consider any documents relating to superconducting field effect devices to be relevant to this end.

4.7. The appellant also submitted that the skilled person would arrive at the claimed subject matter in an obvious manner by combining the teaching of document D1 with that of document D9. Such a combination would, in the Board's opinion, be even more unlikely, since not only do the two devices operate according to different principles, but also, neither device is a field effect device: Whereas the device of D1 is a tunnel injection device where a tunnel current is injected into the superconducting channel, the current in the channel layer of the device of document D9 is controlled by subjecting the channel layer 3 to a magnetic field H induced by a control current Is flowing through a conducting line 20 perpendicular to the channel layer (cf. D9, column 5, lines 39 to 52). Therefore, contrary to the submission of the appellant, the device of document D9 cannot be regarded as a "field effect device", since the term "field effect device" is in the technical field of electronic devices --regardless whether they are based on superconductor or semiconductor technology-- reserved for devices where the conductivity in the channel is controlled by an applied electric field.

4.8. During the oral proceedings, the appellant also submitted that document D3 could equally be considered as the closest prior art, and that it would be obvious to a skilled person to combine the teaching of document D3 with that of document D10 to arrive at the claimed device.

4.8.1. Document D3 relates to a so-called SFET, i.e. a superconducting device where superconducting source and drain electrodes are separated by a semiconductor channel where the distance between the source and drain is small enough that superconductivity is induced in the channel through a phenomenon known as the "proximity effect". The device of document D3 comprises a channel layer formed in a step 5 perpendicular to the main surface of a semiconductor substrate 2, source and drain electrodes 3, 4 formed above and below the step, a gate insulating layer 7 formed on the channel, and a gate electrode 7 formed on the step. The current through the channel is controlled by an insulated gate, i.e. field effect. The purpose of placing the channel along the step is to adjust accurately the small distance between the source and drain (cf. D3a, page 3, first paragraph). The gate electrode can be made of aluminum or of a superconducting layer. As superconductor material for the channel layer and gate electrode, oxide superconductors, such as BiSrCaCuO is suggested (cf. D3a, page 5, penultimate paragraph).

4.8.2. The device of claim 1 differs from that of document D3 not only in that the channel is made of a superconducting layer instead of a semiconductor, but also in that the gate electrode is abutting the opposite face of the channel layer to that contacted by the source and drain electrodes, whereas in the device of document D3, the gate electrode is abutting the same face of the channel layer as the source and drain electrodes. Moreover, as the gate electrode in the device of document D3 is overlapping the source and drain electrodes, the gate length of the device is not determined by the physical size of the gate electrode, but by the source-drain distance.

4.8.3. Thus, although the device of document D10 is described therein to have advantages over devices working according to the proximity effect, such as the device of document D3, the resulting device from such a combination of documents D3 and D10 would neither have the gate length determined by the thickness of the gate electrode layer, nor would the gate electrode be on the opposite face of the channel layer to that of the source and drain electrodes. The Board also does not see any indication that the modifications of the above-mentioned features required for arriving at the claimed subject matter could be carried out by the skilled person in a routine manner.

4.10. Therefore, in the Board's judgement, the subject matter of claim 1 involves an inventive step within the meaning of Article 56 EPC.

4.11. Since the method of independent claim 12 results in a device having the same features as that of claim 1, the subject matter of claim 12 involves an inventive step for the reasons given above in respect of claim 1. The patent in suit therefore meets the requirements of Article 52(1) EPC.