T 0032/84 (Redefining an invention) of 6.8.1985

Summary of Facts and Submissions

I. European patent application No. 80 400 098.2 filed on 22 January 1980 claiming the priority of an earlier application of 26 January 1979 and with publication No. 0 014 149, was refused by a decision of the Examining Division of 26 September 1983. This decision was taken on the basis of Claims 1 to 5 filed with a letter dated 6 October 1982.

II In its decision the Examining Division stated that the invention was not described in the application documents in such a manner as to enable a person skilled in the art to carry it out as required by Article 83 EPC.

III. The applicants filed a notice of appeal against this decision on 18 November 1983 and a statement setting out the grounds for appeal which was received by the EPO on 16 January 1984.

4. The applicants maintained their application unamended in the form in which it had been considered by the Examining Division together with Claims 1 to 5 as filed with the letter dated 6 October 1982. The Board considered the case on the basis of the following documents. p. - Pages 2 to 13 of the description as originally filed and page 1 as filed with the letter dated 6 October 1982.

- Claims 1 to 5 filed with the letter dated 6 October 1982.

- Three sheets of drawings as filed with the original application.

The claims read as follows:

1. A reference voltage generator comprising two MOS transistors T' and T'2 having the same substrate, characterised in that the channels of these transistors are of lengths L1 and L2 and widths Z1 and Z2 respectively, one of the transistors T'2 having a channel of which one dimension only is of the same order of size as the corresponding dimension of the extension relative to the source (S'2) and drain (D'2) of this transistor, of the space charge zone appearing around the said source and the said drain when the transistor is in operation, the other dimension being large relative to the corresponding dimension of the said extension of the space charge zone, the other transistor T'1 having a channel, both dimensions of which are large by comparison with the dimension of the corresponding space charge zone and in that it comprises means for fixing the common voltage (VBS) of the source and substrate of the transistors T'1 and T'2, the sources of which (S2, S'2) are connected to a common point (s), means for producing the difference in threshold voltages of the transistors T'1 and T'2 by measuring the difference of the respective gate source voltages of the transistors T'1 and T'2, which means comprise a capacitor (C'), one terminal of which is connected to a first point (D') common to the gate (T'2) and the drain (D'2) of the transistor (T'2) and the other terminal to a second point (D") which is common to the gate (T'1) and the drain (D'1) of the transistor T'1 by means of two switches (I3, I'3) controlling the charge and discharge operations, the first and second points (D' and D") being connected to a DC voltage source (V'1) by means respectively of two potential applying switches (I'1 and I"1) so that when the two charge/discharge switches (I3, I'3) and the two potential applying switches (I'1 and I"1) are closed each of the plates of the capacitor (C') becomes charged to the supply potential (V'1) and so that when the potential applying switches (I'1 and I"1) are open and the charge/discharge switches (I3 , I'3) are closed the capacitor plates discharge into the transistors T'2 and T'1 until the difference in potential available at the terminals of the capacitor (C') is equal to the difference in the threshold voltages of the transistors T'1 and T'2, the difference in the threshold voltages of the transistors T'2 and T'1 being made available at the terminals of the capacitor (C') by means of two output switches (I2 and I'2) which are closed once the charge/discharge switches (I3 and I'3) have been opened when the difference in potential at the terminals of the capacitor (C') is equal to the difference in the threshold voltages of the transistors T'2 and T'1.

2. A reference voltage generator as claimed in Claim 1, characterised in that the charge/discharge switches (I3 and I'3) the potential applying switches (I'1 and I"1) and the output switches (I2 and I'2) are MOS transistors, the gates of which are connected to control means (03) in order to render the charge/discharge transistors (I3 and I'3) and the potential applying transistors (I'1 and I"1) conductive for long enough to charge the plates of the capacitor (C') to the supply potential (V'1) and then to block the potential applying transistors (I'1 and I"1) so that the plates of the capacitor (C') discharge into the transistors T2 and T'1 until the potential difference at its terminals is equal to the difference in the threshold voltages of the transistors T'2 and T'1 and then to render the output transistors (I2 and I'2) conductive when this threshold voltage difference has been achieved, the said voltage being available between the sources of the output transistors (I2 and I'2) whose drains are connected to the terminals of the capacitor (C'), the drain of the transistor T'2 being connected to the drain of the charge/discharge transistor I3 and the drain of the transistor T'1 to the drain of the other charge/discharge transistor I'3, the sources of both charge/discharge transistors being connected to the said DC voltage source (V'1).

3. A reference voltage generator as claimed in Claim 2 characterised in that the source of one of the output transistors (I'2) is connected to means to bring this source to a predetermined polarisation potential (Vo) relative to the potential of the substrate of the transistors T'2 and T'1.

4. A reference voltage generator as claimed in Claim 3 characterised in that the length L1 of T'1 is greater than 30 and the length L2 or width Z2 of the channel of T'2 is less than 5.

5. A reference voltage generator as claimed in Claim 4 characterised in that the capacitor (C') and the MOS transistors take the form of an integrated circuit on a substrate. V.In their appeal statement the appellants maintained that the application disclosed the invention in a manner sufficiently clear and complete for it to be carried out by a person skilled in the art.

VI. Detailed account of the procedure before the Examining Division:

The application as originally filed had nine claims. In his first communication of 12 August 1982 the examiner expressed a number of objections, one being the statement on page 2, paragraph 3 that "It would appear that the circuit shown in Figure 5 does not function as described on page 12, lines 16 to 24, for if the switches I'1, I"1 and I3 and I'3 are closed the capacitor terminals are at the same potential and no charging can take place." In a reply dated 6 October 1982 the applicants filed a new version of page 1 together with a new set of five claims to replace those originally filed.

In his communication of 12 January 1983 the examiner repeated his objection concerning the impossibility of the circuit shown in Figure 5 functioning as described, stating that "It was pointed out in the communication of 12 August 1982 (page 2, paragraph 3) that the circuit shown in Figure 5 to which Claim 1 now relates does not function as described so that the invention as claimed in Claim 1 does not meet the requirements of Articles 52 and 57." In a reply dated 20 January 1983 the applicants contested this argument, maintaining that the transistors T'1 and T'2 had different threshold voltages, enabling the capacitor to charge up and that this difference in threshold voltages and the charging of the capacitor C' were in fact the essential points of the generator of the invention.

In his communication of 16 June 1983 the examiner once again concentrated on the functioning of the circuit as shown in Figure 5 of the application. He insisted that the circuit definitely could not function as described without additional capacitors and that the figure did not show any capacitance other than the capacitor C'. He therefore concluded that the subject-matter of the application, namely generation of a reference voltage, was not feasible using the circuit proposed and that consequently the invention was not disclosed in the application in a manner sufficiently clear and complete to enable a person skilled in the art to carry it out, as required by Article 83 EPC.

In their reply of 8 July 1983 the applicants detailed the functioning of the circuit concerned and explained the timing of the opening and closing of the transistor switches under the influence of the control signals 01', 02 and 03. On 26 September 1983 the Examining Division refused the application on the sole grounds of failure to function of the circuit as shown in Figure 5, pointing out that there was no reference in the text to a capacitance other than (C').

Reasons for the Decision

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

2. The Examining Division considered that the circuit constituting the subject-matter of the application did not enable a reference voltage to be obtained and consequently decided to refuse the application solely on the basis of Article 83 EPC (page 4, last paragraph). It therefore concerned itself only with the alleged failure to function of the circuit shown in Figure 5 of the application and did not express any opinion as to the claims. The appellants request that the patent be granted on the basis of the claims in their present form (page 1, lines 2 and 3 of the appeal statement).

A reading of Claim 1 makes clear that the decisive objection is that based on Article 83 EPC as set out in the decision to refuse the application. Before reaching a decision on the present appeal the Board considered it necessary to reconsider the question of whether the application as initially filed provided sufficient information to a person skilled in the art to enable him to carry out the invention, as required by Article 83.

3. This makes it necessary to consider the functioning of the circuit shown in Figure 5 and the description as filed. The circuit and its mode of operation are covered by the present Claim 1. Pages 12 and 13 of the description explain how the reference voltage is generated in the circuit shown in Figure 5 when the switching signals 0'1, 02 and 03 are applied in three successive stages. The following is a brief description of the three stages designated (a), (b) and (c) using the terminology of the application. (a) Stage 1: The switches I', I", I3, I'3 are closed. The switches I2 and I'2 remain open as long as the switches I'1, I"1, I3, I'3 are closed. Consequently the plates of the capacitor C' are charged to the supply potential V'1; (b) Stage 2: The switches I3 and I'3 remain closed as long as the switches I'1, I"1, I2 and I'2 are open. This causes the capacitor C' to discharge through the transistors T'2 and T'1 until the voltage VD'1-VS between the gate and source of the transistor T'1 is equal to the threshold voltage VT'1' of this transistor and the voltage VD'2-VS between the gate and the source of the transistor T'2 is equal to the threshold voltage VT'2 of this transistor. Consequently the difference in potential VD'1-VD'2 between the gates or drains of the transistors T'1 and T'2 is equal to the difference in threshold voltage VT'1-VT'2 of the two transistors T'1 and T'2. (c) Stage 3: The switches I'1, I"1, I3, I'3 remain open as long as the switches I2, I'2 are closed. The difference in threshold voltage VT'1-VT'2 occurs between the terminals S' and S".

The Board doubts whether at stage (a) the two electrodes of the capacitor (C') are at the same potential. The effect of closing the switches I' and I" is to produce a parallel arrangement between the terminal at potential V'1 and the terminal S, the first branch being composed of the two transistors I'1 and T'2 in the conducting state in series and the second the transistors I"1 and T'1 also in the conducting state. As these transistors have different characteristics and different threshold voltages the current passing through the two branches should also differ. Since, therefore, the branches act as voltage dividers through which different currents flow, the potentials of points D' and D" should be different. The Board considers the essential aspect of the circuit's operation to be the fact that at stage (b) the switches I'1 and I"1 are open while I3 and I'3 are closed. In these circumstances and allowing that the capacitor C' has been charged by the non-zero difference in potential between points D' and D", it could discharge into the series circuit comprising I3, T'2, T'1 and I'3. Since the transistors T'1 and T'2 have different threshold voltages one of the them - namely that having the higher threshold voltage, for example T'1 - would be blocked before the other one. There would then no longer be any continuity in the series circuit, the capacitor C' would no longer discharge and the source/drain voltage and current would disappear. At the end of stage (b), therefore, there would be a difference in potential between the two electrodes of the capacitor C' equal to the higher threshold voltage of the two transistors T'1 and T'2. It is thus impossible for a reference voltage VT'1 and VT'2 to be obtained in this manner at the terminals of the capacitor C'. This conclusion bears out the Examining Division's decision that the circuit concerned did not function as described unless accompanied by other means such as additional capacitances enabling the gate/source voltages of the transistor T'1 and T'2 to be maintained at their threshold level. It would seem important that during stage (a) closure of the switches I1' and I" makes the transistors T'1 and T'2 become conductive with gate/source voltages higher than their threshold value while during stage (b) the opening of switches I'1 and I" leads to a decrease in both the gate/source voltages of the transistors T'1 and T'2 and in their gate/source currents until they are both blocked, two events which are probably not simultaneous. To enable this circuit to function as described the gate/source voltage of the two transistors, which is equal to the threshold voltage, must remain constant long enough for the potential of each electrode of the capacitor C' to become equal to that of the transistor gate to which that electrode is connected via one of the switches I3 or I'3. The invention therefore makes it possible to obtain a reference voltage equal to the difference between the true threshold voltages of the transistors T'1 and T'2. This principle is not to be found in any of the citations contained in the search report.

4. The idea of using the true threshold voltage of a transistor is disclosed in that portion of the description relating to Figure 4 which gives a detailed explanation also applicable to Figure 5. It describes how the switch I1 is only closed for a fairly short periodt and then opens, enabling the threshold voltage T2 to be used after a certain time as a reference voltage. It is also clear that the capacitor (c) is an essential component of this circuit since once the switch I1 has opened it is this capacitor that enables a voltage to exist between the gate or drain (point A) on the one hand and the source (point E) of the transistor T2 on the other. Since at the moment the switch I1 opens this voltage is higher than the threshold voltage of the transistor T2, the capacitor (c) will discharge until its voltage is equal to the threshold voltage of the transistor T1. The difference in potential at the terminals of the capacitor (c) that is available as a reference voltage is thus similarly equal to the threshold voltage of the transistor T2.

5. In the description as filed the aim of the invention is stated on page 2, lines 13 to 21 and its subject-matter defined on page 2, line 22, to page 3, line 5. It is clear from the latter passage that, in the words of Claim 1 as initially filed, the invention relates to "means for producing a difference in the threshold voltages of the transistors T'1 and T'2, which difference constitutes the said reference voltage". (i) This shows that in its original form the application was concerned with the generation of a reference voltage equal to the non-zero difference in the threshold voltages of two transistors specifically chosen for this purpose. Since the applicant used the references T'1 and T'2 in specifying the subject-matter of the application on pages 2 and 3 it is clear that Figure 5 of the application, as the only one showing these references, illustrates the subject-matter.

(ii) In order to explain the functioning of this circuit in which the threshold voltages of two transistors (T'1 and T'2) are generated the applicants begin by describing the mode of generation of a threshold voltage for a single transistor (T1 in Figure 4). As is clear from the description and the explanations given above the capacitor (c) is an essential component of this circuit. Moreover a continued reading of the application reveals the following statement on page 11, lines 33 to 37:

"The operation of the reference voltage generator shown in Figure 5 will be better understood from the following explanation. The generator utilises two threshold voltage measuring circuits of the type previously described when referring to Figures 4a and 4b." The Board of Appeal considers that just as the circuit of the transistor T'1 shown in Figure 5 has an additional capacitor equivalent to the capacitor (c) in Figure 4a, the circuit of the transistor T'2 should have its own capacitor equivalent to (c).

(iii) The Board considers that a person skilled in the art having read the application, and in particular lines 33 to 37 of page 11, would be suprised not to find illustrated in Figure 5 - in addition to the capacitor (C') - capacitors similar to those shown in Figure 4a. He will then find that the circuit shown in Figure 5 cannot function unless he takes account of the statement made on page 11, lines 33 to 37 quoted above which involves the existence of additional capacitors equivalent to the capacitor (c) shown in Figure 4a. (iv) The Board of Appeal is of the opinion that in view of what has been said above the application provides sufficient information to enable a person skilled in the art to form a complete concept of the invention and to carry it out. (v )Since the teaching given in the application is quite unambiguous to a person skilled in the art the fact that the applicants have omitted to refer to the additional capacitors and to show them in Figure 5 should not be interpreted against them.

6. In paragraphs 3 to 5 above the Board has considered whether the application as originally filed complied with the requirements of Article 83 EPC. It has not taken into account the information, arguments and explanations advanced by either the examiner or the applicants. Since as pointed out above the application provides sufficient information to enable the invention to be carried out in practice this question must be answered in the affirmative. The Board does not, therefore, need to consider the question of compliance with Article 123(2) EPC.

7. The additional capacitances involved here are, for example, the parasitic capacitances that occur in an integrated circuit and an inherent feature thereof.

A person skilled in the art is perfectly aware of this fact and also knows that it is possible to ensure a very specific value for these capacitances during circuit manufacture utilising this knowledge when designing them. Further information on this subject will be found on pages 30 and 31 of "Principes et applications des circuits intégrés" (1972) by H. Lilen, to which the appellants refer and in which such capacitances are described as "structural capacitances". This thinking is also reflected in the application when it states on page 9, lines 31 to 33, that "the small-geometry transistor T2, the MOS transistor I1, and the capacitor C may take the form of an integrated circuit on a semi-conductor substrate".

8. These comments should not be allowed to mask the deficiencies in the text of the application which requires close study for a clear understanding. Thus on page 12, lines 20 to 24 it contains the statement that:

During the first stage the plates of the capacitor C' are charged to the source potential V'1. However three lines later (27 to 29) it is stated that During the second stage the capacitor C' discharges via the transistors T'2 and T'1. The first sentence must be interpreted as meaning that the two terminals of the capacitor C' are at the same potential, thus that the capacitor C' is not charged. C' cannot therefore discharge via the transistors T'1 and T'2 as the second sentence states. This textual error in the application explains why the Examining Division concluded that the circuit shown in Figure 5 could not function as described. It is worth pointing out that such additional capacitances exist either between the gate and source of the transistor (see Figure 4a) or between the gate and the substrate, or - since the gate is connected to the drain - between drain and source or drain and substrate. The capacitance between the drain and substrate may in the case of the circuit shown in Figure 5 be replaced by the capacitance between one electrode of the capacitor C' and the substrate without impairing the circuit's operation. This is because the switches I3 and I'3 are closed while the capacitor is discharging to the level of the threshold voltage of the transistor concerned.

9. The Examining Division's decision to refuse the application is based on alleged non-compliance with Article 83 EPC. Since this is sufficient grounds for a refusal the Examining Division had no reason to continue the procedure and consider the other conditions for patentability, namely novelty and inventive step. The fact that the second communication of 12 January 1983 refers to Articles 52 and 57 EPC does not mean that the application was examined in the light of any criteria other than that of Article 83 EPC.

Since, therefore

- the grounds for refusal under Article 83 EPC have been shown to be incorrect,

- the application has not yet been examined in the light of patentability criteria other than that of Article 83,

- the applicants are entitled to demand equitable treatment, the Board of Appeal is referring the application back to the Examining Division under Article 111(1) EPC.

ORDER

For these reasons, it is decided that:

1. The disputed decision is set aside.

2. The matter is referred back to the Examining Division for further examination of the application.