T 0009/91 (Programming Read Only Memory/TOSHIBA) 14-07-1992

1. Allowability of the Amendments

Although the features in new Claim 1 have been rearranged, the claim differs in substance from the published version only in that (i) a contact hole is formed to expose the second conductivity type region of some of the transistors (cf. feature II(a) of the claim) and (ii) an aluminium contact electrode is formed on the insulating film away from the transistors (cf. feature II(b) of the claim).

New feature (i) is disclosed on page 7,lines 12 and 13 of the application documents as originally filed. Similarly it is evident from the original disclosure on page 7, lines 13 to 16 in combination with Figure 5B that an aluminium contact electrode (80) is formed away from the transistors in the sense that it does not overlie the gate regions of the transistors.

The amendments therefore meet the requirements of Article 123(2) EPC.

2. Inventive step

The only issue which remains to be considered in the present appeal is, therefore, the question of inventive step.

2.1. The patent in suit concerns a method of manufacturing a mask type Read Only Memory (ROM) comprising series connected MOS transistors, which is programmed according to customer's specifications at a late stage of device fabrication, so that the delay in supplying the programmed ROM to the customer is reduced. In the method, after the formation of a field insulating layer (65) and a gate insulating layer (67), a gate electrode (69,70), and source and drain regions of MOS transistors are formed using the gate electrode as a mask so that it is self- aligned to the source and drain regions and the source regions of the adjacent transistors are interconnected (cf. features I(a) to I(d)). Subsequently, a second insulating film (78) is formed on the surface of the substrate in which the transistors are formed, a contact opening (79) exposing the source or drain region is provided in the second insulating film and a contact electrode (80) contacting the source or drain region through the opening and a bonding pad (81) are provided. The contact electrode is provided on the second insulating film away from the transistors in the sense that it does not overlie the gate regions of the transistors (cf. features I(e), II(a) to II(c)). At this stage of the device fabrication, therefore, the transistors are covered by the second insulating film, and the memory device is ready for programming according to customer's specifications. For programming, the second insulating film is selectively removed to expose the gate electrode and source and drain regions of some of the selected transistors and ions having the same conductivity type as the source and drain regions are implanted so that the source and drain regions of the exposed transistors are interconnected by the channel regions (cf. features III(a) and III(b)). A final protective film (84) is then provided on the bonding pads (81), contact electrode (80) and the transistors, and the bonding pads are subsequently exposed and outer leads (86) are provided on the bonding pads (cf. feature IV).

2.2. In the Board's view, the prior art coming closest to the invention is disclosed in document D2 which, as correctly pointed out by the Respondent, is concerned with the manufacture of a ROM device wherein programming is carried out at a late stage of the device fabrication whereby delay in supplying the ROM device programmed according to customer's specifications is reduced. It is evident from Figure 2 that MOS transistors of this known device are connected in parallel.

In one embodiment described with reference to Figures 4a to 4f (see page 3, line 93 to page 6, line 9), after the formation of field oxide regions (24), openings in an oxide layer (31) defining source and drain regions of ROM array transistors are formed. Subsequently, the source and drain regions (16,17) are formed using the field oxide regions and a nitride layer (32) as a mask, a gate oxide (19) is formed and a polysilicon layer (35) is deposited over the entire substrate. As is evident from Figure 1, the source regions of the adjacent transistors are interconnected. The polysilicon layer is then patterned to form inter alia gate electrodes (11). The gate electrode is thus not self-aligned to source and drain regions as in the method according to the invention. A composite insulating film, which can be regarded as the second insulating film (78) employed in the method according to the invention, consisting of a nitride film (37) and an overlying multilevel oxide (38) is deposited over the entire substrate and is then removed so that the entire ROM array area, a metal-to-polysilicon contact area (39), and a metal-to-source contact area (41) are exposed. Metal contacts in the contact areas (39,41), interconnections and bonding pads are then formed. At this stage in the process when the ROM devices are waiting customer's specifications for programming, the gate electrodes (11) in the ROM array are not covered by the composite film (37,38) (see the right-hand side of Figure 4f) and are exposed to the environment.

For programming according to customer's specifications, a protective oxide layer (21) covering the entire substrate is deposited and then patterned so that an aperture (22) is formed in the protective oxide layer (21) over each of the transistors to be programmed. Ions having the same conductivity type as the substrate are then implanted through the apertures in the channel area so as to raise the threshold voltage of the selected transistors. Although in connection with the programming step, a reference is made to Figure 4g in the description, there is no Figure 4g in the document.

In the above method of programming using the protective oxide layer (21) as the implant mask, the transistors which are programmed are left without a covering of the protective oxide layer, and it is recognised in the document that this might have detrimental effects on the transistors. As an alternative, therefore, a photoresist is used as the ion-implant mask, and after the programming, the protective oxide layer (21) is deposited and patterned to expose only the bonding pads.

It is stated at page 5, lines 115 to 119 of document D2 that to provide additional protection, instead of removing the multilevel oxide coating (38) from the entire cell array area as shown in Figure 4f, it may be removed only over the gates of the transistor (10). There is no disclosure as to whether or not the underlying nitride layer is left over the gates of the transistors. The Board agrees with the Appellant that it is not clear from this passage whether the multilevel oxide is removed over the gates of all the transistors or only over the transistors to be programmed. In the Board's view, no matter how the above disclosure is interpreted, during programming a protective oxide or a photoresist is always required as a mask for ion-implantation.

2.3. In summary, therefore, the claimed method differs from the closest prior art in that

(i) the MOS transistors formed are connected in series ;

(ii) the second conductivity type regions, that is, the source and drain regions (74,76) are formed using the gate electrodes as a mask so that the latter are self-aligned to the former;

(iii) at the device fabrication stage after the formation of metal contacts to the second conductivity type regions, interconnections and bonding pads, the second insulating film is left on the surface of the substrate in which the transistors are formed;

(iv) the second insulating film is selectively removed to expose the gate electrode, source and drain regions of the transistors to be programmed, and

(v) subsequently, the second insulating film is used as a mask during ion-implantation of an impurity in the channel regions of the exposed transistors,

(vi) the impurity being of the second conductivity type so that the source and drain regions are interconnected.

In this connection, it is to be noted that features (iv) to (vi), and also feature (iii) in so far as it influences subsequent programming steps, are concerned with the programming of the ROM device at a late stage in the manufacturing process, whereby the number of process steps required to complete the fabrication of the device after programming, and consequently the time in supplying the finished device is reduced. Moreover, since at the end of the device fabrication when the device is awaiting to be programmed, the second insulating film according to feature (iii) covers the MOS transistors, whereby these are protected from detrimental effects of the environment.

Features (i) and (ii), on the other hand, are not at all concerned with programming, but relate respectively to the type of ROM device and formation of source and drain regions of the MOS transistors in a manner whereby not only is the gate electrode accurately aligned to the source and drain regions but additional masking and etching steps required in the prior art method of document D2 (for defining the source and drain regions) are avoided. These features, therefore, do not contribute to the solution of the problem of reducing the number of process steps after programming, and are therefore to be considered separately from the distinguishing features (iii) to (vi) in the assessment of inventive step.

2.4. As regards the distinguishing feature (ii), the Board agrees with the Respondent that this is a well-established technique in the art and is normally employed with a view to self-aligning the gate electrode with the source and drain regions (cf. document D3, Figure 10.1 and the accompanying text). Moreover, in document D4 this technique is employed in the manufacture of a ROM having MOS transistors which are connected in series between a supply voltage VDD and the ground (cf. column 5, line 49 to column 6,line 25; column 3, lines 32 to 41; Figures 3, 5 and 6a to 6f). The Board is therefore of the view that the incorporation of the features (i) and (ii) in the method according to document D2 would have been regarded as obvious by a skilled person.

2.5. Document D1 relates in general to a method of fabricating a ROM device in which data programming is carried out at or near the last stage of fabrication. According to the method described with reference to Figures 1(a) to 1(g) in column 3, line 58 to column 5, line 66, before the last stage of programming, a metallisation pattern (30) remains over all rows of the ROM matrix defined by the field effect transistors that have been formed, and thus over all portions of the gate regions and a passivation film (32) covered by a photoresist is provided over the surface of the wafer. For programming the device by ion- implantation, the passivation film (32) as well as the underlying metallisation (30) need to be selectively removed to expose the desired gate locations. In a photolithographic operation subsequent to the ion- implantation, the passivation layer is again selectively etched to define the areas for pad locations. Thus, contrary to the invention, in the above method the final passivation film, corresponding to the claimed protective film (84), and additionally an underlying metallisation are used as ion-implantation mask during the programming. Moreover, the bonding pads are formed not before but after the ion implantation. Thus the features (iii) to (v) are not suggested by the above embodiment.

In the method described with reference to Figures 4 and 5 (see, column 7, line 30 to column 8, line 3), contact openings (100) for source, drain and gate are formed in a phosphosilicate glass layer corresponding to the second insulating layer of the claimed invention, and simultaneously openings (102) over selected gate regions are formed for programming by ion implantation. The ion- implantation is followed by deposition of metal contacts in the openings (100) for the source and gate. Similarly, in the method described with reference to Figures 6 and 7 in document D1 (see column 8, lines 4 to 36), contact openings (108,110) for source, drain and gate electrode and openings over selected gate regions for programming are formed simultaneously in a phosphosilicate glass layer (98). After ion-implantation for programming, metallisation patterns (118,122,120) contacting the source, drain and gate are formed. In the method according to the embodiment illustrated in Figures 10 and 11 in document D1, contact openings for pad locations and openings over selected gate regions for programming are formed simultaneously. Thus, in the above embodiments openings for programming on the one hand and for metal contacts or bonding pads on the other are formed in the same photolithographic operation. As a result, in the above embodiments, contrary to the features (iii) to (v) of the invention, the metal contacts or bonding pads are formed always after the step of programming a selected transistor by ion-implantation, so that features (iii) to (v) cannot be regarded as obvious in the light of these embodiments.

2.6. In the contested decision, it was held that starting with the method as claimed in document D1, it was a matter of routine design procedure to introduce a separate photolithographic step for programming and thereby accept the drawback of having an additional fabrication step. The Board, however, cannot follow this line of argumentation, firstly because it entails going against the teaching of document D1 and secondly, the problem underlying the invention, as implied in the above line of argumentation, also includes part of its solution, namely, providing a photolithographic step for programming which is in addition to that for forming contact openings, indicating a hindsight analysis which is not permissible in the assessment of inventive step.

2.7. For the foregoing reasons, in the Board's judgment, the subject-matter of Claim 1 as amended is not rendered obvious by the cited prior art within the meaning of Article 56 EPC. Claim 1 is, therefore, allowable under Article 52(1) EPC. Dependent Claims 2 to 4 relate to particular embodiments of the method according to Claim 1 and are, therefore, likewise allowable.