PROCESS FOR THE PRODUCTION OF BIPOLAR DEVICES

Abstract: In order to limit lateral oxidation and to produce a substantially planar surface of a bipolar device, particularly devices of 1 mu m, or less, local oxidation is carried out on a device on which a layer of polysilicon is deposited on a substrate containing a buried n<+> collector, mesa and base regions, a silicon nitride layer is deposited on the polysilicon layer and selectively etched after a resist layer has been applied, p<+> boron is implanted to penetrate the base region and form a base contact, the resist layer is removed and an oxide layer of pyrox is deposited. The local oxidation is performed at moderately low temperature &tilde& 950 DEG C and elevated pressure ( &tilde& 10 atm) so that lateral oxidation and base distribution is minimised, so that near vertical oxide walls result.

Description: PROCESS FOR THE PRODUCTION OF BIPOLAR DEVICES This invention relates to a process for the production of bipolar devices and is particularly concerned with a process for the production of υm minimum feature size bipolar devices. In designing a or sub icron bipolar device, attention must be paid to those aspects of processing that will maximise performance and improve manufacture. Generally, one is sacrificed to some extent by the other and usually a compromise is reached depending on the particular application. For example, at small geometries the high speed capability of an npn bipolar transistor can be governed by the p + iffusion-to-emi ter edge spacing. The auto registration of the p + diffusion-to-emitter edge then results in considerable improvement in device speed but has, to date, resulted in substantially more complex processing. Furthermore, the inclusion of auto registration may completely determine the method of fabrication and give rise to additional difficulties for subsequent processing stages. For example, surface planarity may be completely lost giving rise to device metallisation and interconnect failures. It is also important to ensure that, especially with 1 >τn bipolar devices, lateral oxidation beneath a nitride mask is kept to the minu um. The present invention aims to provide a process for the production of a bipolar device in which lateral oxidation is minimised, and, preferably, a process in which the p + diffusion-to-emitter edge separation is au oregistered enable the device to be quickly and easily producd. According to the invention, there is provided a process for the production of a semiconductor device which includes the steps of depositing a layer of polysilicon on a substrate containing a base region, depositing a nitride layer on said polysilicon layer and selectively etching said nitride layer, implanting a p + implant into the base region to form a base contact, and applying an oxide layer followed by local oxidation of the polysilicon layer, wherein the oxidation of the polysilicon layer is carried out at elevated pressure. Preferably, the nitride layer is selectively etched by a dry etching technique using a resist mask which is applied to the nitride layer. The resist layer may also serve as an implant mask when the p + implant, which is desirably boron, is implanted in the base region. The resist layer is then removed before the deposition of the oxide layer which is preferably pyrox. The pyrox is desirably deposited to a thickness of 1000 A to 2000 A. The resist mask used for the p + implant may comprise the resist mask applied to the nitride layer. Local oxidation of the polysilicon layer is desirably carried out at moderately low temperature of about 750°C to 950°C and at an elevated pressure, which may be about 10 atmospheres. After a first high pressure ox.idation, the oxide layer is removed to expose the nitride layer after which a base cover-up mask is applied and an implant such as As + carried out to form an emitter. The cover-up mask, which serves as a resist layer for the emitter implantation, is then removed and a further oxidation, which may also be at high pressure, is carried out. Finally, the nitride layer may be removed followed by emitter drive-in and metallisation. The invention also extends to a bipolar device when made by the above-described process. The invention will now be further described by way of example, with reference to the drawings, in which:- Figure 1 is a section through a bipolar device to be produced by the process according to the invention; Figure 2 is a section, to an enlarged scale, of the area indicated by the letter A in Figure 1; and Figures 3 to 7 are sections, corresponding to Figure 2, showing various steps in the production of a bipolar device according to the invention. Referring to the drawings, following formation of the buried n + collector, mesa and base regions, as shown in Figure 1, ^1000 i. of polysilicon is blanket deposited. Silicon nitride -v 250 A thick is deposited and patterned by standard dry etching techniques using a resist mask. A deep p + boron implant is performed to penetrate the base region and forms the base contact using the resist as the implant mask. This condition is shown in Figure 2 which shows the emitter-base region of the device since the emitter is the most critical region. The resist is then removed and ^ 2000 A thick layer of pyrox is deposited followed by local oxidation of the polysilicon layer. Oxidation of the polysilicon layer is performed at moderately low temperature (750°C to 950°C) and elevated pressure (5 to 25 atm). Elevated pressure is necessary in order to prevent excessive base redistribution. The oxidation is continued for 10 to 60 minutes until the single crystal substrate has just been penetrated and it has been found that, with the aforesaid structure and conditions, the extent of lateral oxidation is minimal with near vertical oxide side walls as shown in Figure 3 of the drawings. The underlying silicon nitride layer is then exposed by chemically removing the uppermost surface oxide layer to produce the structure shown in Figure 4 and a base cover-up mask (not shown) is located over the base regions after which arsenic is implanted through the silicon nitride layer into the surface region of the polysilicon layer. The alignment tolerance of the cover-up mask over the base contact is not critical and the arsenic does not penetrate the single crystal layer underlying the polysilicon layer as shown in Figure 5 of the drawings. A further wet oxidation is then performed at 800°C low temperature to exploit the enhanced oxidation rate of arsenic doped polysilicon. The oxidation may also take place at elevated pressure. Arsenic redistribution is confined to the polysilicon at this low temperature with no significant arsenic penetration into the single crystal as indicated in Figure 6. Furthermore, boron redistribution from the p + and base regions is, therefore, small when compared to known processes. An oxidation rate enhancement of up to 10X has been achieved for arsenic doped polysilicon compared to boron doped single crystal. Thus, lateral oxidation extending about 1000 - 2000 A may be achieved with only slight vertical oxidation. Finally, chemical removal of the silicon nitride layer is performed and the emitter-base junction formed by subsequent emitter drive-in. The emitter region is therefore separated from the p + base contact by the extent of lateral oxidation as shown in Figure 7 of the drawings. The device can then be metallised using known techniques. It has been found that in a bipolar device produced by the process according to the invention there is reduced base diffusion, minimal lateral oxidation and a near-planar surface. The process itself is simple and permits "add-on" autoregistration incorporating selective enhanced oxidation in localised areas of the polysilicon layer. The process is eminently suitable for the production of l/-m, and possibly also sub-micron, bipolar devices and achieves the aims of high speed and ease of manufac ure. Although the present invention has been described with respect to a particular embodiment modifications may be effected within the scope of the invention. For example, process temperatures and pressures other than those recited may be utilised. Furthermore, the resist mask used for the p + implant may also be used as the resist mask applied to the nitride layer.

Claims: CLAIMS : 1. A process for the production of a bipolar device which includes the steps of deposting a layer of polysilicon on a substrate containing a base regiαπ depositing a nitride layer on said polysilicon layer and selectively etching said nitride layer, implanting- a.g 4 " implant into the base region to form a base, contact,, and applying an oxide layer followed by local oxidation of the polysilicon layer, wherein the oxidation of the polysilicon layer is carried out at elevated pressure. 2. A process according to claim 1, wherein the nitride layer is selectively etched by a dry etching technique using a resist mask which is applied to the nitride layer. 3. A process according to claim 2, wherein the resist layer serves as an implant mask when the p + implant is implanted into the base region. 4. A process according to claim 3 wherein the resist mask for the p + implant comprises the resist mask applied to the nitride layer. 5. A process according to claim 4, wherein the p + implant is boron. 6. A process according to claim 4 or claim 5, wherein, after implantation into the base region, the resist layer is removed prior to deposition of the oxide layer. 1, A process according to any preceding claim, wherein the oxide layer is pyrox. 8. A process according to any preceding claim, wherein the oxide layer is deposited to a thickness of Λ ^2000 .. 9. A process according to any preceding claim, wherein the local oxidation of the polysilicon layer is carried out at a temperature not exceeding about 950°C. 10. A process according to any preceding claim, wherein the local oxidation of the polysilicon layer is carried out at an elevated pressure. 11. A process according to claim 10, wherein the local oxidation is carried out for between 10 to 60 minutes. 12. A process according to any preceding claim, wherein nitride layer consists of silicon nitride deposited to a thickness of ^- 250 A. 13. A process according to any preceding claim, wherein the oxide layer is removed, after oxidation, a base cover-up mask is applied and implantation is carried out to form an emitter in the base region. 14. A process according to claim 13, wherein arsenic is implanted into the base region to form the emitter. 15. A process according to claim 13 or claim 14, wherein, after emitter implantation, further oxidation is carried ou . 16. A process according to claim 15 wherein the further oxidation is carried out at approximately 800°C. 17. A process according to claim 15 or 16, wherein said further oxidation is also carried out at elevated pressure. 18. A process according to any one of claims 15 to 17, wherein, after said further oxidation, the nitride layer is removed and an emitter-base junction formed by subsequent emitter drive-in. 19. A process according to claim 18, wherein the nitride layer is chemically removed. 20. A process according to claim 18 or claim 19, wherein, after formation of the emitter-base junction, the device is metallised. 21. A process for the production of a bipolar device substantially as described herein with reference to the drawings. 22. A bipolar device when made by the process claimed in any one of the preceding claims. 23. A bipolar device substantially as described herein with reference to the drawings.

Inventors: CHILDS PETER ANTHONY, HUNT PETER CHARLES, WILSON MARTIN CLIVE, CHILDS, PETER, ANTHONY, HUNT, PETER, CHARLES, WILSON, MARTIN, CLIVE
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1987
Main classification: H01L21
IPC codes: H01L21, H01L29