Abstract:
A method in which an oxide layer is formed on material defining and surrounding an emitter window. The technique comprises depositing a non-conformal oxide layer on the surrounding material and in the emitter window, whereby the thickness of at least a portion of the oxide layer in the emitter window is smaller than the thickness of the oxide layer on the surrounding material outside the emitter window; and removing at least a portion of the oxide layer in the emitter window so as to reveal at least a portion of the bottom of the emitter window whilst permitting at least a portion of the oxide layer to remain on the surrounding material. The technique can be used in the manufacture of a self-aligned epitaxial base BJT (bipolar junction transistor) or SiGe HBT (hetero junction bipolar transistor).

Description:
The present invention relates to improvements in transistor manufacture. The invention finds particular application in the manufacture of a bipolar junction transistor (BJT) or a hetero junction bipolar transistor (HBT). Whilst not limited thereto, the invention is particularly useful in the manufacture of a self-aligned epitaxial base BJT or SiGe HBT. 
     BACKGROUND 
     In some transistor manufacturing processes an emitter window is formed, and it is then desired to form an oxide layer on material surrounding the emitter window, whilst it is also desired that the bottom of the emitter window is not covered by the oxide layer. Complicated processes are known in the art which aim to achieve this. 
     SUMMARY 
     In one embodiment, the present inventor has devised a novel method of providing an oxide layer on material surrounding an emitter window whilst ensuring that at least a portion of the bottom of the emitter window is not covered by the oxide layer. 
     One or more aspect(s) is/are set out in the independent claim(s). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
         FIG. 1  shows a first step of a processing sequence according to an embodiment of the invention; 
         FIG. 2  shows a second step of a processing sequence according to an embodiment of the invention; 
         FIG. 3  shows a third step of a processing sequence according to an embodiment of the invention; 
         FIG. 4  shows a fourth step of a processing sequence according to an embodiment of the invention; 
         FIG. 5  shows a fifth step of a processing sequence according to an embodiment of the Invention; 
         FIG. 6  shows a sixth step of a processing sequence according to an embodiment of the invention; 
         FIG. 7  shows a seventh step of a processing sequence according to an embodiment of the invention; 
         FIG. 8  shows an eighth step of a processing sequence according to an embodiment of the invention; 
         FIG. 9  shows a ninth step of a processing sequence according to an embodiment of the invention; 
         FIG. 10  shows a tenth step of a processing sequence according to an embodiment of the invention; 
         FIG. 11  shows a SEM (Scanning Electron Microscopy) image of a first stage of a preliminary experiment according to the present invention; and 
         FIG. 12  shows a SEM image of a second stage of the preliminary experiment of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     The principle of the present invention will now be illustrated with reference to the manufacture of a self-aligned epitaxial base BJT, but it will be understood that the invention is not limited thereto. 
     As shown in  FIG. 1 , a heavily doped n+ buried layer  2 , which serves as the sub-collector, is provided. A n-type Si epitaxial layer  4 , which acts as a collector, is grown on the heavily doped n+ buried layer  2 . 
     As shown in  FIG. 2 , silicon oxide is deposited so as to form a pad oxide layer  6  on the n-type Si epitaxial layer  4 . Subsequently a poly Si layer  8  is deposited on pad oxide layer  6 , which poly Si layer  8  is then p-type heavily doped by high dose ion implantation. 
     As shown in  FIG. 3 , an opening  10  is etched into the p+ poly Si layer  8  and pad oxide layer  6 . The opening  10  can be generally of rectangular cross section as shown in  FIG. 3 , with side walls  12  and bottom portion  14 . Other geometries are also possible. A SIC  16  (selectively ion-implanted collector) is then formed primarily in n-type Si-epitaxial layer  4  by ion implantation through the opening  10 . 
     As shown in  FIG. 4 , a Si epitaxial layer  18  is then grown on the bottom portion  14  of the opening  10  so as to form a base material. A corresponding polycrystalline layer  20  is also deposited on top of poly Si layer  8  and on side wall  12  formed by the poly Si layer  8  and the pad oxide layer  6 . As a result of subsequent thermal cycles, the polycrystalline layer  20  will be p-type heavily doped by dopant-diffusion from poly Si layer  8 . Eventually, layers  20  and  8  will constitute an extrinsic base layer. 
     As shown in  FIG. 5 , a thin silicon oxide layer  22  is then thermally grown on Si epitaxial layer  18  and extrinsic base layer  20  so as to form a surface passivation layer. 
     The opening resulting after the forming of oxide layer  22  will be referred to as emitter window  100 . This has a bottom portion  114  and side walls  112 , as shown in  FIG. 5 . 
     As shown in  FIG. 6 , an intentionally non-conformal silicon oxide cap layer  24  is then deposited on surface passivation layer  22  so as to form a dielectric layer inside the emitter window  100  and on the material surrounding the emitter window. This dielectric layer  24  is thinner inside the emitter window than outside the emitter window. Preferably, at least over a portion of the bottom  114  of the emitter window  100  the dielectric layer is thinner than outside the emitter window. Preferably, the dielectric layer  24  is thinner substantially over the entire bottom portion  114  of the emitter window than over the material (immediately) surrounding the emitter window. 
     A technique of non-conformal coverage of a step with an SiO 2  film using a depositer has been disclosed on pages 185- 187 of “Silicon Processing for the VLSI Era”, Volume 1: Process Technology, by S. Wolf and R. N. Tauber, Lattice Press, Post Office Box 340, Sunset Beach, Calif. 90742, USA. the entire contents of which are hereby incorporated by reference. This technique can be applied to the present invention, subject to any necessary modifications, which will be clear to one skilled in the art. 
     Preferably, the oxide cap layer  24  is about 50 to 300% thicker outside the emitter window than inside the emitter window. More preferably, the oxide cap layer  24  is about 100 to 200% thicker outside the emitter window than inside the emitter window. For example, the thickness of oxide cap layer  24  at the bottom of the emitter window may be 100 mm and outside the emitter window the thickness may be 200-300 mm when cap layer  24  has been deposited. 
     As shown in  FIG. 7 , nitride side wall spacers  26  are then created along the side walls  112  of the emitter window  100 . This can be achieved by depositing a nitride layer and subsequently performing an etch-back. 
     As shown in  FIG. 8 , a HF wet etch is then carried out using a wet etcher or remover. This removes the cap oxide layer  24  and surface passivation layer  22  at least at a central portion  28  of the emitter window  100 , i.e. where, due to the non-conformality, the silicon oxide cap layer  24  is thinner than in other regions. During this HF wet etch the silicon oxide cap layer  24  is also etched outside the emitter window, i.e. on the material surrounding the emitter window, but a sufficiently thick portion of the cap oxide layer  24  remains on top of the p+polycrystalline layer  20 . Preferably the HF wet etch is stopped as soon as substantially all of the silicon oxide cap layer  24  and surface passivation layer  22  has been removed between the nitride side wall spacers, although the HF wet etch may also continue until a small portion of surface passivation layer  22  and silicon oxide cap layer  24  has been removed from “under” the nitride side wall spacers. As shown in  FIG. 8 , a structure results which has the Si epitaxial layer  18  exposed (at least partially) and which is formed with a sufficiently thick cap oxide layer  24  and surface passivation layer  22  outside the emitter window  100 . Preferably, the combined thickness of cap oxide layer  24  and passivation layer  22  remaining outside the emitter window  100  is at least 50 nm, preferably 50-150 nm, more preferably 80-120 nm and most preferably about 100 nm. 
     Preferably, the etch rate in the emitter window  100  is the same as outside the emitter window. 
     As shown in  FIG. 9 , the emitter poly  30  is then deposited, doped (in this example it is heavily n+ doped in situ during deposition) and patterned. The doping could alternatively be carried out by ion implant. Due to the oxide/nitride side wall spacers (note that now part of oxide layer  24  and passivation layer  22  can be regarded as forming part of the (oxide/nitride) side wall spacers) and the sufficiently thick cap oxide layer  24  remaining between the emitter poly material  30  and the extrinsic base poly material  20  and  8  it is possible to keep the emitter-base capacitances sufficiently low. 
     As shown in  FIG. 10 , the p+ extrinsic base poly layer  8  is patterned, and this is followed by an emitter drive-in anneal. This results in dopant out-diffusion from the emitter poly  30 , which in turn results in the formation of an intrinsic emitter  32 , at the bottom portion of emitter window  100 . The p+ extrinsic base  20  and  8  is spaced from the emitter edge by means of the oxide/nitride side wall spacer in a self-aligned manner. The spacing is preferably chosen relatively small so that extrinsic base resistances are kept small. However, the spacing is preferably chosen large enough so that breakdown and leakage between emitter and extrinsic base is avoided. 
     Finally, the manufacture of the self-aligned epitaxial base BJT is completed by conventional backend processes, including the forming of contacts and vias, and metallization steps. 
     The invention is also applicable to the manufacture of a HBT, such as a SiGe HBT. In this case the epitaxial layer  18  which is grown as base material (see  FIG. 4 ) is a Si/SiGe/Si epitaxial stack layer. 
     Whilst in the specific description reference has been made to a NPN BJT, the invention is also applicable to a PNP BJT. Any necessary modifications will be clear to one skilled in the art. 
     Whilst in the above detailed description of a preferred embodiment it is stated that the opening shown in  FIG. 5  is an emitter window it will be appreciated that an emitter window can be formed by processes other than the technique described with reference to  FIGS. 1 to 5 . It will further be appreciated that during the wet etch shown in  FIG. 8  the emitter window  100  changes in form. Nevertheless, the opening shown in  FIG. 8  is still to be regarded as an emitter window, and it is intended that the term “emitter window” as used in the claims will be interpreted in a similarly “flexible” manner. 
     A preliminary experiment was carried out, using a short cycle lot, to demonstrate the feasibility of the deposition and the subsequent wet etch-back of the non-conformal oxide for realising a self-aligned SiGe HBT device architecture according to an embodiment of the invention. 
     An approximately 400 nm poly layer was deposited and dry-etched on top of an approximately 50 nm thermal oxide layer so as to form a trench of about 0.4 μm width to imitate the emitter window. This was followed by a BOE (buffered oxide wet etch) removal of the bottom oxide layer, followed by the intentionally non-conformal oxide deposition (SILOX CVD technique, using SiH 4  and N 2 O as reactants). The non-conformality (thickness ratio of oxide outside to inside emitter window) attained was more than 200%, with a target thickness near 400 nm for the SILOX CVD. This stage of the process is shown in  FIG. 11 . 
     Subsequently a BOE wet etch-back was carried out, as a result of which a cap layer still as thick as 190 nm SILOX CVD was remaining on top of the poly outside the window while no oxide was left inside the window, as shown in  FIG. 12 . 
     Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.