PNP Type Lateral Transistor With Minimal Substrate Operation Interference

This invention is a novel structure and method for forming a VLSI PNP type lateral dielectrically isolated transistor utilizing VLSI chip processing steps to fabricate hundreds of such devices simultaneously.

Techniques extracted from the prior art come from the field of VLSI fabrication technology wherein the dimensional resolution is now comparable to or even smaller than the thicknesses of the thermally grown silicon oxide layers on silicon. From these techniques, the novel structure for the PNP lateral transistor with greatly reduced parasitic capacitance and resistances is achieved.

The invention is a novel lateral PNP type transistor formed on a substrate by orthogonal slots which define each active region and permit doping of the active regions via opposed slots while supported by oxidation achieved through underlying etching of each active region in a semi-array of regions supported at spaced apart positions and totally isolated from the substrate by substrate oxidation. Electrical connections are conventionally made to the P emitter, N+N base and P collector.

FIG. 1 is a view in section through an N type substrate with photoresist applied through a slotted mask;

FIG. 2 shows the structure of FIG. 1 with slots formed in the substrate where not protected by the resist;

FIG. 3 is a perspective view of the substrate showing the first set of longitudinal slots;

FIG. 4 is a view in section showing the application of an angle evaporation etch resist layer partially protecting the edges of the slots;

FIG. 5 is a view in section with the etch resist layer in place and showing the effects of the etch having been applied via the slots to undercut the longitudinal regions which will become active regions of the transistors being formed;

FIG. 6 shows the structure of FIG. 5 after the etch resist has been removed and following oxidation of the substrate to fill in the voids created by the etch;

FIG. 7 is a view in plan of the structure of FIG. 6 showing the orthogonal set of slots relative to the original set of slots;

FIG. 8 is a view in section taken across the orthogonal set of slots showing the phosphorus ion implant step to produce the N+ doping shown;

FIG. 9 shows the structure of FIG. 8 taken after phosphorus drive-in and the application of boron deposit or implant;

FIG. 10 shows the structure of FIG. 9 following boron drive-in from both sides to produce the P regions and following oxidation of the substrate to fill the slots and protect the top;

FIG. 11 is a top plan view of the completed structure showing in dashed outline the electrode connections for the emitter, base and collector for a single active region of a single transistor;

FIG. 12 is a view in perspective of the transistor active region, per se.

FIG. 13 shows an alternate embodiment using substrate doping change in lieu of oxide for device isolation; and,

FIG. 14 shows you the steps of the process of FIG. 13.

An alternate method of forming contacts is set forth in the following figures.

FIG. 15 is a view in top plan of a bipolar device;

FIG. 16 shows the device of FIG. 15 in cross-section in field oxide;

FIG. 17 shows the application of a polysilicon layer covered by a nitride layer to the structure of FIG. 16;

FIG. 18 shows a contact mask;

FIG. 19 shows the nitride buttons remaining over each of the bipolar elements atop the polysilicon layer;

FIG. 20 shows a portion of the polysilicon layer removed;

FIG. 21 shows the polysilicon layer converted to polyoxide except where the nitride buttons preserve the polysilicon;

FIG. 22 shows the mask for activating the base electrode polysilicon;

FIG. 23 shows the boron implant to achieve the doping of the polysilicon electrode;

FIG. 24 shows a mask for opening regions to the collector and emitter (N regions);

FIG. 25 shows these regions opened and the polysilicon therein being doped by phosphorus implant;

FIG. 26 shows a layer of metal over top of the structure;

FIG. 27 shows the arrangement in top plan view as delineated by the mask of FIG. 28;

FIG. 29 shows the completed device following sintering; and,

FIG. 30 shows relative sizes for the bipolar device the elements themselves, such as the collector measuring 2.0 by 2.0 and the contact regions measuring 0.7 by 0.7 microns.

A further alternate embodiment for making submicron contacts is shown in FIGS. 31-45.

FIG. 31 depicts the transistor elements under a layer of polysilicon covered by nitride;

FIG. 32 shows a nitride button mask;

FIG. 33 depicts the isolated nitride buttons over contact regions;

FIG. 34 shows etching away of part of the polysilicon;

FIG. 35 reveals the plasma oxide covering the contacts, and in turn covered by glass (SOG);

FIG. 36 planarizes the structure of FIG. 35 down to the tops of the nitride buttons;

FIG. 37 shows the structure of FIG. 36 with the nitride buttons removed;

FIG. 38 shows the mask to be used in making a doping opening to the base contact;

FIG. 39 shows the doping step for the base contact;

FIG. 39A shows a mask for making openings to the collector and emitter contacts;

FIG. 40 shows the doping step for the collector and emitter contacts;

FIG. 41 includes the metal interconnect layer;

FIG. 42 shows the delineated metal interconnects;

FIG. 43 shows the mask for effecting the delineation;

FIG. 44 shows the completed device; and,

FIG. 45 is an example of relative sizes available from this method.

In the device produced in accordance with the present invention, the effects of the substrate are minimized relative to the operation of the device. It should be realized that the active region may be completely contained in substrate with a surface area of only 2d.times.6d or 12d.sup.2 wherein d is approximately 1.0 micrometers, or 3d.times.5d=15d.sup.2, as is also pictured. The isolated region is completely separated from the substrate by oxidation and the parasitic capacitances and resistances are almost completely reduced to a minimum value attainable because the transistor is reduced essentially to just its electronically active region whereas in the conventional (vertical) transistor, the electrically active region is only a very small fraction of the total transistor area. The dielectric isolation between transistors offered by the same substrate oxidation has advantages with respect to high frequency performance, high voltage integrated circuits, radiation resistance, circuit flexibility, i.e., the process can provide NPN, PNP, JFET, MOSFET's etc. on the same integrated circuit chip.

In FIG. 1, there is shown a silicon substrate of the N type, having patterned photoresist applied to an upper surface thereof. The pattern may simply comprise longitudinal slots for rendering the photoresist 23 alternately soluble and it may be seen that the soluble photoresist has been removed to provide exposure of the substrate 21 so that the slots 25 and 27 may be milled therein or formed by O.D.E. etching process. These slots 25 and 27 penetrate the upper surface of the silicon substrate 21 sufficiently deeply that they extend beneath the depth of the region which will ultimately become the active regions for the various transistors.

In FIG. 3, the photoresist layer 23 has been removed and the structure is shown in perspective thereby showing the slots 25 and 27 extending inwardly of the upper surface of silicon substrate 21.

In FIG. 4, it may be appreciated why the slots 25 and 27 are necessary. They permit angle evaporation of etch-resist 31 to the edges of the slots 25 and 27. The result provides a protective covering over the tops of the active regions being formed. Also, it should be noted that due to the angle of evaporation of the etch resist 31, this protective layer only extends to a certain depth within slots 25 and 27, but this depth must be adequate to protect the active regions being formed. Thus, in FIG. 5, it may be seen that the active regions are generally shown at 35, 37 and 39 above the silicon substrate etch level 41 and also above the etch levels 35', 37' and 39' where the etch introduced through the slots 25 and 27 removed the substrate from beneath the active regions 35, 37 and 39.

Actually, there are plurality of active regions aligned in the remaining portions 35, 37 and 39 which represent semi-arrays of transistors being formed. Obviously, the semi-arrays are supported at their ends or spaced apart positions so that the active regions do not collapse into the substrate etched space 41'.

In FIG. 6, it may be seen that the etch resist 31 has been removed and the silicon has been oxidized to SiO.sub.2 shown at 41' as filling or partly filling the voids between regions 35, 37 and 39 and the silicon substrate 21. Thus, with the regions supported now by the silicon oxide 41', the orthogonally related sets of slots 45 and 47 may be formed as illustrated in the top plan view of FIG. 7. The procedure for forming these slots is the same as before, requiring a photoresist layer patterned in the form of spaced apart slots and the silicon material either ion milled away or etched using O.D.E. techniques, as best shown in FIG. 8.

In FIG. 8, the application of N+ doping is achieved by phosphorus implant shown by the arrows 49 to produce the N+ region 51. It should be noted that this implanting is done only into the corresponding right hand edges of slots 45 and 47 to provide the N+ layers 51 and 53.

In FIG. 9, it may be seen that the N+ regions have been driven in, such as by using high heat so that they are expanded as shown by 51' and 53'. Also, a further N+ region is visible to the left at 55', having been produced by a slot to the left of the margin of the page.

Next, in FIG. 9, it will be seen that boron is implanted (or deposited) into both edges of each slots as shown by the arrows 57 to provide the P or P+ regions 61, 62, 63 and 64. It may also be pointed out that the N regions 21', 21" and 21'" are the remaining regions from the original P type silicon substrate.

Proceeding from FIG. 9 to FIG. 10, it may be seen that the boron P or P+ regions 62 and 63 have now been driven in to comprise the expanded regions 62' and 63'. Accordingly the N+ region has been changed in size and is now designated 51" as is also true of the remaining N region now identified as 21"". The region 62' will comprise the emitter with the regions 51" and 21"" comprising a graded base region and finally region 63' being the collector region.

Also, in FIG. 10, it may be seen that the substrate has again been oxidized to fill in the slots 45 and 47 and provide a covering over the active region of the electrodes generally shown at 75. The doping gradient in the base region is similar to conventional double diffused transistors.

A finished transistor is shown in FIG. 11 wherein the contacts 101, 102 and 103 are provided from a metallization layer for the emitter 111, base 112 and collector 113, respectively. These contacts are fabricated simply by conventional techniques for patterning or metallization.

It may be seen that the transistor comprising these three electrodes is surrounded by moats of silicon oxide formed in the slots 25, 27 and 45, 47. Thus, the transistor is isolated from its substrate, adjacent transistors or other elements.

The active region of the transistor is visible in FIG. 12 in the perspective view showing the emitter 111 base 112 and collector 113.

If it is desired to use doping opposite that of the substrate beneath the active region as further isolation, a P substrate can be doped N in the active regions and the use of O.D.E. avoided by skipping from FIG. 3 to FIG. 13. First, the active regions are doped N (see FIG. 13) via the slots 45 and 47; and next, phosphorus or arsenic is ion angle implanted so only regions 51', 53' become N+ doped. Then, boron is implanted from all edges of slots 45' and 47' (FIG. 14) to form P+ regions 61', 62', 64' and 64'. Thereafter, the slots 45' and 47' are deepened into the original P substrate and filled with oxide. Thus, each active region is P+ N+ N and P+ doped in an N doped region but is underlaid by P doped substrate for total isolation without using etching techniques.

The principles of the present invention are compatible with use of the Perkin Elmer Projection Printer equipment for 2 micron dimensions, the 10:1 stepper for 1 micron rated dimensions, and electron beam or X-ray lithography or under 1 micron dimensions, even utilizing a portion of a line as a contact, all utilizing accepted existing registration and alignment techniques.

Contacts are established to each emitter, base and collector by metallization techniques.

The following method of making extremely small contacts is an alternative method. It is applied to an NPN lateral bipolar transistor for illustrating versatility, but the principles apply equally to PNP, and the changes are set forth hereinafter.

In FIG. 15, the collector, base and emitter 201, 202 and 203 are depicted in a lateral arrangement as seen in top plan.

In FIG. 16, the collector 201 is shown as being N doped, the base 202 P doped and the emitter 203 N doped, all surrounded by oxide.

The first step in building sub-micron contacts (i.e., 0.7 micron by 0.7 micron) for these devices is to deposit undoped polysilicon to approximately 5000 angstroms. This layer is shown at 205. This step may be applied to the structure of FIG. 10, once the upper oxide is removed. Only the doping for the substrate and the steps of FIGS. 23 and 25 would be switched for the PNP device. The graded base 112 (FIG. 12) is a single N doped electrode and presents no problem in this alternate method.

Returning to FIG. 17, a layer 206 of nitride is deposited to a thickness of approximately 1000 angstroms over layer 205.

The mask 207 of FIG. 18 is employed with photoresist (not shown) to delineate the nitride 206, thereby leaving the nitride buttons 211, 212 and 213 over the collector 201, base 202 and emitter 203, respectively (as shown in FIG. 19).

To insure ultimate planarization, a portion of the polysilicon is etch removed (FIG. 20). This may comprise 2500 angstroms of polysilicon. A non-selective or anisotropic etch is employed so that only the polysilicon is removed.

Next, the resist is stripped and the polysilicon oxidized to become polyoxide 221 shown in FIG. 21. The polyoxide 221 also fills in the spaces between the nitride buttons 211, 212 and 213 and rises slightly above the nitride to insulate the contacts from each other.

FIG. 21 also shows the structure after the nitride buttons 211, 212 and 213 have been removed by etching or stripping.

In FIG. 22, mask 250 is used, together with photoresist 251 (FIG. 23), to open aperture 253 into the P doped base 202. For boron implanting into the base electrode 260 to make contact to base 202, implanting is carried out using a 60 Kev implanter with a 1 to 5.times.10.sup.15 ions per cm.sup.2 dose. (See FIG. 23.)

Next, the photoresist 251 is stripped and a new layer applied for use with the mask 270 (FIG. 24) to open the new layer of photoresist 271 above the collector 201, and emitter 203. The phosphorus ions are implanted, the photoresist 271 is stripped and the device of FIG. 25 is annealed for the implants at 800.degree. C. for 40 minutes in a nitrogen atmosphere.

In FIG. 26, a metal layer 300 has been applied over the structure, photoresist and the metal mask of FIG. 28 are applied, and the metal is etched to provide a metal interconnect 311 (FIG. 27) to the doped poly electrode 315 of collector 201, interconnect 312 to the doped poly electrode 260 of base 202 and interconnect 313 to the doped poly electrode 317 of emitter 203.

FIG. 29 shows a completed device.

FIG. 30 shows relative sizes for the bipolar device, the elements themselves, such as the collector measuring 2.0 by 2.0 microns and the contact regions measuring 0.7 by 0.7 microns.

Yet another alternative method for forming the contacts is disclosed, beginning with FIG. 31 wherein the three elements collector 201, base 202 and emitter 203 are first covered by polysilicon 205 to a depth of, e.g., approximately 3000 angstroms, and nitride 325 is deposited to a depth of, e.g., approximately 1000 angstroms. Mask 327 serves to form the nitride buttons 211, 212 and 213 (the same as before). These nitride buttons 211-213 are preferably centered over the collector 201, base 202 and emitter 203 as seen in FIG. 33, although considerable leeway is available for any misalignment, as is apparent from FIG. 45.

In FIG. 34, the unprotected polysilicon 205 is removed in conventional manner, such as by etching to leave undoped polysilicon contacts 205, 205', and 205".

In FIG. 35, plasma oxide 329 is deposited using the conventional planarization process to a depth of approximately 3000 angstroms. This step is followed by spinning on glass 327 (SOG) so as to look substantially planar.

In FIG. 36, a reactive ion etching (RIE) conventional step is carried out for effecting planarization which exposes the nitride buttons 211, 212 and 213 and is terminated approximately at the level of the tops of these buttons.

In FIG. 37, the nitride buttons 211, 212 and 213 have been removed from the contacts 205, 205' and 205" using a conventional step to prepare the contacts for doping to make them properly conductive.

In FIG. 38, mask 250 is shown for delineating photoresist 251, in the previous manner, to provide the opening 253 for the boron implant, as shown by the arrows in FIG. 39.

In similar fashion, mask 270 of FIG. 39A, is again utilized to delineate the photoresist 271 for the phosphorus implants, shown by the arrows (FIG. 40), thereby concluding the doping of the three polysilicon contacts 205, 205' and 205" in the same N or P doping as their underlying collector, base and emitter.

In FIG. 41, the metal layer 300 has been applied over this structure, and the metal mask of FIG. 43, shown at 300', is used and the metal is etched to provide the metal interconnects 311 (FIG. 42) to the doped poly electrode 205 of collector 201, interconnect 312 to the doped poly electrode 205 of the base 202 and interconnect 313 to the doped poly electrode 205" of emitter 203.

FIG. 44 shows the completed device made in this fashion.

Finally, FIG. 45 shows, by way of example, relative sizes for the bipolar device, the elements themselves such as the collector measuring 2.0 by 2.0 microns and the contact regions measuring 0.7 by 0.7 microns.

The use of an E-beam fabricator, guided by the masks successively used herein, makes the VLSI contacts of the order of these dimensions. Of course, larger contacts might be readily utilized using these principles.

If desired, single MOS or bipolar devices may be made, and it is only necessary to insulate them by isolation, usually in lieu of the field oxide, illustrated.

This method is also applicable to MOS, GaAs and conventional bipolar minimal size contacts.

In summary, the following advantages are achieved by this alternate method:

1. Sub-micron contact technology.

2. Contact integrity without the need of excessive silicon content in the metal or contact barriers such as Titanium Nitride, etc.

3. Planar structures without severe steps to hinder metal step coverage, or restrictions for proximity of contact to vias to gates, thereby improving packing density.

4. Principle applicable to MOS (NMOS, PMOS, CMOS, etc.), Bipolar devices and GaAs technologies.

5. Contact technology compatible with shallow junctions and other state of the art processes such as selective silicides.

6. Minimum overlap of metal for interconnects.

7. Via overlap of contact realizable with this technique.