Thermally stable BiCMOS fabrication method and bipolar junction transistors formed according to the method

A method for forming BiCMOS integrated circuits and structures formed according to the method. After forming doped wells and gate stacks for the CMOS devices and collector and base regions for the bipolar junction transistor, an emitter layer is formed within an emitter window. A dielectric material layer is formed over the emitter layer and remains in place during etching of the emitter layer and removal of the etch mask. The dielectric material layer further remains in place during source/drain implant doping and activation of the implanted source/drain dopants. The dielectric material layer functions as a thermal barrier, to limit out-diffusion of the emitter dopants during the activation step.

FIELD OF THE INVENTION

The present invention relates generally to bipolar junction transistors and more specifically to a thermally stable process for forming bipolar junction transistors and bipolar junction transistors fabricated according to the process.

BACKGROUND OF THE INVENTION

Integrated circuits typically comprise semiconductor devices, such as bipolar junction transistors (BJTS) and metal-oxide semiconductor field effect transistors (MOSFETS) formed from doped regions within a semiconductor layer or substrate. Interconnect structures overlying the substrate connect the BJTS and MOSFETS to form circuits. The interconnect structures comprise conductive runners or traces alternating with dielectric layers. Conductive vias disposed in the dielectric layers connect overlying and underlying conductive runners and traces.

BiCMOS integrated circuits comprise both bipolar junction transistors and CMOS (complementary metal oxide semiconductor field effect) transistors with the fabrication process steps for both devices integrated into one fabrication sequence. Integrating BJTS and MOSFETS on the same integrated circuit die allows the circuit designer to take advantage of both the BJT advantages of high speed and high frequency operation and the MOSFET advantages of low power consumption, high noise immunity and small die area.

The overall process sequence and the individual process steps (e.g., implanting dopants, depositing and patterning material layers, forming openings, epitaxially growing material layers, forming interconnect structures) must be carefully designed and executed to ensure that the process steps for forming the BJTS do not adversely affect the MOSFETS and vice versa, as such adverse effects can lower fabrication yields and increase costs. Thus semiconductor manufactures desire to implement a BiCMOS process flow for fabricating properly operable BJTS and MOSFETS on the same die with a high fabrication yield.

According to conventional BiCMOS processes, a BJT emitter is formed from a polysilicon material layer deposited in an emitter window. First a hard mask layer is formed overlying the emitter polysilicon layer and patterned. The polysilicon layer is etched according to the patterned hard mask and the hard mask is removed. During polysilicon etching the hard mark layer thickness is gradually reduced and the etch process is terminated when the etchant reaches the emitter polysilicon layer. However, during etching an upper region of the emitter polysilicon layer may be consumed, possibly impairing device functionality since a minimum emitter polysilicon thickness is required for proper operation. In an extreme case the bipolar junction transistor can be rendered nonfunctional by excessive erosion of the polysilicon emitter layer.

Known techniques to overcome the effects of emitter layer erosion include forming a thicker polysilicon emitter layer such that notwithstanding erosion the layer is sufficiently thick to present an acceptable resistance. Disadvantageously, this technique increases the stack height of the bipolar junction transistor material layers. Also, it is more difficult to etch a thicker polysilicon layer to form the required emitter region shape.

After forming the BJT emitter, the fabrication sequence forms MOSFET source and drain regions by implanting dopants into the substrate. The substrate is then annealed to repair crystal lattice damage resulting from collisions between the implanting dopants and the lattice atoms and to electrically activate the implanted dopants (i.e., to transfer the implanted dopant atoms from an interstitial to a substitutional state within the silicon crystal). A rapid thermal anneal (at about 1000° C. for about 20 seconds) is required for dopant activation. During the activation process, dopants in the emitter polysilicon tend to out-diffuse to the adjacent base, reducing the effectiveness of the BJT and potentially causing device failure.

It is known to reduce the duration and/or the temperature of the thermal anneal step to reduce dopant out-diffusion, but device functionality may be compromised if the dopant atoms are not completely activated.

BRIEF SUMMARY OF THE INVENTION

A method according to the present invention comprises forming a bipolar junction transistor and a metal oxide semiconductor field effect transistor in a semiconductor layer. The method further comprises providing the semiconductor layer; forming first MOSFET structures in a MOSFET region of the semiconductor layer; forming bipolar junction transistor structures, including an emitter material layer, in a bipolar junction transistor region of the semiconductor layer; forming a first material layer overlying the emitter material layer; patterning the first material layer to form a thermal barrier structure; patterning the emitter material layer to form a patterned emitter; forming second MOSFET structures in the MOSFET region and heating the semiconductor layer.

Another method according to the present invention comprises providing the semiconductor layer; forming first MOSFET structures in a MOSFET region of the semiconductor layer; forming bipolar junction transistor structures including forming an extrinsic base material layer and an emitter material layer overlying the extrinsic base material layer in a bipolar junction transistor region of the semiconductor layer; forming a first material layer over the emitter material layer; patterning the first material layer to form a thermal barrier structure; patterning the emitter material layer to form a patterned emitter; patterning the extrinsic base material layer; forming second MOSFET structures in the MOSFET region and heating the semiconductor layer.

In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Reference characters denote like elements throughout the figures and text. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail an exemplary method for forming bipolar junction transistors and metal-oxide semiconductor field effect transistors according to a thermally stable BiCMOS process of the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements and process steps. So as not to obscure the disclosure with details that will be readily apparent to those skilled in the art, certain conventional elements and steps have been presented with lesser detail, while the drawings and the specification describe in greater detail other elements and steps pertinent to understanding the invention.

The following embodiments are not intended to define limits as to the structure or method of the invention, but only to provide exemplary constructions. The embodiments are permissive rather than mandatory and illustrative rather than exhaustive.

The teachings of the present invention apply to silicon NPN and PNP BJTS and to heterojunction bipolar transistors (HBTS) wherein the three material regions comprise silicon, silicon-germanium, gallium-arsenide or another material. The present application refers to an exemplary silicon-germanium NPN BJT to describe the invention.

The BiCMOS integration scheme of the present invention avoids the thermal budget and the layer erosion issues described above, without impairing the functionality of the bipolar junction transistors formed during the BiCMOS process.

A process sequence for forming single-layer polysilicon bipolar junction transistors (also referred to as implanted base bipolar junction transistors) and MOSFETS in a BiCMOS process is described below in conjunction withFIGS. 1-9, which show cross-sectional views of formed structures according to a sequence of exemplary fabrication steps. The process forms an NPN bipolar junction transistor in a region6, an NMOSFET in a region7and a PMOSFET in a region8of a semiconductor layer10formed on a substrate11. SeeFIG. 1. The NMOSFET and the PMOSFET form a complementary MOSFET (CMOS) pair.

To avoid performance degradation and electrical cross talk between devices, it is desirable to electrically isolate bipolar junction transistors and CMOS devices in the semiconductor layer10. The exemplary process ofFIG. 1employs LOCOS (local oxidation of silicon) isolation regions20, but in other embodiments shallow and deep trench isolation structures can be used alone or in combination with the LOCOS isolation regions20. The LOCOS regions20are formed by oxidizing regions of an upper surface24of the p-type silicon semiconductor layer10that are exposed through openings formed in overlying material layers. The overlying material layers are then chemically stripped from the semiconductor layer10and a sacrificial screen silicon dioxide layer (not shown inFIG. 1) is formed on the upper surface24.

Next, a first photoresist layer is deposited and patterned to form a mask (not shown inFIG. 1) through which a p-type dopant is implanted to form a p-tub27in the NMOS region7of the semiconductor layer10. A wet clean process removes the first photoresist implant mask

A second photoresist implant mask (not shown) is formed through which an n-type dopant is implanted, forming an n-tub40in the PMOS region8and a collector sinker region42for an NPN bipolar junction transistor device. The collector sinker region42bridges a later-formed collector surface region and a subcollector region.

A wet clean process removes the second photoresist implant mask. A gate oxide layer, a polysilicon layer (doped in situ or by an implant process) and a tungsten silicide layer are blanket deposited on the upper surface24of the semiconductor layer10. The tungsten silicide layer provides improved ohmic contact (reducing sheet resistivity and contact resistance) between the gate electrode formed from the polysilicon layer and a later-formed overlying conductive via.

A silicon dioxide layer is deposited and patterned to form a hard mask for etching the gate silicon dioxide layer, the polysilicon layer and the tungsten silicide layer to form a gate stack44over the p-tub27(in the NMOS region7) and the n-tub40(in the PMOS region8). Each gate stack44comprises a gate oxide structure50, a polysilicon structure52and a tungsten silicide structure54. Other integration schemes with different gate and contact structures can be used, for example, a gate comprising in situ doped amorphous silicon and contacts comprising one of titanium silicide, cobalt silicide and nickel silicide.

Using a first patterned implant mask (not shown), n-type lightly doped drain regions61are formed in the p-tub27and an n-type lightly doped collector surface region62is formed in the bipolar junction transistor region6by implanting through the implant mask.

Using a second patterned implant mask (not shown), p-type lightly doped drain regions67are formed in the n-tub40.

A TEOS (tetraethylorthosilicate) deposited silicon oxide layer68(having a thickness of about 18 nm in one embodiment) is formed overlying the semiconductor layer10. SeeFIG. 2.

Using a subcollector mask that shields the NMOSFET region7and the PMOSFET region8, a collector region69is implanted in the bipolar junction transistor region6. To simplify the Figures, the collector sinker region42within the collector region69is not separately illustrated. It is to be understood that the collector region69can comprise several subregions, including the collector sinker region42, a deep buried layer and a pedestal layer, each having different dopant concentrations.

Using an appropriate mask, a p-type base region70is formed in an upper region of the collector69by counterdoping with a p-type implant, e.g., boron.

As shown inFIG. 3, a spacer TEOS-based silicon dioxide layer71is deposited over the semiconductor layer10. In one embodiment the layer71is about 120 nm thick. A TEOS-based silicon dioxide hard mask layer72is deposited over the silicon dioxide layer71according to a PECVD process, for example, and patterned to form a hard mask for a subsequent wet etch step.

As is known by those skilled in the art, the various silicon dioxide layers referred to in the process steps of the present invention may be formed by depositing silane according to an LPCVD how pressure chemical vapor deposition), a PECVD (plasma enhanced chemical vapor deposition) or a CVD (chemical vapor deposition) process or according to known TEOS-based processes.

A photoresist layer (not shown) is deposited and patterned for use in anisotropically etching an opening72A in the hard mask layer72. SeeFIG. 3. Using the opening72A as a pattern, a wet etch process forms an emitter window73in the silicon dioxide layer68and the TEOS-based silicon dioxide layer71.

As illustrated inFIG. 4, an emitter polysilicon layer80having a thickness of about 240 nm is deposited over the substrate surface, filling the emitter window73. The polysilicon layer80is implanted (or doped in-situ) with arsenic or another n-type dopant.

The wafer is cleaned using an RCA cleaning process. A silicon oxynitride layer81(having a thickness of about 30 nm to 60 nm) is deposited over the polysilicon layer80as illustrated inFIG. 4. According to an exemplary embodiment, the silicon oxynitride layer81is deposited according to a PECVD process using silane (SiH4) at 125 SCCM, ammonia (NH3) at 350 SCCM and helium at 2500 SCCM. Any of these parameters can be varied by at least 10%.

Those skilled in the art recognize that other processes can be used to deposit the silicon oxynitride layer81and other materials can be used in lieu of the silicon oxynitride. For example, silicon nitride or silicon carbide can be substituted for silicon oxynitride. Deposition of silicon nitride uses the same process parameters as the deposition of silicon oxynitride, except nitrogen dioxide (NO2) is substituted for ammonia at about the same flow rate.

A silicon dioxide hard mask layer82is deposited over the silicon oxynitride layer81. In one embodiment the hard mask layer82is about 300 nm thick, but according to certain prior art processes the thickness is increased to about 600 nm to limit erosion of the polysilicon emitter layer during subsequent etch steps. The increased thickness is advantageously avoided according to the process of the present invention.

Next, a photoresist layer is deposited and patterned to form a mask structure84as illustrated inFIG. 5. During the step of photolithographically exposing the photoresist layer, prior to patterning the layer, the silicon oxynitride advantageously functions as an antireflective coating to improve the accuracy of the photolithographic patterning process.

Using the mask structure84, the hard mask layer82is etched (patterned) to form a hard mask structure82A. SeeFIG. 6. Preferably, the hard mask layer82is etched according to a plasma etching process using CHF3 at about 54 SCCM, CF4 at about 27 SCCM and argon at about 20 SCCM. The chamber pressure is about 120 mTorr and the RF power is about 900 W. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the hard mask layer82. After etching the hard mask layer82, a plasma resist strip and clean process removes the mask structure84.

An etch process using the hard mask structure82A defines a silicon oxynitride structure81A formed from the silicon oxynitride layer81, an emitter polysilicon structure80A formed from the emitter polysilicon layer80and a hard mask structure72B formed from the hard mask layer72, as illustrated inFIG. 6. An etch chemistry comprising a plasma-containing fluorine material etches the silicon oxynitride layer81, and an etch chemistry comprising a plasma-containing chlorine material and hydrogen bromine etches the emitter polysilicon layer80and the hard mask layer72.

According to an exemplary embodiment, the silicon oxynitride layer81is etched (according to the pattern in the hard mask structure82A) using a plasma etch process employing C2F6 at about 100 SCCM and oxygen at about 10 SCCM. The chamber pressure is about 120 mTorr and an RF power of about 600 W is supplied. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the silicon oxynitride layer81.

According to an exemplary embodiment, the emitter polysilicon layer80and the hard mask layer72are etched (according to the pattern in the hard mask structure82A) using a plasma etch process employing chlorine at about 50 SCCM, hydrogen bromine at about 150 SCCM and a mixture of helium and oxygen at about 10 SCCM. The chamber pressure is about 10 mTorr. For an etch tool comprising two power supplies, an upper power is about 250 W and a lower power is about 100 W. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the emitter polysilicon layer80and the hard mask layer72.

The substrate11is cleaned and a plasma resist strip and clean process removes the hard mask structure82A, during which the silicon oxynitride structure81A acts an etch stop to avoid etching the emitter polysilicon structure80A. SeeFIG. 7.

Gate stack spacers100for the NMOSFET device in the region7and gate stack spacers102for the PMOSFET device in region8are formed by anisotropically etching the TEOS-based silicon dioxide layers68and71. SeeFIG. 7. During the gate stack etch process material layers formed underlying the emitter polysilicon structure80A include silicon oxide structures71A and68A. After forming the gate stack spacers100and102, the substrate11is cleaned.

A photoresist layer (not illustrated) is deposited and patterned to form openings therein. Implanting arsenic through the openings forms n+ source/drain regions106and108(seeFIG. 8) into the p-tub27for the NMOSFET in the region7and forms an n+ collector contact110in the bipolar junction transistor collector region69. The relatively high dopant concentration of the collector contact110minimizes contact resistance with a subsequently-formed overlying collector contact.

A high dose of a p-type (p+) dopant is implanted through a patterned mask (not illustrated) into the n-tub40to forms source/drain regions114and116into the n-tub40for the PMOSFET in the region8. Simultaneously, an extrinsic base region118of the NPN bipolar junction transistor is formed by implanting the p-type dopant.

The substrate11is annealed to repair crystal lattice damage resulting from collisions between the n-type and p-type implanted dopants and the lattice atoms, and to electrically activate the implanted dopants. The annealing is performed at about 1000° C. for about 20 seconds.

During the annealing step the silicon oxynitride structure81A acts as a thermal and a diffusion barrier that limits out-diffusion of dopants from the emitter polysilicon structure80A. In particular, the thermal barrier limits the temperature of the emitter polysilicon structure81A to minimize dopant diffusion from the emitter80A to the base118.

The silicon oxynitride structure81A is removed during a wet or dry etch. An exemplary dry etch comprises a plasma etch process employing C2F6 at about 100 SCCM and oxygen at about 10 SCCM. The chamber pressure is about 120 mTorr and an RF power of about 600 W is supplied. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the silicon oxynitride layer81.

The silicon oxynitride structure81A can also be removed using a wet etch process as known by those skilled in the art.

FIG. 9illustrates the final device structure. Overlying alternating dielectric layers and conductive layers are formed to interconnect the doped regions formed in the semiconductor layer10.

In another embodiment (not illustrated), the silicon oxynitride structure81A is not removed and the alternating dielectric layers and conductive layers are formed thereover. It is necessary in this embodiment to form an opening in the silicon oxynitride structure81A to permit electrical contact to the underlying emitter polysilicon structure80A.

An exemplary double-polysilicon BiCMOS process (forming an NPN bipolar junction transistor comprising two polysilicon layers, one layer for the emitter and one layer for the extrinsic base) includes the process steps set forth above and illustrated inFIGS. 1 and 2(with the exception that the intrinsic base70illustrated inFIG. 2is not formed according to the double-polysilicon process) and the process steps illustrated inFIGS. 10-22, the latter illustrating cross-sectional views of the formed structures according to sequential processing steps.

With the initial steps set forth inFIGS. 1 and 2above completed (with the exception that the intrinsic base70illustrated inFIG. 2is not formed according to the double-polysilicon process) a TEOS-based silicon dioxide layer144and a base polysilicon layer146are formed on an upper surface of the semiconductor layer10. The base polysilicon layer146is heavily doped (in situ or implanted) with boron. An extrinsic base is formed from the doped base polysilicon layer146as described below.

As illustrated inFIG. 11, a silicon nitride layer156and a silicon dioxide layer158(in one embodiment the latter formed according to a TEOS-based deposition process) are deposited in a stacked relation over the base polysilicon layer146. A patterned photoresist layer160defines a window162through which the silicon dioxide layer158, the silicon nitride layer156and the base polysilicon layer146are anisotropically etched, stopping on the TEOS layer144. A lower region of the window162is referred to as an emitter window163. In certain bipolar junction transistor embodiments an implant may be made through the window162to create a collector region166.

The photoresist layer160is removed and a layer of silicon nitride is deposited overlying the silicon dioxide layer158and anisotropically etched to form sidewall spacers170. SeeFIG. 12. A wet etch process removes the silicon dioxide layer158across the semiconductor layer10and removes regions of the silicon dioxide layers68and144within the emitter window163. Removal of these material layers forms a primary cavity174having laterally disposed recesses175within the emitter window163, as illustrated inFIG. 12.

An intrinsic base and cap region, both referred to by a reference character176(seeFIG. 13), are formed in the cavities174and recesses175during a silicon-germanium epitaxial growth step. Voids177may form in regions where the silicon-germanium does not grow.

A silicon nitride spacer180and an underlying silicon oxide spacer182are formed in the window163as illustrated in the close-up view ofFIG. 14. The spacers, which isolate a later-formed emitter and extrinsic base, are formed by depositing a silicon oxide layer and an overlying silicon nitride layer. The layers are anisotropically etched back to form the spacers180and182, with the etch stopping on a region of the silicon oxide layer182A formed on an upper surface of the intrinsic base and cap region176. In another embodiment the spacers180and182are not required if the previously formed spacers170provide sufficient isolation between the extrinsic base and the emitter. Following spacer formation, regions of the TEOS silicon dioxide layer182A overlying the upper surface of the intrinsic base and cap region173are removed by a wet etch process.

A polysilicon layer190having a thickness of about 240 nm is blanket deposited. SeeFIGS. 15 and 16. The layer190is later etched to define the bipolar junction transistor emitter. The polysilicon layer190may be implanted or doped in-situ with arsenic or another n-type dopant.

The wafer is cleaned using an RCA cleaning process. A silicon oxynitride layer191having a thickness between about 30 nm and about 60 nm is deposited over the polysilicon layer190. See the resulting structure illustrated inFIGS. 15 and 16. In an exemplary embodiment, the silicon oxynitride layer191is deposited according to a PECVD (plasma enhanced chemical vapor deposition) using silane (SiH4) at 125 SCCM and ammonia (NH3) at 350 SCCM and helium at 2500 SCCM. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other processes can be used to deposit the silicon oxynitride layer191.

In other embodiments, silicon nitride or silicon carbide is substituted for silicon oxynitride. Deposition of silicon nitride uses the same process parameters as the deposition of silicon oxynitride, except nitrogen dioxide (NO2) is substituted for ammonia at about the same flow rate.

According to a preferred embodiment, a hard mask silicon dioxide layer (having a thickness of about 300 nm and formed according to a plasma-enhanced TEOS process) is deposited overlying the silicon oxynitride layer191. A photoresist layer (not shown) is deposited over the hard mask layer and patterned.

During photolithographic exposure of the photoresist layer, the silicon oxynitride advantageously functions as an antireflective coating to improve the accuracy of the exposure process, which in turn improves the dimensional accuracy of the patterned hard mask, and the patterned silicon oxynitride layer191and the patterned emitter polysilicon layer190formed using the patterned hard mask.

The photoresist pattern is transferred to the hard mask silicon dioxide layer by etching the hard mask layer to form a patterned hard mask192(seeFIG. 16). Preferably, the hard mask layer is etched according to a plasma etching process using CHF3 at about 54 SCCM, CF4 at about 27 SCCM and argon at about 20 SCCM. The chamber pressure is about 120 mTorr and the RF power is about 900 W. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the hard mask layer.

After forming the patterned hard mask192a plasma resist strip and clean process removes the patterned photoresist.

Using the patterned hard mask192, the silicon oxynitride layer191and the emitter polysilicon layer190are etched to form a silicon oxynitride structure191A and a polysilicon emitter structure190A as illustrated inFIG. 17. In one embodiment, an etch chemistry comprising a plasma-containing fluorine material etches the silicon oxynitride layer191, and an etch chemistry comprising a plasma-containing chlorine material and hydrogen bromine etches the emitter polysilicon layer190.

According to an exemplary embodiment, the silicon oxynitride layer191is etched according to the pattern in the patterned hard mask192using a plasma etch process employing C2F6 at about 100 SCCM and oxygen at about 10 SCCM. The chamber pressure is about 120 mTorr and an RF power of about 600 W is supplied. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the silicon oxynitride layer191. The resulting silicon oxynitride structure191A is illustrated inFIG. 17.

According to an exemplary embodiment, the emitter polysilicon layer190is etched according to the pattern in the patterned hard mask192using a plasma etch process employing chlorine at about 50 SCCM, hydrogen bromine at about 150 SCCM and a mixture of helium and oxygen at about 10 SCCM. The chamber pressure is about 10 mTorr. For an etch tool comprising two power supplies, an upper power is about 250 W and a lower power is about 100 W. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the emitter polysilicon layer190. The resulting emitter polysilicon structure190A is illustrated inFIG. 17.

The silicon nitride layer156is etched according to any one of several known silicon nitride etch processes to form the silicon nitride structure156A ofFIG. 17.

The substrate11is cleaned and a plasma resist strip and clean process removes the patterned hard mask192. During this resist strip and clean process, the silicon oxynitride structure191A acts an etch stop to avoid etching of the polysilicon emitter structure190A.

In an alternative embodiment, a photoresist layer is patterned to form a mask for directly etching (i.e., without using a hard mask layer) portions of the silicon oxynitride layer191, the polysilicon layer190and the silicon nitride layer156to form the resulting structures as illustrated inFIG. 17.

A photoresist layer is patterned to form a mask200(seeFIG. 18) for etching regions of the base polysilicon layer146to form extrinsic base polysilicon structures146A illustrated inFIG. 19.

The base polysilicon photoresist mask200is removed during a plasma resist strip and clean step. The TEOS-based silicon dioxide layer144and the silicon dioxide layer68are anisotropically etched to form gate stack spacers210for the NMOSFET device in the region7and gate stack spacers212for the PMOSFET device in the region8. SeeFIG. 20. Structures144A of the TEOS-based silicon dioxide layer144and structures68A of the silicon dioxide layer68remain underlying the extrinsic base polysilicon structures146A. After forming the gate stack spacers210and212the substrate11is cleaned.

The substrate11is annealed to repair crystal lattice damage resulting from collisions between the implanted n-type and p-type dopants and the lattice atoms, and also to electrically activate the implanted dopants. The annealing, typically comprising a rapid thermal anneal, is performed at about 1000° C. for about 20 seconds.

During the annealing step the silicon oxynitride structure191A acts as a thermal and a diffusion barrier that limits out-diffusion of dopants from the emitter polysilicon structure190A. In particular, the thermal barrier limits the temperature of the emitter polysilicon structure190A to minimize dopant diffusion from the emitter to the intrinsic base176and to the extrinsic base structures146A.

The silicon oxynitride structure191A is removed during a wet or dry etch step. An exemplary dry etch comprises a plasma etch process employing C2F6 at about 100 SCCM and oxygen at about 10 SCCM. The chamber pressure is about 120 mTorr and an RF power of about 600 W is supplied. Any of these parameters can be varied by at least 10%. Those skilled in the art recognize that other etch materials, parameters and processes can be employed to etch the silicon oxynitride structure191A. Wet etch processes for removing the silicon oxynitride structure191A are known in the art.

FIG. 22illustrates the final device structure. Alternating dielectric and conductive layers are formed overlying the substrate11to interconnect the doped regions within the substrate11.

In another embodiment (not illustrated), the silicon oxynitride structure191A is not removed and the alternating dielectric layers and conductive layers are formed thereover. It is necessary in this embodiment to form an opening in the silicon oxynitride structure191A to permit electrical contact to the underlying emitter polysilicon structure190A.

While the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for the elements thereof without departing from the scope of the invention. The scope of the present invention further includes any combination of elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.