# Patent

## Title
Self-aligned mask formed utilizing differential oxidation rates of materials

## Abstract

A self-aligned oxide mask is formed utilizing differential oxidation rates of different materials. The self-aligned oxide mask is formed on a CVD grown base NPN base layer which compromises single crystal Si (or Si/SiGe) at active area and polycrystal Si (or Si/SiGe) on the field. The self-aligned mask is fabricated by taking advantage of the fact that poly Si (or Si/SiGe) oxidizes faster than single crystal Si (or Si/SiGe). An oxide film is formed over both the poly Si (or Si/siGe) and the single crystal Si (or Si/siGe) by using an thermal oxidation process to form a thick oxidation layer over the poly Si (or Si/siGe) and a thin oxidation layer over the single crystal Si (or Si/siGe), followed by a controlled oxide etch to remove the thin oxidation layer over the single crystal Si (or Si/siGe) while leaving the self-aligned oxide mask layer over the poly Si (or Si/siGe). A raised extrinsic base is then formed following the self-aligned mask formation. This self-aligned oxide mask blocks B diffusion from the raised extrinsic base to the corner of collector.

## Background
BACKGROUND OF THE INVENTION
The present invention relates generally to a self-aligned mask formed utilizing differential oxidation rates of different materials, and more particularly to a method for forming a self-aligned oxide mask on a polycrystal field of a base layer of a CVD grown base NPN transistor wherein the base layer compromises single crystal Si (or Si/SiGe-which means a layer of single crystal silicon on top of a layer of single crystal silicon germanium) at active area and polycrystal Si (or Si/SiGe-which means a layer of polycrystal silicon on top of a layer of polycrystal silicon germanium) on the field isolation.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide a self-aligned mask formed by utilizing differential oxidation rates of different materials. A self-aligned oxide mask is formed on a CVD grown base NPN transistor base layer which comprises single crystal Si (or Si/SiGe) at an active area and polycrystal Si (or Si/SiGe) on the surrounding field, which is followed by the formation of a raised extrinsic base. This self-aligned mask is fabricated by taking advantage of the fact that poly Si (or Si/SiGe) oxidizes faster than single crystal Si (or Si/SiGe). In addition, the fact that SiGe oxidizes faster than Si, and doped Si (or SiGe) oxidizes faster than intrinsic Si (or SiGe) can also be utilized to form the self-aligned mask. An oxide film is formed over both the poly Si (or Si/SiGe) and the single crystal Si (or Si/SiGe) by using a thermal oxidation process to form a thick oxide layer over the poly Si (or Si/SiGe) and a thin oxide layer over the single crystal Si (or Si/SiGe). This is followed by a controlled oxide etch to remove the thin oxide layer over the single crystal Si (or Si/SiGe) while leaving an oxide mask layer over the poly Si (or Si/SiGe) which blocks B diffusion from the highly doped raised extrinsic base to the corner of the collector.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing objects and advantages of the present invention may be more readily understood by one skilled in the art with reference being had to the following detailed description of several embodiments thereof, taken in conjunction with the accompanying drawings wherein like elements are designated by identical reference numerals throughout the several views, and in which:
FIG. 1 illustrates a structure composed of first and second different materials with different oxidation rates, a first fast oxidation rate for the first material and a second slower oxidation rate for the second material.
FIG. 2 illustrates the structure of FIG. 1 after oxidation of the two different materials at the two different oxidation rates, which shows a thick oxide layer over the first material and a thin oxide layer over the second material.
FIG. 3 illustrates the structure after a controlled oxide etch which etches both oxide layers at approximately the same rate, such that after the second oxide layer is completely etched, a self-aligned mask remains over the first material.
FIG. 4 illustrates a prior art CVD grown base NPN transistor having a raised extrinsic base construction which is used for an HBT (heterojunction bipolar transistor) base contact.
FIG. 5 shows a CVD grown base NPN transistor similar to FIG. 4, wherein pursuant to the present invention an oxide layer is formed over poly Si (or Si/SiGe) on the field around a central single crystal Si (or Si/SiGe) of the base.
FIG. 6 illustrates a prior art CVD grown base NPN transistor similar to FIG. 4, without the oxide pull down at the side edge of the STI.
FIG. 7 shows a CVD grown base NPN transistor similar to FIG. 6, wherein pursuant to the present invention an oxide layer is formed over the poly Si (or Si/SiGe) on the field around a central single crystal Si (or Si/SiGe) of the base.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates a structure composed of first and second different materials 10,12 with different oxidation rates, a first fast oxidation rate for the first material and a second slower oxidation rate for the second material.
FIG. 2 illustrates the structure of FIG. 1 after oxidation of the two different materials at the two different oxidation rates, which shows a thick oxide layer 20 over the first material and a thin oxide layer 22 over the second material.
FIG. 3 illustrates the structure after a controlled oxide etch which etches both oxide layers at approximately the same rate, such that after the second oxide layer is completely etched, a self-aligned mask 30 remains over the first material.
The present invention uses this principle to form a self-aligned oxide mask on a CVD grown base NPN transistor base layer which comprises single crystal Si (or Si/SiGe) at an active area and polycrystal Si (or Si/SiGe) on the surrounding field. This self-aligned mask is fabricated by taking advantage of the fact that poly Si (or Si/SiGe) oxidizes faster than single crystal Si (or Si/SiGe). An oxide film is formed over both the poly Si (or Si/SiGe) and the single crystal Si (or Si/SiGe) by using an oxidation process to form a thick oxide layer over the poly Si (or Si/SiGe) and a thin oxide layer over the single crystal Si (or Si/SiGe). A HIPOX (HIgh Pressure OXidation) process is used for reduced total thermal cycle due to its faster oxidation rate, but the invention is not limited to HIPOX.
The oxidation process is then followed by a controlled oxide etch to remove the thin oxidation layer over the single crystal Si (or Si/SiGe) while leaving an oxide mask layer over the poly Si (or Si/SiGe) which blocks B diffusion from the highly doped raised extrinsic base to the corner of the collector.
FIG. 4 illustrates a prior art CVD grown base NPN transistor having a B (boron) doped raised extrinsic base construction which is now used for HBT (heterojunction bipolar transistor) base contact [B. Jagannathan et al., “Self-aligned SiGe NPN transistors with 285 GHz fMAX and 207 GHz fT in a manufacturable technology”, IEEE Electron Device Lett. 23, 258 (2002)]. The device comprises a collector, a highly B doped raised extrinsic base contact, an in situ B doped single crystal base layer, and a central emitter CE with CE spacers separating emitter and extrinsic base, with the device being isolated by shallow trench isolation STI on each side. Its advantage compared to a CVD grown base NPN transistor having an implanted base [S. Jeng et al., “Impact of extrinsic base process on NPN HBT performance and polysilicon resistor in integrated SiGe HBTs”, Proceedings of 1997 BCTM, P. 187-190], is that it reduces the total base-collector capacitance Ccb, which helps to achieve an extremely high fMAX [B. Jagannathan et al., “Self-aligned SiGe NPN transistors with 285 GHz fMAX and 207 GHz fT in a manufacturable technology”, IEEE Electron Device Lett. 23, 258 (2002)].
At a side corner 50 of the device, STI oxide is pulled down during a pre-CVD HF (hydrofluoric acid) etch, which is necessary for high quality CVD film growth. The side corner 50 is then filled with poly Si (or Si/SiGe). This introduces a fast corner diffusion path for B from the raised extrinsic base to diffuse into the collector, and causes an additional base-collector capacitance Ccb ext2.
FIGS. 4-7 illustrate the structure after the raised extrinsic base formation, and CE is an emitter pedestal, which will be opened in a later process and emitter poly will be deposited in the opening. Since the emitter process is not directly related to this invention, the details of the emitter process are omitted here. Several options of emitter processes can be adopted, one example is an emitter self-aligned to the extrinsic base as described by prior art [B. Jagannathan et al., “Self-aligned SiGe NPN transistors with 285 GHz fMAX and 207 GHz fT in a manufacturable technology”, IEEE Electron Device Lett. 23, 258 (2002)], but this invention is not limited to this specific emitter process.
The present invention provides a method for eliminating the corner diffusion path for B to reduce the total Ccb of the device. The inventive method leaves an oxide layer in between the raised extrinsic base and the CVD grown poly Si (or Si/SiGe), which blocks B diffusion from the raised extrinsic base, as shown in FIG. 5.
FIG. 5 shows a CVD grown base NPN transistor similar to FIG. 4, wherein pursuant to the present invention an oxide layer 50 is formed over the poly Si (or Si/SiGe) 51 on the field around the central single crystal Si (or Si/SiGe) base 52. The device comprises a collector 53, a B doped raised extrinsic base contact 54, an in situ B doped single crystal base layer 52, and a central emitter CE (to be formed later) with CE spacers separating the emitter and the extrinsic base 54, with the device being isolated by shallow trench isolation STI on each side. At a side edge corner 55 of the device, STI oxide is pulled down during a pre-CVD HF etch, which is necessary for high quality CVD film growth. The side edge corner 55 is then filled with poly Si (or Si/SiGe).
Pursuant to the present invention, the self-aligned mask is fabricated by taking advantage of the fact that poly Si (or Si/SiGe) oxidizes faster than single crystal Si (or Si/SiGe). An oxide film is formed over both the poly Si (or Si/SiGe) 51 and the single crystal Si (or Si/SiGe) 52 by using a thermal oxidation process to form a thick oxide layer over the poly Si (or Si/SiGe) and a thin oxidation layer over the single crystal Si (or Si/SiGe).
The oxidation process is followed by a controlled oxide etch to remove the thin oxide layer over the single crystal Si (or Si/SiGe) 52 while leaving an oxide mask layer 50 over the poly Si (or Si/SiGe) 51. This oxide mask blocks B diffusion from the raised extrinsic base through the side edge corner 55 to the collector.
The oxide layer is also initially formed thicker on the single crystal facet face 56 than on the (100) face 57, which is due to the higher oxidation rate of the single crystal facet face compared to the (100) face, such that after the controlled etch, an oxide mask layer also remains over the single crystal facet face.
The differential oxidation between poly and single crystal can be further enhanced by the fact that SiGe oxidizes faster than Si, and doped Si (or SiGe) oxidizes faster than intrinsic Si (or SiGe). For example, if the base layer comprises a SiGe layer followed by an intrinsic Si cap layer, and oxidation thickness is chosen properly so that in the single crystal area oxidation is limited to the top intrinsic Si cap of base layer, and oxidation in the polycrystal area consumes all the intrinsic Si cap and further oxidizes SiGe underneath the Si cap. Faster oxidation rate in SiGe further enhances the differential oxidation. B doping in the base layer beneath an intrinsic Si cap works similarly as SiGe discussed above.
The present invention reduces or eliminates Ccb ext2, which is caused by B diffusion through the poly at the side edge corner 55. Ccb ext2 is significant because the doping concentration in the collector is higher when it's deeper (closer to the subcollector).
In the above example, STI oxide at the side edge corner 55 of the device is pulled down during pre-CVD HF etch, and that corner is then filled with poly Si (or Si/SiGe) during the CVD process. However, the present invention will still work and help to reduce Ccb even when there is no such pull down, as illustrated in FIG. 6 and FIG. 7.
FIG. 6 illustrates a prior art CVD grown base NPN transistor similar to FIG. 4, but without the oxide pull down (similar to 55) at the side edge of the STI.
High concentration dopant in the extrinsic base still sees a fast diffusion path of poly Si (or Si/SiGe) near the edge of single crystal Si (or Si/SiGe). The dopant diffuses to the side of single crystal area and causes additional Ccb. A self-aligned mask formed over poly Si (or Si/SiGe) area blocks this diffusion path, as illustrated in FIG. 7.
FIG. 7 shows a CVD grown base NPN transistor similar to FIG. 6, wherein pursuant to the present invention an oxide layer 70 is formed over the poly Si (or Si/SiGe) 71 on the field around the central single crystal Si (or Si/SiGe) base 72. The device comprises a collector 73, a B doped raised extrinsic base contact 74, an in situ B doped single crystal base layer 72, and a central emitter CE (to be formed later) with CE spacers separating the emitter and the extrinsic base 74, with the device being isolated by shallow trench isolation STI on each side.
Pursuant to the present invention, the self-aligned mask is fabricated by taking advantage of the fact that poly Si (or Si/SiGe) oxidizes faster than single crystal Si (or Si/SiGe). An oxide film is formed over both the poly Si (or Si/SiGe) 71 and the single crystal Si (or Si/SiGe) 72 by using a thermal oxidation process to form a thick oxide layer over the poly Si (or Si/SiGe) and a thin oxidation layer over the single crystal Si (or Si/SiGe).
The oxide layer is also initially formed thicker on the single crystal facet face 76 than on the (100) face 77, which is due to the higher oxidation rate of the single crystal facet face compared to the (100) face, such that after the controlled etch, an oxide mask layer also remains over the single crystal facet face
The oxidation process is followed by a controlled oxide etch to remove the thin oxide layer over the single crystal Si (or Si/SiGe) 72 while leaving an oxide mask layer 70 over the poly Si (or Si/SiGe) 71. This oxide mask blocks B diffusion from the raised extrinsic base to the collector.
In addition, it's easy to see that the STI is not essential for the present invention to work.
While several embodiments and variations of the present invention for a self-aligned mask formed utilizing differential oxidation rates of materials are described in detail herein, it should be apparent that the disclosure and teachings of the present invention will suggest many alternative designs to those skilled in the art.

## Claims

1. A method for forming a self-aligned oxide mask on a composite base of a bipolar transistor comprised of a collector, an emitter, and the composite base which includes a polycrystal Si on a field and a single crystal Si at an active area, wherein the method comprises:
forming an oxide layer over both the poly Si and the single crystal Si of the composite base by using a thermal oxidation process to form a thick oxide layer over the poly Si and a thin oxide layer over the single crystal Si; 
performing a controlled oxide etch to remove the thin oxide layer over the single crystal Si while leaving a self-aligned oxide mask layer over the poly Si.

2. The method of claim 1, wherein differential oxidation is enhanced due to faster oxidation of doped Si compared to intrinsic Si of the bipolar transistor, and wherein the oxide layer thickness is formed so that in the single crystal Si, oxidation is limited to a top intrinsic Si cap of the base, and oxidation in the polycrystal Si consumes all the intrinsic Si cap and further oxidizes doped Si underneath the Si cap.

3. The method of claim 1, wherein the oxide layer is also initially formed thicker on a single crystal facet face of the single crystal Si than on a (100) face of the single crystal Si because of a higher oxidation rate of the single crystal facet face compared to the (100) face, such that after the controlled etch, an oxide mask layer also remains over the single crystal facet face.

4. The method of claim 3, wherein a side edge portion of isolation oxide of the transistor is pulled down during a pre-CVD HF etch provided for high quality CVD film growth, and the side edge portion is then filled with poly Si, and the self-aligned oxide mask blocks a diffusion path for B from the raised extrinsic base through the poly Si filled side edge corner to diffuse into the collector.

5. The method of claim 1, wherein a side edge portion of isolation oxide of the transistor is pulled down during a pre-CVD HF etch provided for high quality CVD film growth, and the side edge portion is then filled with poly Si, and the self-aligned oxide mask blocks a diffusion path for B from the raised extrinsic base through the poly Si filled side edge corner to diffuse into the collector.

6. A method for forming a self-aligned oxide mask on a composite base of a bipolar transistor comprised of a collector, an emitter, and the composite base includes a polycrystal Si/SiGe, which means a layer of polycrystal silicon on top of a layer of polycrystal silicon germanium on a field, and a single crystal Si/SiGe, which means a layer of single crystal silicon on top of a layer of single crystal silicon germanium, at an active area, wherein the method comprises:
forming an oxide layer over both the poly Si/SiGe and the single crystal Si/SiGe of the composite base by using a thermal oxidation process to form a thick oxide layer over the poly Si/SiGe and a thin oxide layer over the single crystal Si/SiGe; 
performing a controlled oxide etch to remove the thin oxide layer over the single crystal Si/SiGe while leaving a self-aligned oxide mask layer over the poly Si/SiGe.

7. The method of claim 6, wherein differential oxidation is enhanced due to faster oxidation of SiGe compared to Si of the bipolar transistor.

8. The method of claim 6, wherein differential oxidation is enhanced due to faster oxidation of doped SiGe compared to intrinsic SiGe of the bipolar transistor, and wherein the oxide layer thickness is formed so that in the single crystal, oxidation is limited to a top intrinsic Si cap of the base, and oxidation in the polycrystal consumes all of the intrinsic Si cap and further oxidizes doped SiGe underneath the Si cap.

9. The method of claim 6, wherein the oxide layer is also initially formed thicker on a single crystal facet face of the single crystal layer than on a (100) face of the single crystal layer because of a higher oxidation rate of the single crystal facet face compared to the (100) face, such that after the controlled etch, an oxide mask layer also remains over the single crystal facet face.

10. The method of claim 9, wherein a side edge portion of isolation oxide of the transistor is pulled down during a pre-CVD HF etch provided for high quality CVD film growth, and the side edge portion is then filled with poly Si/SiGe, and the self-aligned oxide mask blocks a diffusion path for B from the raised extrinsic base through the poly Si/SiGe filled side edge corner to diffuse into the collector.

11. The method of claim 6, wherein a side edge portion of isolation oxide of the transistor is pulled down during a pre-CVD HF etch provided for high quality CVD film growth, and the side edge portion is then filled with poly Si/SiGe, and the self-aligned oxide mask blocks a diffusion path for B from the raised extrinsic base through the poly Si/SiGe filled side edge corner to diffuse into the collector.