# Patent

## Title
Method to reduce diode capacitance of short-channel MOSFETS

## Abstract

This application claims the priority benefit of Taiwan application Ser. No. 87106241, filed Apr. 23, 1998, the full disclosure of which is incorporated herein by reference.

## Background
1. Field of Invention

The present invention relates to a method for fabricating a semiconductor device substrate. More particularly, the present invention relates to a method for fabricating a high-voltage device substrate capable of preventing junction breakdown and leakage current in the source/drain regions of a MOS transistor.

2. Description of Related Art

In general, most high-voltage MOS devices utilize the thickening of an isolating layer between the gate and the source/drain regions as a means of lowering the horizontal electric field within the channel. Alternatively, the drift regions below the isolation layer and the graded regions beneath the source/drain regions are lightly doped to provide the necessary voltage gradient. The two above measures are capable of increasing junction breakdown voltage in the source/drain regions so that the MOS transistor is able to operate normally despite the application of a high voltage.

As the level of integration for semiconductor devices continues to increase, rules for forming high-voltage devices become more stringent. Besides having to increase the degree of integration, the distance of separation between two neighboring devices has to be carefully planned. The most commonly used method for preparing a MOS device for high voltages is to increase the distance between neighboring devices and/or adding one more lightly doped layer to provide the necessary voltage gradient. Therefore, before a high-voltage device is formed, the substrate has to undergo a number of processing treatments to enhance its voltage bearing capacity. However, the difference in the height between substrate surfaces in different doped regions for a conventionally processed high-voltage MOS device substrate is small. Hence, when an epitaxial layer is grown on the substrate, the alignment of masks in subsequent processing operations is difficult.

To get a better understanding of the processing steps in fabricating a conventional high-voltage device substrate, the progression of manufacturing steps is illustrated using FIGS. 1A through 1K.

First, as shown in FIG. 1A, a substrate 100 such as a P-type substrate is provided. Then, a pad oxide layer 102 is formed over the substrate 100 using a thermal oxidation method. Thereafter, a chemical vapor deposition (CVD) method is used to form a silicon nitride layer 104 over the pad oxide layer 102.

Next, as shown in FIG. 1B, using photolithographic and etching techniques, the silicon nitride layer 104 and the pad oxide layer 102 are patterned to form a stacked layer 106. The stacked layer 106 is composed of a silicon nitride layer 104a and a pad oxide layer 102a.

Next, as shown in FIG. 1C, a field oxide (FOX) layer 112 is formed between two neighboring stack layers 106 by a thermal oxidation process such as wet oxidation.

Next, as shown in FIG. 1D, the silicon nitride layer 104a is removed using a wet etching method, for example. Subsequently, ions are implanted to form a heavily doped P.sup.+  region 108 in the substrate underneath the pad oxide layer 102a. For example, the implantation can be carried out using boron (B.sup.11) ions with an implantation energy level of about 70 KeV and a dosage level of about 2.0.times.10.sup.15 /cm.sup.2.

Next, as shown in FIG. 1E, the pad oxide layer 102a and the field oxide layer 112 are removed to expose the entire substrate surface 100. Thereafter, an atmospheric pressure chemical vapor deposition (APCVD) method is used to deposit an oxide layer 122 over the substrate 100. Preferably, the oxide layer 122 is formed using tetra-ethyl-ortho-silicate (TEOS) as a gaseous reactant in an APCVD process. Since TEOS oxide layer requires densification, the oxide layer 122 also needs to be densified. For example, densification is carried out using a temperature of about 1000.degree. C. for about 10 to 30 minutes. After the densification operation, the oxide layer 122 will contract a little making it somewhat denser.

Next, as shown in FIG. 1F, a patterned photoresist layer 110 is formed over the oxide layer 122. Subsequently, the oxide layer 122 is anisotropically etched using the photoresist layer 110 as a mask. Hence, an oxide layer 122a is formed and at the same time the substrate region 100 is exposed so that ions of opposite polarity to the ions already in the doped region 108 are ready to be implanted.

Next, as shown in FIG. 1G, another ion implantation is carried out to implant ions having polarity opposite to the heavily doped P.sup.+  region 108. In other words, N-type ions are implanted into the substrate 100 to form a heavily doped N.sup.+  region 118. For example, the implantation can be carried out using arsenic (As.sup.75) ions with an implantation energy level of about 100 KeV and a dosage level of about 4.0.times.10.sup.14 /cm.sup.2.

Next, as shown in FIG. 1H, the photoresist layer 110 is removed. Then, an APCVD method is used to deposit another oxide layer 132 over the substrate 100, covering the heavily doped N+ region 118 and the oxide layer 122a. Preferably, the oxide layer 132 having a thickness of about 3000 .ANG. is deposited using TEOS as the gaseous reactant in an APCVD process.

Next, as shown in FIG. 1I, oxidation and dopant drive-in operations are carried out simultaneously. The silicon substrate 100 is placed inside a reaction chamber having some moisture in the atmosphere, and then heated to initiate thermal oxidation. The heat also drives the ions, implanted earlier in the doped regions, deeper into the substrate. Since the heavily doped N.sup.+  region 118a is only covered by a very thin layer of oxide 132 above it, silicon within the heavily doped N.sup.+  region 118a is able to oxidize into silicon dioxide. Consequently, an oxide layer 142 is formed between the silicon substrate 100 and the oxide layer 132, where the oxide layer 142 includes the former oxide layer 122a as well.

Next, as shown in FIG. 1J, the oxide layer above the silicon substrate 100 is completely removed. Both oxide layers 132 and 142 are removed to expose the entire silicon substrate surface 100.

Finally, as shown in FIG. 1K, an epitaxial layer 150 is formed over the substrate surface 100 so that the fabrication of a silicon substrate suitable for forming high-voltage devices is complete. The epitaxial layer 150 can be formed using, for example, a chemical vapor deposition (CVD) method. The epitaxial layer 150, preferably having a thickness of about 19 .mu.m, can be turned into an N-type epitaxial layer if doping of N-type ions is also carried out during the CVD operation.

In the aforementioned fabrication process for a high-voltage device substrate, the difference in height level between the highest (in the P-doped region 108a) and lowest point (in the N-doped region 118a) on the substrate surface is small. Therefore, when the epitaxial layer 150 is formed over the substrate 100, the peak and the trough of the epitaxial layer will also be quite small. Consequently, when alignment marks are subsequently formed over the surface for alignment of photomasks in the fabrication of high-voltage devices, alignment will be inaccurate. This can lead to processing errors and an increase in product failure.

In light of the foregoing, there is a need to provide an improved method of fabricating high-voltage device substrate.

Accordingly, the present invention provides a method for fabricating a high-voltage device substrate that has a greater difference in topographical level between substrate surfaces in the heavily doped N.sup.+  region and the heavily doped P.sup.+  region, through which subsequent alignment of photomasks is more accurate and processing time is reduced.

To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method for fabricating a high-voltage device substrate. The method comprises the steps of forming a pad oxide layer and a mask layer over a substrate. Then, the pad oxide layer and the mask layer are patterned to define a region for a first ion implantation. Next, the exposed substrate is oxidized to form a field oxide layer. Thereafter, the mask layer is removed, followed by a first ion implantation. Next, a portion of the field oxide layer is removed. Subsequently, a photoresist layer is formed over the first ion implanted region and the remaining portion of the field oxide layer. This is followed by a second ion implantation. Then, a conformal oxide layer is formed covering the substrate surface. Next, a high temperature drive-in and oxidation operation is carried out. This serves to drive the ions implanted into the first ion implanted region and the ions implanted into the second implanted region deeper into the substrate interior, and at the same time oxidizes the substrate surface of those regions. Finally, the oxide layer on the substrate surface is removed, and then an epitaxial layer is formed over the substrate.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,

FIGS. 1A through 1K are cross-sectional views showing the progression of manufacturing steps in fabricating a conventional high-voltage device substrate; and

FIGS. 2A through 2J are cross-sectional views showing the progression of manufacturing steps in fabricating a high-voltage device substrate according to one preferred embodiment of this invention.

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

FIGS. 2A through 2J are cross-sectional views showing the progression of manufacturing steps in fabricating a high-voltage device substrate according to one preferred embodiment of this invention.

First, as shown in FIG. 2A, a pad oxide layer 202 having a thickness of about 400 .ANG. is formed over a substrate 200, such as a P-type silicon substrate, using a thermal oxidation method. Then, a chemical vapor deposition (CVD) method is used to form a mask layer 204 over the pad oxide layer 202. The mask layer 204 has a thickness of about 1500 .ANG. and can be a silicon nitride layer 204.

Next, in FIG. 2B, photolithographic and etching methods are used to pattern the silicon nitride layer 204 and the pad oxide layer 202. Consequently, stack layers 206, each composed of a silicon nitride layer 204a and a pad oxide layer 202a, are formed.

Next, in FIG. 2C, a thermal oxidation, for example, a wet oxidation is carried out to form a field oxide layer 212 having a thickness of about 11000 .ANG. between two neighboring stack layers 206.

Next, in FIG. 2D, the silicon nitride layer 204a is removed using, for example, a wet etching method. Subsequently, an ion implantation is conducted to form a heavily doped P.sup.+  region 208 in the substrate underneath the pad oxide layer 202a. For example, the implantation can be carried out using boron (B.sup.11) ions with an implantation energy level of about 70 KeV and a dosage level of about 2.0.times.10.sup.15 /cm.sup.2.

Next, as shown in FIG. 2E, a patterned photoresist layer 210 is formed over the field oxide layer 212 and the substrate 200. Thereafter, using the photoresist layer 210 as a mask, the field oxide layer 212 is anisotropically etched to form a field oxide layer 212a that exposes a portion of the substrate 200. Thus, a region for implanting ions opposite in polarity type to the heavily doped region 208 is created. Since thickness of the field oxide layer 212a is rather large (about 11000 .ANG.), it is difficult to oxidize portions of the substrate 200 covered by the field oxide layer 212a in subsequent oxidation and drive-in operations. Therefore, the difference between the height of the substrate 200 and the lower heavily doped N.sup.+  region is further increased.

Next, as shown in FIG. 2F, a second ion implantation is carried out implanting ions having polarity opposite to the heavily doped P.sup.+ region 208. In other words, N-type ions are implanted into the exposed substrate 200 to form a heavily doped N.sup.+  region 218. For example, the implantation can be carried out using arsenic (As.sup.75) ions with an implantation energy level of about 100 KeV and a dosage level of about 4.0.times.10.sup.14 /cm.sup.2.

Subsequently, as shown in FIG. 2G, the photoresist layer 210 is removed. Then, a conformal oxide layer 222 is deposited over the substrate 200 using an atmospheric pressure chemical vapor deposition (APCVD) method. The conformal oxide layer 222 covers the N.sup.+  heavily doped region 218, the pad oxide layer 202a and field oxide layer 212a. The conformal oxide layer 222 preferably having a thickness of about 3000 .ANG. is formed using tetra-ethyl-ortho-silicate (TEOS) as a gaseous reactant in APCVD operation.

Next, as shown in FIG. 2H, oxidation and dopant drive-in operations are carried out simultaneously. The silicon substrate 200 is placed inside a reaction chamber having some moisture in the atmosphere so that a portion of the silicon substrate 200 is oxidized. In the meantime, the heat generated by the oxidation also drives the N.sup.+  and P.sup.+  ions in the respective ion implanted regions deeper into the substrate 200.

Since the heavily doped N.sup.+  region 218 is formed near the substrate surface and is only covered by a rather thin APCVD oxide layer 222, silicon in the N.sup.+  region 218 will react to form silicon dioxide. Therefore, an oxide layer 242 is formed between the substrate 200 and the oxide layer 222. Furthermore, since the heavily doped P.sup.+  region 208 is only covered by the thin pad oxide layer 202a and the thin oxide layer 222, silicon in the P.sup.+  region 218 will react to form silicon dioxide. Therefore, the pad oxide layer 202a between the substrate 200 and the APCVD oxide layer 222 will expand to form an oxide layer 202b.

Consequently, besides forming an oxide layer 242 on the substrate 200 above the heavily doped N.sup.+  region 218a, an oxide layer 202b is also formed on the substrate 200 above the heavily doped P.sup.+  region 208. However, as the thickness of the field oxide layer 212a is rather large (about 11000 .ANG.), it is more difficult to oxidize portions of the substrate 200 covered by the field oxide layer 212a in subsequent oxidation and drive-in operations. Therefore, the difference in height between the substrate 200 and the lower heavily doped N.sup.+  region is larger than that produced by conventional methods. When an epitaxial layer is subsequently formed over the substrate layer, difference in level of height between the high points and low points on the epitaxial layer will be much greater than the one formed by a conventional means. When alignment marks for aligning photomasks are subsequently formed over the epitaxial layer, alignment accuracy will be greatly increased.

Next, as shown in FIG. 2I, the oxide layers 222 and 242 above the silicon substrate 200 are removed to expose the entire substrate surface.

Finally, as shown in FIG. 2J, an epitaxial layer 250 is formed over the substrate surface 200 so that the fabrication of a silicon substrate suitable for forming high-voltage devices is complete. The epitaxial layer 250 can be formed using, for example, a chemical vapor deposition (CVD) method. The epitaxial layer 250 preferably having a thickness of about 19 .mu.m can be turned into an N-type epitaxial layer if doping of N-type ions is also carried out during the CVD operation.

In summary, the method to form the high-voltage device substrate in this invention includes the following advantages: by an APCVD process. Furthermore, this APCVD oxide layer requires performance of subsequent densification and etching procedures. Therefore, this invention is able to save up to one or two days of processing time. Moreover, the densification of APCVD oxide layer is difficult to control, and the amount of waste material resulting from abnormal formation is high. In other words, conventional processing method costs more and takes longer. greater difference in height between the highest and lowest points, when compared to one formed by a conventional method. Therefore, when alignment marks for aligning photomasks are subsequently formed over the epitaxial layer, alignment accuracy is greatly increased. existing fabricating procedures. Therefore, the method can be immediately run in a factory production line.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.

## Claims

1. A method for fabricating high-voltage device substrate, comprising the steps of: in the substrate where oxidation is required; first oxide layer; forming a first ion doped region in the substrate underneath the pad oxide layer; where a second implantation is required; the second region of the substrate;

2. The method of claim 1, wherein the step of forming the pad oxide layer includes a thermal oxidation method.

3. The method of claim 1, wherein the pad oxide layer has a thickness of about 400 .ANG..

4. The method of claim 1, wherein the step of forming the mask layer includes a chemical vapor deposition method.

5. The method of claim 1, wherein the mask layer has a thickness of about 1500 .ANG..

6. The method of claim 1, wherein the step of performing thermal oxidation includes a wet oxidation method.

7. The method of claim 1, wherein the first oxide layer has a thickness of about 11000 .ANG..

8. The method of claim 1, wherein the step of removing the mask layer includes a wet etching method.

9. The method of claim 1, wherein the step of performing the first implantation includes implanting boron (B.sup.11) ions with an implantation energy level of about 70 KeV and a dosage level of about 2.0.times.10.sup.15 /cm.sup.2.

10. The method of claim 1, wherein the step of performing the second implantation includes implanting arsenic (As.sup.75) ions with an implantation energy level of about 100 KeV and a dosage level of about 4.0.times.10.sup.14 /m.sup.2.

11. The method of claim 1, wherein the ions used in the first ion implantation are of the opposite polarity to the ions used in the second ion implantation.

12. The method of claim 1, wherein the step of forming the second oxide layer includes an atmospheric pressure chemical vapor deposition method.

13. The method of claim 1, wherein the second oxide layer has a thickness of about 3000 .ANG..

14. The method of claim 1, wherein the step of performing high temperature drive in and oxidation reaction is for forming a third oxide layer over the first ion doped region and the second ion doped region, and at the same time driving the ions in the first ion doped region and the second ion doped region deeper into the substrate interior.

15. The method of claim 14, wherein the third oxide layer is formed on the first ion doped substrate region and on the second ion doped substrate region after performing the high temperature drive-in and oxidation reaction.

16. The method of claim 1, wherein the step of forming an epitaxial layer over the substrate further includes implanting ions of the same polarity type as the second ion doped region.

17. The method of claim 1, wherein the epitaxial layer has a thickness of about 19 .mu.m.