# Patent

## Title
Semiconductor device and method for manufacturing the same

## Abstract

An epitaxial layer is formed on an n+ semiconductor substrate by epitaxial growth. A gate trench is formed to the surface of gate trench so that the bottom of gate trench reaches middle of the epitaxial layer. A gate insulator is formed on the inner wall of gate trench and a polysilicon is formed in the gate trench with the gate insulator interposed therebetween. An HTO film is formed on the surface of the polysilicon and the n− epitaxial layer. At this time, an ion plantation is performed to the epitaxial layer through the HTO film. Hence, a p diffused base layer, an n+ diffused source layer, an n+ diffused source layer is formed. A CVD oxide film is formed on the HTO film. After a BPSG having flowability is deposited on the CVD oxide film, the BPSG film is planarized with a heat treatment of 900-1100 degree Celsius.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device and a method for manufacturing semiconductor device. Particularly, the present invention relates to a vertical MOSFET having a trench gate electrode and a method for manufacturing the same.
2. Description of Related Art
With rapid development of microfabrication technology, a semiconductor device continues to be integrated highly. Especially, it is well known that a vertical MOSFET (UMOSFET) having a gate electrode buried in a trench has low on-resistance and high breakdown voltage. Further, high integration is required for lower on-resistance and cost reduction (Japanese Unexamined Patent Application Publication No. 2005-86140 and No. 2001-36074). As one of methods for high integration, it is known that the gate trench is formed deeply in an epitaxial layer so as to shorten an aperture of the gate trench. For another method, it is known that an interlayer insulator is buried completely in the gate trench to shorten the aperture of the trench (Japanese Unexamined Patent Application Publication No. 2003-101027, No. 2000-252468 and U.S. Pat. No. 6,351,009).
Hereinafter, a related manufacturing process of UMOSFET, having the interlayer insulator buried in the gate trench completely, will be described. An N-channel type of UMOSFET is taken for instance. As shown in FIG. 9, an n− type epitaxial layer 82 is formed on a semiconductor substrate 81 by an epitaxial growth. A gate trench 83 is formed to the surface of the n− type epitaxial layer 82 so that the gate trench 83 reaches to the inner of the n− epitaxial layer 82. A gate insulator 84 is formed on the inner side of the gate trench 83. Further, a polysilicon 85 as a gate electrode is buried in the gate trench 83 with the gate insulator interposed therebetween. A high temperature oxide film (an HTO film) 86 is formed on the polysilicon 85 and the surface 82a of the n− type epitaxial layer.
A p type diffused base layer 87 and an n+ type diffused source layer 88 are formed on the surface 82a of the n− type epitaxial layer with ion implantation doping though the HTO film 86. A boron phosphorus silicate glass film (a BPSG film) 89 is formed on the HTO film 86. The BPSG film 89 has a flowability. Hence, the surface of the BPSG film 89 is planarized by a heat treatment after forming the BPSG film 89. An etch-back process is performed from the surface of the planarized BPSG film 89 to the depth of an aperture of the gate trench. So, the HTO film 86 and the BPSG film 89 formed on the n− type epitaxial layer 82 are removed. As shown in FIG. 10, a source electrode is formed on the entire surface of the semiconductor device. A drain electrode 91 is formed on the back side of semiconductor substrate 81. Hence, the cell pitch can be reduced, because the interlayer insulator (the BPSG film 89) between the gate electrode (the polysilicon 85) and the source electrode 90 is buried wholly in the gate trench 83.
In the UMOSFET configured as described above, the polysilicon 85 as the gate electrode is positioned in the lower portion of the gate trench 83. It is because the BPSG film 89 as the interlayer insulator is buried in the gate trench completely. Hence, it needs to form the n+ type diffused source layer 88 in the lower portion of the gate trench 83 depending on the position of the polysilicon 85. The process of heat treatment to planarize the BPSG film 89 includes the process to diffuse the n+ type diffused source layer 88 also in order to reduce number of process. Here, this process needs high temperature as to diffuse the n+ diffused source layer 88 sufficiently. However, the thickness of the HTO film 86 between the BPSG film 89 and the n− type epitaxial layer 82 is formed to be thin. It is because the p type diffused base layer 87 and the n+ type diffused source layer 88 are formed by ion implantation doping though the HTO film 86 as described above. Hence, if the heat treatment to planarize the BPSG film 89 is set to be high temperature, the diffusion of boron and phosphorus from the BPSG film 89 to the n− type epitaxial layer 82 is promoted. So, it makes the controllability of the manufacturing the semiconductor device worse.
In this way, the UMOSFET having the interlayer insulator buries in the gate trench has the process lower controllability, because impurity like boron and phosphorus diffuse from the BPSG film at the heat treatment.
SUMMARY
According to one aspect of this invention, there is provided a method for manufacturing a semiconductor device comprising: forming a first oxide film on a surface of a semiconductor layer and a polysilicon in a trench, the trench formed in the semiconductor layer; forming a first diffused layer of a first conductivity type and a second diffused layer of a second conductivity type through the first oxide film; forming a second oxide film on the first oxide film; forming a flowable insulator film on the second oxide film; performing a heat treatment for planarizing the insulator film and diffusing the second diffused layer to prescribe depth; and etching the insulator film.
According to another aspect of this invention, there is provided a semiconductor device comprising: a semiconductor layer of a second conductive type; a first diffused region of a first conductive type formed in the semiconductor layer; a second diffused region of the second conductive type selectively formed in the first diffused region; a trench formed in the semiconductor layer; a polysilicon formed in the trench with an insulator intervening; a first oxide film formed on the polysilicon so that the first oxide film is buried in the trench; a second oxide film formed on the first oxide film so that the second oxide film is buried in the trench; a flowable insulator film on the second oxide film.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which:
FIG. 1 shows a cross sectional view of the semiconductor device 10 according to a first embodiment;
FIG. 2 shows the first process of forming the semiconductor device 10;
FIG. 3 shows the second process of forming the semiconductor device 10;
FIG. 4 shows a relationship between temperature (degree Celsius) at a heat treatment and a minimum film thickness t (angstrom);
FIG. 5 shows a cross sectional view of the semiconductor device 40 according to a second embodiment;
FIG. 6 shows the first process of forming the semiconductor device 40;
FIG. 7 shows the second process of forming the semiconductor device 40;
FIG. 8 shows a cross sectional view of another semiconductor device 40′ according to the second embodiment;
FIG. 9 shows the first process of the related forming process of the semiconductor device; and
FIG. 10 shows the second process of the related forming process of the semiconductor device.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The invention will now be described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes.
First Embodiment
FIG. 1 shows a cross sectional view of a semiconductor device according to a first embodiment of the invention. Hereinafter, “n+” means n type semiconductor which n type impurity heavily doped. “n−” means n type semiconductor which n type impurity lightly doped. Likewise, “p+” means p type semiconductor which p type impurity heavily doped. “p−” means p type semiconductor which p type impurity lightly doped. “X direction” means a horizontal direction of drawing sheet and “Y direction” means a vertical direction of drawing sheet.
As shown in FIG. 1, a semiconductor device 10 comprises an n+ type semiconductor substrate 11. An n− type epitaxial layer 12 is formed on the n+ type semiconductor substrate 11. A p type diffused base layer 17 (a first diffused layer) is formed on the n− type epitaxial layer 12. A gate trench 13 is formed on the surface of the p diffused base layer. A plurality of the gate trenches 13 are formed in the X direction. A bottom of the gate trench 13 reaches to the n− epitaxial layer 12. A gate insulator 14 is formed on an inner wall of the gate trench 13. A polysilicon 15 is formed on an inner aspect of the gate insulator 14. The HTO film 16 (a first oxide film) is formed in a lower portion in the Y direction than a surface of p type diffused base layer on the polysilicon 15. A chemical vapor deposition oxide film (a CVD oxide film) 20 (a second oxide film) is formed so that the CVD oxide film 20 reaches around an aperture of the gate trench 13. Beside the aperture of the gate trench 13, an n+ type diffused source layer 18 (a second diffused layer) is formed. A source electrode 21 is formed on the n+ type diffused source layer 18. The source electrode 21 is connected electrically to the n+ type diffused source layer 18 and the p type diffused base layer 17. A drain electrode 22 is formed on the backside of the n+ type semiconductor substrate 11.
Next, a method for manufacturing the semiconductor device 10 configured as above is explained hereinafter. FIG. 2 shows a first process for manufacturing semiconductor device 10. Firstly, the epitaxial layer 12 is formed on the n+ type semiconductor substrate 11 by an epitaxial growth. The gate trench 13 is formed to a surface 12a of the epitaxial layer so that a bottom of the gate trench 13 reaches to the epitaxial layer 12. The gate insulator 14 is formed inside the gate trench 13. The polysilicon 15 is buried in the gate trench 13 with the gate insulator 14 interposed therebetween. The surface of the polysilicon 15 is positioned at a lower portion than the surface of the epitaxial layer 12a. The HTO film 16 is formed over the polysilicon 15 and the surface 12a of the n− type epitaxial layer. At this time, as shown in FIG. 2, p type of impurity is implanted to the surface of the epitaxial layer 12a through the HTO film 16 to form the p type diffused base layer 17. In the same way, n type of impurity is implanted to a predetermined portion of the p type diffused base layer 17 through the HTO film 16. Hence, the n+ type diffused source layer 18 is formed beside the aperture of the gate trench 13.
Next, as shown in FIG. 3, the CVD oxide film 20 is formed on the HTO film 16. At this time, the CVD oxide film 20 is formed along a shape of lower layer. Hence, a CVD oxide film 20a located above the gate trench 13 is deposited with lower position than a CVD oxide film 20b located above the surface of the epitaxial layer 12a. The BPSG having a flowability is deposited on the CVD oxide film 20. A surface of the deposited BPSG film 19 has an asperity along a surface (20a, 20b) of the CVD oxide film 20 below the BPSG film 19 (not shown).
At this time, a heat treating is performed to planarize the BPSG film 19, as shown in FIG. 3. This process of heat treating combines the process to diffuse the n+ diffused source layer 18 injected by ion implantation so that the n+ source layer 18 is diffused as high as the polysilicon 15. This is for cutting the number of the processes. An etch-back process is performed to the surface of the BPSG film 19 until the surface of CVD oxide film 20 is positioned as high as around the aperture of the gate trench 13. Hence, the semiconductor device 10 is formed as shown in FIG. 1. The BPSG film 19 is used for planarization the surface of the CVD oxide film 20 and the HTO film 16 which are not flat as shown in FIG. 1.
For the semiconductor device formed in this way, the CVD oxide film 20 (as shown in FIG. 3) formed below the BPSG film 19 can prevent boron and phosphorus of the BPSG film 19 from diffusing to the semiconductor layer (such as p base layer 17, the n+ diffused source layer 18 and the n− epitaxial layer 12). Hence, the n+ diffused source layer 18 is diffused adequately by the heat treating. Concurrently, it can reduce the diffusion of boron and phosphorus the BPSG film 19 includes to the semiconductor layer. As a result, it can enhance a controllability of manufacturing the semiconductor device 10.
It is necessary to set a thickness t of the CVD oxide film 20, so that the CVD oxide film 20 prevent adequately boron and phosphorus of the BPSG film 19 from diffusing to the semiconductor layer. At a high temperature treatment where process temperature is from 900 to 1100 degree Celsius, a diffusion coefficient of phosphorus is larger than a diffusion coefficient of boron. Hence, it may determine the thickness t of the CVD oxide film 20 considering the diffusion coefficient of phosphorus and production tolerance. Here, phosphorus concentration of the BPSG film 19 is about from 3 to 5 mol % and boron concentration of the BPSG film 19 is about from 10 to 11 mol %. A diffusion coefficient of phosphorus in SiO2 is about 1×10−14 (cm2/sec) at 1000 degree Celsius. A diffusion coefficient of phosphorus in Si is about 5×10−13 (cm2/sec) at 1000 degree Celsius. A diffusion coefficient of phosphorus in Si at 1000 degree Celsius is about fifty fold of in SiO2.
On the other hand, in analysis of SIMS (Secondary Ionization Mass Spectrometer), a depth of phosphorus diffusion in Si after 30 minutes of the heat processing at 1000 degree Celsius is about 1.0 μm. Based on the result in this analysis, it is estimated that a depth of phosphorus diffusion in SiO2 after 30 minutes of the heat processing at 1000 degree Celsius is about 200 angstrom that is one-fifty of the depth of phosphorus diffusion in Si. As described above, it is estimated that the preferable thickness t of the CVD oxide film 20 is more than 200 angstrom at 1000 degree Celsius of the heat processing. A listing as below shows an estimated preferable minimum film thickness t of the CVD oxide film 20 at 900, 950, 1000 and 1100 degree Celsius estimated in the same way described above.














TEMPERATURE




AT HEAT TREATMENT
FILM THICKNESS t









no more than 900 degree Celcius
t > 24 angstrom



no more than 950 degree Celcius
t > 80 angstrom



no more than 1000 degree Celcius
t > 200 angstrom 



no more than 1100 degree Celcius
t > 1200 angstrom 










FIG. 4 shows a relation between temperature (degree Celsius) of the heat process and minimum film thickness t of the CVD oxide film 20. The data of the relation between temperature at the heat process and minimum film thickness in the listing above is plotted on a semi-logarithmic graph. This plotted data is approximated by expression line L. Based on the graph in FIG. 4, the thickness t of the CVD oxide film 20 can be set more than the value of expression line L for processing temperature after forming the BPSG film 19. Furthermore, considering an embeddability, cost of manufacturing, variation of etching process to remove the CVD oxide film 20, the preferable thickness t of the CVD oxide film 20 is 24-10000 angstrom.
In the first embodiment, an n channel type of UMOSFET is explained for example, but this invention can be applied to a p type of UMOSFET. Applied to a p type of UMOSFET, advantages of this invention can be obtained. When this embodiment is applied to the p type of UMOSFET, conductivity type of semiconductor device in FIG. 1 is inverted.
Second Embodiment
FIG. 5 shows a cross sectional diagram of semiconductor device 40 according to a second embodiment of this invention. One feature of the second embodiment is that an NSG film 41 (None-doped Silicate Glass film) (a third oxide film) is formed below the HTO oxide film 16. Hereinafter, the same number is given to the same composition as the first embodiment.
As shown in FIG. 5, the semiconductor device 40 comprises the n+ type semiconductor substrate 11. The n− type epitaxial layer 12 is formed on the n+ type semiconductor substrate 11. The p type diffused base layer 17 is formed on the n− epitaxial layer 12. The gate trench 13 is formed at the surface of the p type diffused base layer 17. A plurality of the gate trenches 13 are formed in the X direction. The gate insulator 14 is formed on the sidewall of the gate trench 13. The polysilicon 15 is formed on the gate insulator 14.
An NSG film 41 is formed on the polysilicon 15 in the gate trench 13. A dielectric strength of the NSG film 41 is as strong as the CVD oxide film, and the NSG film 41 has a reflowability. Hence, the NSG film 41 is preferable material for an interlayer insulator formed in the gate trench 13. The HTO film 16 is formed on the NSG film 41 in the gate trench 13. The CVD oxide film 20 is formed on the HTO film 16 so as to reach the aperture portion of the gate trench 13. The n+ diffused layer 18 is formed beside the aperture of the gate trench 13.
Next, a manufacturing method of the semiconductor device 40 configured as above is described hereinafter. FIG. 6 shows the first manufacturing process of the semiconductor device 40. First, the epitaxial layer 12 is formed on the n+ semiconductor substrate 11 by the epitaxial growth. A plurality of the gate trenches 13 are formed in the X direction so that the bottom of the gate trench 13 reaches the epitaxial layer 12. The gate insulator 14 is formed on an inner aspect of the gate trench 13. The polysilicon 15 is formed on an inner aspect of the gate insulator 14. The NSG film 41 is deposited to the polysilicon 15. Here, the NSG film 41 is formed in the gate trench 13, and not on the surface 12a of the epitaxial layer. The HTO film 16 is deposited on the NSG film 41 and the epitaxial layer 12. At this state, an impurity is implanted to the n− epitaxial layer 12 through the HTO film 16 so that p diffused base layer 17 and the n+ diffused source layer 18 are formed in the n− epitaxial layer 12.
As shown in FIG. 7, the CVD oxide film 20 is formed on the HTO film 16. The BPSG film 19 is deposited on the CVD oxide film 20. As described above, after depositing the BPSG film 19, the surface of the BPSG film 19 has the ragged asperity along the surface of the BPSG film 19 (not shown). With the high heat processing, the ragged surface of the BPSG film 19 having a reflowability is planarized. An etch-back process is performed to the planarized surface of BPSG film 19 until the surface of CVD oxide film 20 is positioned as high as around the aperture of the gate trench 13. So, the BPSG film 19, the CVD oxide film 20 and the HTO film 16 on the epitaxial layer 12 are removed. In this way, the semiconductor device 40 as shown in FIG. 5 is formed. The source electrode 21 and the drain electrode 22 are formed as same as the first embodiment.
In the semiconductor device 40 configured as above, as the NSG film 41 is formed between the HTO film 16 and the polysilicon 15, the gap between the HTO film 16a on the gate trench 13 and the HTO film 16b on the surface 12a of epitaxial layer is less than the first embodiment (see FIG. 6). Hence, at the process of forming the p type diffused base layer 17 and the n+ type diffused source layer 18, it can prevent an impurity from diffusing through the sidewall of the gate trench 13. As a result, it can prevent the n+ type diffused layer 18 from entering in deeply around the sidewall of the gate trench 13. So, in the second embodiment, an effect of punch-through phenomena can be reduced more effectively than the first embodiment. Punch-through phenomena become prominent as gate length is shorter. As a result, it can further improve performance of the semiconductor.
For the semiconductor device 10 according to the first embodiment as shown in FIG. 1 and the semiconductor device 40 according to the second embodiment, the BPSG film 19 formed in the process of manufacturing is wholly removed by etching. But, this is the case that the thickness of the HTO film 16 and the CVD oxide film 20 are correctly formed and the etch-back process is performed with required accuracy. However, even when the formed BPSG film 19 is not wholly removed, an advantage of this invention to prevent boron and phosphorus from diffusing from the BPSG film 19 can be obtained.
FIG. 8 shows a semiconductor device 40′ according the second embodiment in the case the BPSG film 19 is not wholly removed. This semiconductor device 40′ has the remained BPSG film 19 on the CVD oxide film 20. In this semiconductor device 40′, even if the etch-back process is excessively performed to the NSG film 41 at the manufacturing process of the second embodiment, the thickness of interlayer insulator is enough ensured. Because the CVD oxide 20, the HTO film 16, the NSG film 41, and the BPSG film 19 are layered on the gate trench 13. Herewith, it can reduce tolerance for etching, and ensure the thickness of the interlayer insulator adequately. As a result, it can diffuse the n+ type diffused source layer 18 to reach the required depth by the heat treatment, and at the time it can restrain diffusing of the impurity. As a result, it can improve performance of UMOSFET having interlayer insulator wholly formed in the gate trench.
The case is described that the BPSG film 19 is remained in the second embodiment, but even if the BPSG film 19 may remain in the first embodiment, the advantage of this invention can be obtained also. Material of an oxide film (as the HTO film 16, the CVD film 20, the NSG film and the like) is not limited that. A variety of material can be applied to the oxide film.
It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.

## Claims

1. A method for manufacturing a semiconductor device comprising:
forming a first oxide film on a surface of a semiconductor layer and a polysilicon in a trench, the trench formed in the semiconductor layer;
forming a first diffused layer of a first conductivity type and a second diffused layer of a second conductivity type through the first oxide film;
forming a second oxide film on the first oxide film;
forming a flowable insulator film on the second oxide film;
performing a heat treatment for planarizing the insulator film and diffusing the second diffused layer to prescribe depth; and
etching the insulator film.

2. The method of manufacturing a semiconductor device according to claim 1, wherein
a thickness of the second oxide film is thicker than 24 angstrom.

3. The method of manufacturing a semiconductor device according to claim 2, wherein
a thickness of the second oxide film is thicker than 80 angstrom when a temperature of the heat treatment is 950 degree Celsius and below.

4. The method of manufacturing a semiconductor device according to claim 2, wherein
a thickness of the second oxide film is thicker than 200 angstrom when a temperature of the heat treatment is 1000 degree Celsius and below.

5. The method of manufacturing a semiconductor device according to claim 2, wherein
a thickness of the second oxide film is thicker than 1200 angstrom when a temperature of the heat treatment is 1100 degree Celsius and below.

6. The method of manufacturing a semiconductor device according to claim 1, wherein
the second oxide film is a CVD film.

7. The method for manufacturing a semiconductor device according to claim 1, wherein
the semiconductor layer, the first diffused layer, and the second diffused layer operate as a drain region, a base region and a source region, respectively.

8. The method of manufacturing a semiconductor device according to claim 1, wherein
the first oxide film is an HTO film.

9. The method of manufacturing a semiconductor device according to claim 1, further comprises a third oxide film between the polysilicon and the first oxide film.

10. The method of manufacturing a semiconductor device according to claim 9, wherein
the third oxide film is NSG film.