JPH0661221A - Method for formation of local field oxide at inside of integrated circuit - Google Patents

Abstract

PURPOSE: To reduce the lateral intrusion of a field oxide into active regions of a semiconductor chip by locally oxidizing an Si substrate. CONSTITUTION: On an Si substrate 40 a pad oxide layer of 2 nm or more thick is laminated and a comparatively thin nitride layer 5-30 nm thick is formed on the top of the oxide layer 38. A polysilicon layer 44 10-300 nm thick is formed on the top of the nitride layer 38, a nitride layer 46 50-500 nm thick is formed on top of the polysilicon layer 44. The nitride layers 46, 42 and polysilicon layer 44 are masked and etched to form windows inside. They are heated in a high oxidative atmosphere to more oxidize the substrate 4o0 near the windows 48 to form a field oxide film 50. This reduces the lateral intrusion (birds beak) B6 of the field oxide film into active regions of a semiconductor chip.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to isolation processes for integrated circuits, and more particularly to a method of forming isolated regions by locally oxidizing a silicon substrate.

[0002]

BACKGROUND OF THE INVENTION Integrated circuits include a number of active devices formed on a common semiconductor substrate. These active devices must be isolated from each other to prevent electrical interference between them. MOS on silicon substrate
In integrated circuits using devices, one way to achieve this isolation is to locally "grow" the field oxide between the active areas. Normally, LOCOS (LOCa
This method, known as Oxidation of Silicon, masks the active areas and then oxidizes the isolated areas. More specifically, the silicon substrate is first heated in an atmosphere of high oxygen concentration to oxidize and form silicon dioxide. This is commonly referred to as the "pad oxide layer". A silicon nitride layer is then deposited over the pad oxide. The isolated regions are covered with photoresist material over the active regions and then etched through the unmasked nitride layer. After the photoresist is removed, the integrated circuit is again heated in a high oxygen concentration atmosphere to grow the field oxide.

The nitride layer prevents oxidation of the underlying substrate area. Nitride cannot be directly deposited on the silicon surface. The reason is that thermal stress occurs between silicon nitride and silicon. This stress increases with the thickness of the nitride layer. The intermediate pad oxide layer acts as a cushion to reduce this stress.

A problem caused by the presence of pad oxide is that it provides a growth path for the field oxide. As oxygen diffuses into the pad oxide layer during the field oxide growth, it forms sharps that penetrate into the regions prepared for active device formation. This sharp object is usually called a "bird's beak." The area occupied by Toribek is a lost area from the perspective of active device manufacturing. In addition, the length of the bird's beak is generally uncontrollable, which makes a wider area unusable due to manufacturing process variations.

As mentioned above, the nitride layer prevents direct oxidation of the underlying substrate. In addition, this nitride layer suppresses lateral penetration of the bird's beak due to the opportunistic stress it places on the pad oxide. Generally speaking, the thicker the nitride layer is, the better the formation of bird's beak can be suppressed. However, the thick nitride layer unduly stresses the underlying pad oxide layer.

Another isolation method that can further reduce bird's beak formation is polycrystalline silicon buffer LOCOS (polybuffer LOC).
OS) process. Poly buffer LOCOS
In the method, a layer of polycrystalline silicon is deposited on the pad oxide, between the pad oxide and the overlying nitride layer. This poly is a buffer between the top nitride layer and the bottom layer.
Work as. This allows a somewhat thicker nitride layer and / or
Alternatively, the use of thin pad oxide is possible. The thick nitride layer reduces lateral penetration of the bird's beak. However, polycrystals themselves oxidize at a faster rate than single crystal substrates.

[0007]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a new and improved local field oxidation method.

Another object of the present invention is to provide a method of reducing lateral intrusion of field oxide into the active area of a semiconductor chip.

Yet another object of the present invention is to provide a method of reducing stress during a local field oxidation process.

Yet another object of the present invention is to provide a local field oxidation process which can reduce bird's beak formation without causing additional defects in the silicon substrate.

[0011]

The present invention is a process or method for forming a local field oxide in a silicon substrate. A pad oxide is formed on the substrate and a relatively thin first nitride layer is formed on the pad oxide. A polycrystalline silicon layer is formed on the first nitride layer, and a second nitride layer is formed on the polycrystalline silicon layer. The first and second nitride and polycrystalline silicon layers are then windowed to allow field oxide growth therein.

[0012]

DETAILED DESCRIPTION FIG. 1 shows an integrated circuit 10 during a prior art LOCOS process. The integrated circuit 10 has a pad oxide layer 12 laminated on a silicon substrate 14. A nitride layer 16 is formed on top of the pad oxide 12 and is masked and etched to leave a window therein.

The integrated circuit 10 is then heated in an atmosphere of high oxygen concentration to further oxidize the silicon substrate 14 near the window 18. This produces the field oxide 20 as shown in FIG. Note that the field oxide 20 extends not only vertically above and below the original surface of the silicon substrate 14, but also laterally to lift the nitride layer 16. The degree of lateral penetration is noted b2. This lateral penetration b2 is usually called the bird's beak.

The polybuffer LOCOS process reduces bird's beak as shown in FIG. FIG. 3 shows an integrated circuit at the stage of processing in a prior art polybuffer LOCOS process. The integrated circuit 22 has a pad oxide layer 24 laminated on a silicon substrate 26. A layer 28 of polycrystalline silicon (polysilicon) is formed over the pad oxide 24. further,. Nitride layer 30 and polycrystalline silicon 28
Is masked and etched to leave a window 32 inside.

The integrated circuit 22 is then further heated in a high concentration oxygen atmosphere to further oxidize the silicon substrate near the window 32. This additional oxidation produces field oxide 34, as shown in FIG. The field oxide 34 extends not only in the vertical direction of the original surface of the silicon substrate 26 but also in the lateral direction to lift the nitride layer 30,
Also note that polysilicon 28 is partially lifted and polysilicon 28 is partially consumed. The degree of lateral intrusion (bird's beak) is marked b4.

Comparing FIGS. 2 and 4, the bird's beak is somewhat reduced in the polybuffer LOCOS process,
That is, it can be seen that b4 <b2.

FIG. 5 illustrates integrated circuit 36 at one stage of processing in accordance with the method of the present invention. The integrated circuit 36 has a pad oxide layer 38 laminated on a silicon substrate 40. A relatively thin nitride layer 42 is formed on top of pad oxide 38. A polysilicon layer 44 is formed on top of the nitride layer 38 and a nitride layer 46 is formed on top of the polysilicon layer 44. Nitride layer 46, polysilicon layer 44, and nitride layer 42 are masked and etched to leave windows 48 therein.

The integrated circuit 36 is then heated in a high oxygen concentration atmosphere to further oxidize the silicon substrate 40 near the window 48. This additional oxidation produces field oxide 50 as shown in FIG. The field oxide 50 extends not only vertically above the original surface of the silicon substrate 14 but also laterally to lift the nitride layer 46, partially lift the polysilicon 44, and the polysilicon 4.
It can be seen that it consumes 4 and also partially consumes the thin nitride layer 42. The degree of lateral intrusion (bird's beak) is b
It is written as 6.

Comparing FIGS. 2, 4 and 6, it is noted that the bird's beak produced by the method of the present invention is smaller than that shown for either prior art, ie, b6 <b4 <b2. I want to.

FIGS. 7-12 show details of steps in a method for achieving the structure shown in FIG. FIG. 7 illustrates forming a pad oxide layer 38 on a silicon substrate 40 by oxidation. The surface of substrate 40 is cleaned and then heated in the presence of oxygen to form silicon dioxide. As is well known, varying the time, temperature, and oxygen controls the thickness of the resulting pad oxide layer 38. In the preferred embodiment, the thickness of the pad oxide layer 38 is between 2 nm and 50 nm.

FIG. 8 shows that a relatively thin nitride layer 42 (Si ₃ N ₄ ) is formed on the pad oxide 38. The nitride layer 42 is preferably formed by a chemical vapor deposition (CVD) method. In the embodiment, the thickness of the nitride layer 42 is 2n.
It is preferably between m and 50 nm and preferably in the range of 5 nm and 30 nm.

FIG. 9 illustrates forming a polysilicon layer 44 (polycrystalline silicon) on the nitride layer 42. The polysilicon layer 44 is preferably formed by the CVD method. In the preferred embodiment, the thickness of polysilicon layer 44 is between 10 nm and 300 nm.

FIG. 10 illustrates forming a nitride layer 46 (Si ₃ N ₄ ) on the polysilicon layer 44. Nitride layer 46 is preferably formed by CVD. In the preferred embodiment, the thickness of nitride layer 46 is between 50 nm and 500 nm.

As shown in FIG. 11, the window 48 is formed by photomasking the surfaces 52 and 54 of the nitride layer 46 which are not to be the window, and then the nitride layer 46 and the polysilicon layer 4 are formed.
4 and nitride layer 42 are etched into nitride layer 46, polysilicon layer 44 and nitride layer 42. In the preferred embodiment, a reactive ion etch (RIE) is utilized to clear the window 48.
If desired, the silicon substrate 40 may be used as an etch stop layer and the pad oxide layer 38 may be removed in a subsequent etching method.

After removing the photoresist from the surface of the nitride layer 42, field impurities are introduced into the substrate 40 by ion implantation to increase the surface concentration of the impurities. The device is then heated in a high oxygen concentration atmosphere to grow field oxide 50 in window 48 as shown in FIG. The resulting structure has a reduced bird's beak as previously described. In addition, the nitride layer 42 is sufficiently thin that it is under stress, if any.

It will be apparent to those skilled in the art that the present invention is not limited to the particular embodiments and illustrations described above. It should be understood that the dimensions, scales, and structural relationships illustrated in the drawings are for illustration purposes and not the actual dimensions, scales, or structural relationships of the local field oxidation method. The present invention may be modified in various ways within the scope of the invention described in the claims.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a prior art integrated circuit before field oxidation in a LOCOS process.

2 is a cross-sectional view of the circuit of FIG. 1 after field oxidation.

FIG. 3 is a cross-sectional view of a prior art integrated circuit prior to field oxidation in a polybuffer LOCOS process.

FIG. 4 is a cross-sectional view of the circuit of FIG. 3 after field oxidation.

FIG. 5 is a sectional view before field oxidation according to the present invention.

6 is a cross-sectional view of the circuit of FIG. 5 after field oxidation.

FIG. 7 is a first integrated circuit cross-sectional view selectively illustrating successive steps of a method in accordance with the present invention.

8 is a second integrated circuit cross-sectional view showing a step subsequent to FIG. 7;

9 is a third integrated circuit cross-sectional view showing a step subsequent to FIG. 8;

10 is a fourth integrated circuit cross-sectional view showing a step subsequent to FIG. 9;

11 is a fifth integrated circuit sectional view showing a step subsequent to FIG. 10; FIG.

FIG. 12 is a sixth integrated circuit cross-sectional view showing a step subsequent to FIG. 11;

[Explanation of symbols]

 10 integrated circuit 18 window 22 integrated circuit 32 window 36 integrated circuit 48 window

Claims (14)

[Claims] 1. A method of forming a localized field oxide in a silicon substrate, the method comprising: forming a pad oxide on the substrate; and forming a first nitride layer on the pad oxide. Forming, forming a polysilicon layer on the first nitride layer, forming a first nitride layer on the polysilicon layer, forming the first and second nitride layers and A method of forming a local field oxide comprising opening a window for growing a field oxide in a polysilicon layer. 2. The method of claim 1, wherein the first nitride layer is relatively thin. 3. The method according to claim 2, wherein the thickness of the first nitride layer is less than 50 nm. 4. The method of claim 3, wherein the pad oxide layer has a thickness greater than 2 nm. 5. The method according to claim 4, wherein the thickness of the first nitride layer is between 5 nm and 30 nm. 6. The method of claim 5, wherein the polysilicon layer has a thickness between 10 nm and 300 nm. 7. The method according to claim 6, wherein the thickness of the second nitride layer is between 50 nm and 500 nm. 8. The method of claim 1, wherein the first nitride layer is chemical vapor deposited Si ₃ N ₄ . 9. The method of claim 1 wherein the polysilicon layer is formed by chemical vapor deposition. 10. The method of claim 1, wherein the second nitride layer is Si ₃ N ₄ formed by chemical vapor deposition. 11. The method of claim 1, wherein the method of opening the window comprises photomasking non-windowed regions of the second nitride layer, the second nitride layer, a polysilicon layer, and A method comprising etching a first nitride layer. 12. The method of claim 11 wherein the etching is reactive ion etching. 13. The method of claim 11, further comprising introducing field impurities by ion implantation into the silicon substrate facing the window. 14. The method of claim 13, further comprising growing the field oxide within the window.