Semiconductor structure and manufacturing method thereof

The invention provides a semiconductor structure, the semiconductor structure includes a substrate, two shallow trench isolation structures are located in the substrate, a first region, a second region and a third region are defined between the two shallow trench isolation structures, the second region is located between the first region and the third region. Two thick oxide layers are respectively located in the first region and the third region and directly contact the two shallow trench isolation structures respectively, and a thin oxide layer is located in the second region, the thickness of the thick oxide layer in the first region is greater than that of the thin oxide layer in the second region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductors, in particular to a structure and a manufacturing method of a double diffusion drain metal oxide semiconductor (DDDMOS) with oxide layers with different thicknesses.

2. Description of the Prior Art

Double diffusion drain metal oxide semiconductors (DDDMOS) are usually used as the working element and/or electrostatic protection (ESD) element of high voltage circuits.

In the manufacture of high voltage integrated circuits, double diffusion drain metal oxide semiconductor (DDDMOS) array is often used to provide large output current. Because DDDMOS introduces high voltage, it will produce a strong electric field, especially near the edge of the gate structure, which may cause the electric field to pass through the gate structure and cause damage to the device.

Therefore, an improved DDDMOS structure is needed, which can reduce the probability of the above problems.

SUMMARY OF THE INVENTION

The invention provides a semiconductor structure, the semiconductor structure includes a substrate, two shallow trench isolation structures are located in the substrate, a first region, a second region and a third region are defined between the two shallow trench isolation structures, the second region is located between the first region and the third region, two thick oxide layers are respectively located in the first region and the third region and directly contact the two shallow trench isolation structures respectively, and a thin oxide layer is located in the second region

The invention also provides a manufacturing method of a semiconductor structure, the method includes providing a substrate, forming two shallow trench isolation structures in the substrate. A first region, a second region and a third region are defined between the two shallow trench isolation structures, and the second region is located between the first region and the third region. Next, an oxide layer is formed in the first region, the second region and the third region, and the oxide layer directly contacts the two shallow trench isolation structures. The oxide layer in the second region is then removed, and another oxide layer is formed in the first region, the second region and the third region, so that a thick oxide layer is formed in the first and third regions, and a thin oxide layer is formed in the second region.

According to the embodiment of the present invention, a part of the buffer oxide layer is left on the left and right sides of the DDDMOS structure near the shallow trench isolation, and when another new oxide layer is subsequently formed, an oxide layer with thinner center and thicker left and right sides will be formed under the gate structure. The thick oxide layers on the left and right sides can effectively protect the gate structure from breakdown by high current, while the thin oxide layers remain in the central part, which can also avoid the influence of Kirk effect.

DETAILED DESCRIPTION

To provide a better understanding of the present invention to users skilled in the technology of the present invention, preferred embodiments are detailed as follows. The preferred embodiments of the present invention are illustrated in the accompanying drawings with numbered elements to clarify the contents and the effects to be achieved.

Please note that the figures are only for illustration and the figures may not be to scale. The scale may be further modified according to different design considerations. When referring to the words “up” or “down” that describe the relationship between components in the text, it is well known in the art and should be clearly understood that these words refer to relative positions that can be inverted to obtain a similar structure, and these structures should therefore not be precluded from the scope of the claims in the present invention.

FIG.1toFIG.5are schematic cross-sectional diagrams of fabricating a double diffusion drain metal oxide semiconductor (DDDMOS) according to the preferred embodiment of the present invention. As shown inFIG.1, a substrate10, such as a silicon substrate, is first provided, and then at least two shallow trench isolation structures12are formed in the substrate10. The material of the shallow trench isolation structures12is, for example, silicon oxide, and parts of the shallow trench isolation structures12is located in the substrate10(i.e., penetrates into the substrate10), while the other part protrudes from the surface of the substrate10. In addition, a buffer oxide layer14is formed between the two shallow trench isolation structures12. Preferably, the buffer oxide layer14and the shallow trench isolation structure can be formed at the same time (for example, the buffer oxide layer14can be the pad oxide which is formed with the shallow trench isolation structure12is formed), but the present invention is not limited to this. The buffer oxide layer14serves to protect the substrate10in the subsequent ion doping step.

Then, as shown inFIG.2, an ion doping step P1is performed to form at least one doped region15and a lightly doped region16in the substrate10, the doped region15and the lightly doped region16contain suitable ions, such as III-V ions, etc. The doped region15is a well region, and the lightly doped region16can be used as a lightly doped drain (LDD) in the subsequent DDDMOS. Then, a patterning step P2is performed, for example, using a mask (not shown) combined with an exposure development and etching step to remove part of the buffer oxide layer14. More specifically, the buffer oxide layer14can be defined as a first region R1, a second region R2and a third region R3on the substrate10, the second region R2is located between the first region R1and the third region R3. After the patterning step P2is performed, the buffer oxide layer14in the second region R2is removed, but the buffer oxide layer14in the first region R1and the third region R3still remains.

In the above steps, the ion doping step P1is firstly performed to form the lightly doped region16, and then the patterning step P2is performed. However, in other embodiments of the present invention, the ion doping step P1may also be performed after the patterning step P2is performed, and this process is also within the scope of the present invention.

It is worth noting that in the conventional technology, after the lightly doped region16is formed, the patterning step P2is not performed to partially remove the buffer oxide layer14in the second region R2, but the entire buffer oxide layer14is completely removed, and then a new oxide layer is regenerated. One of the characteristics of the present invention is that after the patterning step P2, only a part of the buffer oxide layer14(that is, the buffer oxide layer14in the second region R2) is removed, and at the same time, the portion of the buffer oxide layer14adjacent to the shallow trench isolation structure12(that is, the buffer oxide layer14in the first region R1and the third region R3) still remains. In this way, a part of the left buffer oxide layer14will subsequently form the thick oxide layers under both sides of the gate structure to better protect the gate structure.

As shown inFIG.3, an oxide layer20is regenerated on the substrate10by a heating step P3. In this embodiment, the heating step is, for example, the in-situ steam generation (ISSG), which raises the temperature to about 140 degrees Celsius in an environment containing oxygen (such as mixed gas containing hydrogen and oxygen, oxygen, ozone, water vapor, etc.), but is not limited to this. The material of the oxide layer20is, for example, silicon oxide, and grows along the surface of the substrate10and the buffer oxide layer14previously left. Therefore, after the heating step P3is performed, the thickness of the buffer oxide layer14in the original first region R1and third region R3increases (because a new oxide layer20is formed on the surface), and the oxide layer20is regenerated in the original second region R2.

In this embodiment, since the buffer oxide layer14and the oxide layer20are made of silicon oxide, they are made of the same material, and the interface between them is indicated by a dashed line. However, it can be understood that the buffer oxide layer14and the oxide layer20are formed separately in different steps. In addition, since the buffer oxide layer14is formed by deposition, its density may be lower than that of the oxide layer20formed by ISSG. However, during the heating step P3, the buffer oxide layer14may be heated again to increase the density. In addition, after the oxide layer20is formed, the buffer oxide layer14and the oxide layer20in the first region R1and the third region R3can be combined and defined as a thick oxide layer22, while the oxide layer20in the second region R2is thinner than the thick oxide layer22, so it can also be defined a thin oxide layer20.

In addition, in this embodiment, except for changing the temperature of the heating step P3, the thickness of the oxide layer20can also be adjusted by adjusting the oxygen-containing ratio of the introduced gas. Preferably, the thickness of the thick oxide layer22is about 10%-30% greater than the thickness of the thin oxide layer20, but not limited to this.

In addition, it is worth noting that during the heating step P3, oxygen reacts with silicon in the substrate to form the silicon oxide layer (i.e., the oxide layer20), so part of the oxide layer20will sink into the surface of the substrate10. Especially in the second region R2, the bottom surface of the oxide layer20will be lower than the top surface of the substrate10in the first region R1or the third region R3. In the present invention, the oxide layer20in the second region R2forms a concave cross-sectional structure, which is helpful to provide better electric field protection for the subsequently formed gate structure.

Then, as shown inFIG.4, a gate structure24is formed on the oxide layer20, the gate structure24is mainly located in the second region R2, but parts of the gate structure24is also located in the first region R1and the third region R3. The gate structure24spans a part of the thick oxide layer22, and the gate structure24is also located on the thin oxide layer20. The gate structure24may be a polysilicon gate, but not limited thereto. In addition, spacers26can be formed on both sides of the gate structure24, which can protect the gate structure24.

Finally, as shown inFIG.5, an interlayer dielectric (ILD)30may be covered over the gate structure24and the spacer26, and then a planarization step (e.g., a chemical mechanical polishing, CMP) may be performed to remove part of the gate structure24, parts of the spacer26and parts of the interlayer dielectric30, so that the gate structure24, the spacer26and the interlayer dielectric30have a flat top surface after the planarization step is performed. These steps belong to the conventional technology in the field, and will not be described in detail here.

To sum up the above paragraphs and drawings, the present invention provides a semiconductor structure, which comprises a substrate100, two shallow trench isolation structures12located in the substrate12, first region R1, a second region R2and a third region R3are defined between the two shallow trench isolation structures12, wherein the second region R2is located between the first region R1and the third region R3, and two thick oxide layers22are respectively located in the first region R1and the third region R3and directly contact the two shallow trench isolation structures12. A thin oxide layer20is located in the second region R2, the thickness of the thick oxide layer22in the first region R1is greater than the thickness of the thin oxide layer20in the second region R2.

In some embodiments, the thick oxide layer22and the thin oxide layer20are formed in different steps.

In some embodiments, the thick oxide layer22and the thin oxide layer20comprise the same material.

In some embodiments, the thickness of the thick oxide layer22in the first region R1is 10% to 30% greater than the thickness of the thin oxide layer20in the second region R2.

In some embodiments, the thickness of the thick oxide layer22in the third region R3is equal to the thickness of the thick oxide layer22in the first region R1.

In some embodiments, a bottom surface of the thin oxide layer20in the second region R2is lower than a bottom surface of the thick oxide layer22in the first region R1.

In some embodiments, a gate structure24is further included on the thin oxide layer20and the thick oxide layer22, wherein the gate structure24is located in the second region R2and partially in the first region R1and the third region R3.

In some embodiments, a doped region (the doped region15or the lightly doped region16) is further included in the substrate10.

The invention also provides a manufacturing method of semiconductor structure, which comprises providing a substrate10, forming two shallow trench isolation structures12in the substrate10, a first region R1, a second region R2and a third region R3are defined between the two shallow trench isolation structures12, the second region R2is located between the first region R1and the third region R3. An oxide layer14is then formed in the first region R1, the second region R2and the third region R3, and the oxide layer14directly contacts the two shallow trench isolation structures12. The oxide layer14is then removed in the second region R2, and another oxide layer20is formed in the first region R1, the second region R2and the third region R3, so that a thick oxide layer22is formed in the first region R1and the third region R3respectively, and a thin oxide layer20is formed in the second region R2

In some embodiments, the oxide layer14is formed simultaneously with the shallow trench isolation structure12.

In some embodiments, the other oxide layer20is formed by a high temperature oxidation step.

In some embodiments, an ion doping step P1is further performed to form at least one doped region (the doped region15or the lightly doped region16) in the substrate.

In some embodiments, the ion doping step P1is performed before removing the oxide layer14in the second region R2.

Compared with the prior art, the advantages of the invention are as follows: because the DDDMOS will introduce high voltage, therefore, a high current will pass through the DDDMOS. Inventors found that when a high current passes through the gate structure of a DDDMOS, it is easy for the current to pass through the oxide layer below the gate structure, and then affect the gate structure. According to the experimental observation results of the inventor, the electric field and current generated on both sides of the gate structure (near the spacer) are the largest. However, if only the thickness of the whole gate dielectric layer (the oxide layer) is increased, the DDDMOS will be easily affected by the Kirk effect, that is, when large current flows, the transistor is not easy to saturate, and the transistor will become characteristic similar to resistance, which may cause leakage. This will also affect the performance of the DDDMOS.

Therefore, according to the embodiment of the present invention, a part of the buffer oxide layer is left on the left and right sides of the DDDMOS structure near the shallow trench isolation, and when another new oxide layer is subsequently formed, an oxide layer with thinner center and thicker left and right sides will be formed under the gate structure. The thick oxide layers on the left and right sides can effectively protect the gate structure from breakdown by high current, while the central part still has a thin oxide layer, which can also avoid the influence of the Kirk effect.