# Patent

## Title
Method for integrally forming an electrical fuse device and a MOS transistor

## Abstract

A method for integrally forming a metal-oxide-semiconductor (MOS) device and an electrical fuse device on a semiconductor substrate includes the following steps. An isolation structure is formed on the semiconductor substrate. A dielectric layer is deposited over the isolation structure and the semiconductor substrate. A metal layer is deposited on the dielectric layer. A polysilicon layer is deposited on the metal layer. The dielectric layer, the metal layer and the polysilicon layer are patterned into a first stack of the dielectric layer, the metal layer and the polysilicon layer on the isolation structure for functioning as the electrical fuse device, and a second stack of the dielectric layer, the metal layer and the polysilicon layer on the semiconductor substrate for functioning as a gate of the MOS device.

## Background
BACKGROUND
The present invention relates generally to semiconductor processing technology, and more particularly to a method for integrally forming an electrical fuse device and a metal-oxide-semiconductor (MOS) device.
An electrical fuse is a device put in a circuit as a fusable link to protect against overloading. Current above a predetermined threshold will melt the fuse and cause an open circuit. Various mechanisms can be used to program the electrical fuse device. There are three mechanisms commonly used in programming the electrical fuse device, namely, electromigration effect, silicide agglomeration and rupture. Given a certain programming current and time, the resistance of the electrical fuse can be adjusted by utilizing these mechanisms.
Conventionally, the electrical fuse device is constructed by depositing and patterning a polysilicon layer on an isolation structure separating the same from its underlying semiconductor substrate. The polysilicon layer usually comes from the same layer that is used to form the poly gate of a MOS device. This allows the electrical fuse device and the MOS device to be constructed in the same series of processing steps.
As the semiconductor manufacturing technology advances, it becomes increasingly popular that the MOS device would include a metal structure in its gate electrode. This poses a new challenge to the construction of the electrical fuse device. If separate processing steps are used to construct the MOS device and the electrical fuse device, the fabrication costs will increase. As such, it is desirable to have a method for forming an electrical fuse device and the MOS device with a metal gate structure integrally.
SUMMARY
The invention discloses a method for integrally forming a metal-oxide-semiconductor (MOS) device and an electrical fuse device on a semiconductor substrate. In one embodiment of the invention, the method includes the following steps. An isolation structure is formed on the semiconductor substrate. A dielectric layer is deposited over the isolation structure and the semiconductor substrate. A metal layer is deposited on the dielectric layer. A polysilicon layer is deposited on the metal layer. The dielectric layer, the metal layer and the polysilicon layer are patterned into a first stack of the dielectric layer, the metal layer and the polysilicon layer on the isolation structure for functioning as the electrical fuse device, and a second stack of the dielectric layer, the metal layer and the polysilicon layer on the semiconductor substrate for functioning as a gate of the MOS device.
The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a cross sectional view of a conventional electrical fuse device and MOS device.
FIGS. 2A through 2F illustrate a number of sectional views of semiconductor structures showing a series of processing steps for integrally fabricating an electrical fuse device and a MOS device in accordance with one embodiment of the present invention.



DESCRIPTION
FIG. 1 illustrates a cross sectional view 100 of a semiconductor structure having a MOS device 104 and a conventional electrical fuse device 108. The MOS device 104 is constructed on top of a semiconductor substrate 102. A gate of the MOS device 104 includes a gate dielectric layer 110, a polysilicon layer 112, and a silicide layer 114. The gate dielectric layer 110 is formed on the semiconductor substrate 102. The polysilicon layer 112 is deposited on the gate dielectric layer 110. The silicide layer 114 is formed on the polysilicon layer 112. Spacers 116 of the MOS device 104 are formed on the sidewalls of the gate dielectric layer 110 and the polysilicon layer 112. Source/drain regions 105 are formed adjacent to the gate dielectric layer 110 of the MOS device 104 in the semiconductor substrate 102.
The conventional electrical fuse device 108 is constructed on top of an isolation structure 106, which is formed on the semiconductor substrate 102. The electrical fuse device 108 includes a stack of layers including a gate oxide layer 110′, polysilicon layer 112′ and silicide layer 114′ in a sequential order. Spacers 116′ of the electrical fuse device 108 are formed on the sidewalls of the gate dielectric layer 110′, the polysilicon layer 112′ and the silicide layer 114′. The electrical fuse device 108 and the gate of the MOS device 104 are formed in the same series of processing steps.
As the semiconductor manufacturing technology advances, it becomes increasingly popular that the MOS device 104 would include a metal structure in its gate structure. This poses a new challenge to the construction of the electrical fuse device 108. Thus, it is desirable to have a method for forming an electrical fuse device and a MOS device with a metal gate structure integrally.
FIGS. 2A through 2F illustrate cross sectional views of semiconductor structures showing the processing steps for fabricating an electrical fuse device in accordance with one embodiment of the present invention. FIG. 2A illustrates a cross-sectional view 202 showing a semiconductor substrate 204, on which an isolation structure 206 is formed. The semiconductor substrate 204 can be made of silicon, germanium, and a combination thereof. The isolation structure 206 can be a shallow trench isolation structure (STI) or a local oxidation of silicon (LOCOS) structure. The STI isolation structure can be formed by steps of trench etching and dielectric material deposition. The LOCOS structure can be formed by locally oxidizing the semiconductor substrate 204.
FIG. 2B illustrates a cross-sectional view 210 of a semiconductor structure, which presents a proposed electrical fuse device in process according to the embodiment of the present invention. A gate dielectric layer 212 is formed on the semiconductor substrate 204 and the isolation structure 206. A metal layer 214 is formed on the gate dielectric layer 212. A polysilicon layer 216 is formed on the metal layer 214. The gate dielectric layer can be formed by oxidizing the surface of the semiconductor substrate 204 or performing a chemical vapor deposition (CVD) process. The metal layer 214 can be formed by methods, such as sputtering, CVP, plasma enhanced chemical vapor deposition (PECVD), Atomic Layer Deposition (ALD), etc. Similarly, the polysilicon layer 216 can be formed by methods such as CVD and PECVD. The metal layer 214 may be made of tantalum carbide or tantalum nitride, with a resistivity below about 100 μohm-cm. Other material choices of the metal layer 214 may be pure metal, metal alloy, metal oxide, metal nitride, metal oxynitride, metal silicide, metal carbide and other metal compound.
FIG. 2C illustrates a cross-sectional view 211 of a semiconductor structure, which presents a proposed electrical fuse device in process according to the embodiment of the present invention. A photoresist layer 213 with one or more openings 217 is formed on the polysilicon layer 216. An etching process is performed, using the photoresist layer 213 as a shield to define stacked structures beneath the photoresist layer 213. The etching processes can use a wet, dry, or plasma enhanced etching technology. Then, the photoresist layer 213 is removed to provide a semiconductor structure as shown in the cross-sectional view 218 in FIG. 2D.
Referring to FIG. 2D, the gate dielectric layer 212, the metal layer 214 and the polysilicon layer 216 are patterned to form a first stack of layers 220 on the substrate 204, and a second stack of layers 222 on top of the isolation structure 206. The first and second stacks of layers include the patterned polysilicon layer 216, metal layer 214 and the gate dielectric layer 212. The first stack of layers 220 functions as a gate of a MOS device, and the second stack of layers 222 functions as an electrical fuse device. An ion implantation process is then performed to form lightly doped source/drain regions 215 adjacent to the gate dielectric layer 212 of the first stack of layers 220 in the semiconductor substrate 204, which is uncovered by the first stack 220 of layers. It is noteworthy that while in this embodiment, the first stack of layers 220 is designed to function as a gate of MOS device; it can be used as a fuse device in other applications, depending on particular design needs.
FIG. 2E illustrates a cross-sectional view 224 of a semiconductor structure, which includes the electrical fuse device in process according to one embodiment of the present invention. Spacers 228 are formed on the sidewalls of the gate dielectric layer 212, the metal layer 214 and the polysilicon layer 216 of the first stack 220. Spacers 230 are formed on the sidewalls of the gate dielectric layer 212, the metal layer 214 and the polysilicon layer 216 of the second stack 222. Then, an ion implantation process is performed to form source/drain regions 219 adjacent to the spacers 228 in the semiconductor substrate 204, which is uncovered by the first stack of layers 220 and the spacers 228 as shown in the cross-sectional view 232 in FIG. 2F. The ion implantation process can be followed by an annealing process to drive the implanted impurities further into the semiconductor substrate 204. A cap layer 236 is deposited over the spacers 228, the polysilicon layer 216, the uncovered isolation structure 206, and the uncovered semiconductor substrate 204. The deposition can be a CVD process using silicon-oxide or silicon-nitride based dielectric materials. The cap layer 236 can insulate the polysilicon layer 216 of the first and second stacked structures from a subsequently formed interconnect structures (not shown in this figure). The cap layer 236 can also help to reduce the parasitic capacitance between the metal layer 214 of the electrical fuse device and the interconnect structures.
One advantage of the present invention is that the process of forming the proposed electrical fuse device is compatible with the process of forming a MOS device with a metal gate structure. Therefore, the proposed electrical fuse device can be formed without using extra masks in addition to the masks used in forming the MOS device.
The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.
Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.

## Claims

1. A semiconductor structure having a metal-oxide-semiconductor (MOS) device and an electrical fuse device integrally constructed, comprising:
an isolation structure formed on a semiconductor substrate;
a first dielectric layer and a second dielectric layer disposed on the isolation structure and the semiconductor substrate, respectively;
a first metal layer and a second metal layer disposed on the first and second dielectric layers, respectively; and
a first polysilicon layer and a second polysilicon layer disposed on the first and second metal layers, respectively,
wherein the first dielectric layer, the first metal layer and the first polysilicon layer function as the electrical fuse device, and the second dielectric layer, the second metal layer and the second polysilicon layer function as the MOS device or the electrical fuse device.

2. The semiconductor structure of claim 1, wherein the isolation structure is a shallow trench isolation (STI) structure or a local oxidation of silicon (LOCOS) structure.

3. The semiconductor structure of claim 1, wherein the first and second metal layers are substantially made of tantalum carbide or tantalum nitride.

4. The semiconductor structure of claim 1, wherein the first or second metal layer has a resistivity below about 100 μohm-cm.

5. The semiconductor structure of claim 1, further comprising a cap layer overlying the electrical fuse device and the MOS device.

6. The method of claim 5, wherein the cap layer reduces parasitic capacitance between the first metal layer and a subsequently formed interconnect structures.