Metal Salicide-CMP-Metal Salicide Semiconductor Process

1. Field of the Invention

The invention relates to a semiconductor process, and, more particularly, to a metal salicide-chemical mechanical polishing (CMP)--metal salicide semiconductor process by which a thicker metal salicide layer is formed on gates to prevent an unacceptable increased gate contact resistance which results from over etching during the etching of shallow contact windows on gates and causes low device efficiencies.

2. Description of the Related Art

Generally, different material layers are formed on a base substrate by depositing and etching in a semiconductor process. As a result, the surface of the substrate on which devices have been already formed is rough, i.e., different positions on the surface of the substrate have different heights. Therefore, etching contact windows having different depths at the same time can not be avoided. In a deep sub-micron semiconductor process, wherein the dimensions of contact windows continue to shrink, Ti self-aligned salicide (Ti salicide) technology is adopted and a chemical mechanical polishing (CMP) method is employed. This results in an increasing severity of the over etching problem that causes material loss inside shallow contact windows during the etching of shallow contact windows.

Referring to FIGS. 1A to 1C, a semiconductor wafer contact window process according to the prior art is illustrated. FIG. 1A shows a part of a cross-sectional view of a semiconductor wafer wherein a metal salicide process and a CMP method for planarization have already been completed. Shown are semiconductor substrate 1, field oxide 2, spacers 3, silicon layer 4 (for example, a poly-silicon layer), Ti salicide (TiSi.sub.2) layer 5 and interlayer dielectric layer 6.

Referring to FIG. 1B, photoresistor layer 7 is deposited over semiconductor substrate 1 of FIG. 1A and a contact window pattern is defined by a photolithography technology. The result shown in FIG. 1B includes a .DELTA.X distance representing the difference between the depths of two expected contact windows for poly-silicon layer 4 and a source (drain) (not shown), respectively.

Referring next to FIG. 1C, shallow contact window 11 is formed in dielectric layer 6 using anisotropic etching. Due to the two expected contact windows having depth difference .DELTA.X, when Ti salicide layer 5 inside shallow contact window 11 begins to be etched during an etching process, there is still the .DELTA.X thickness in deep contact window 12 left for further etching. Therefore, when deep contact window 12 is etched to the end, Ti salicide layer 5 on poly-silicon layer 4, and even poly-silicon layer 4, are seriously etched, resulting in an unacceptable increased gate contact. Typically, Ti salicide layer 5 on polysilicon layer 4 is thinner and more unstable than that on a single-silicon layer, so the selectivity of interlayer dielectric 6 to Ti salicide layer 5 is not preferable.

Based on the above, the invention provides a metal salicide-CMP-metal salicide semiconductor process, suitable for a semiconductor substrate on which gates, sources (drains), spacers, and field oxides are already formed. A first metal layer is formed on the gates, sources (drains), spacers, and field oxides. A first high-temperature process is executed to form a first metal salicide layer on the gates, and sources (drains). A first wet etching is then performed. A first dielectric layer is formed over the semiconductor substrate wherein the horizontal line of the first dielectric layer is over the first metal salicide layer located on the gates. A first CMP is executed until the first metal salicide layer on the gates is reached. A second metal layer is formed on the first dielectric layer and on the first metal salicide layer that is located on the gates. A second high-temperature process is performed in order to form a thicker second metal salicide layer on the gates. A second wet etching is then executed. A second dielectric layer is formed over the semiconductor substrate. A second CMP is then performed. Finally, shallow contact windows and deep contact windows are formed for the gates and sources (drains), respectively. The above-mentioned second metal salicide layer formed on the gates is thicker, thereby preventing an unacceptable increased gate contact resistance resulting from over etching during the etching of shallow contact windows for the gates and causing low device efficiencies.

The objects, characteristics, and advantages of the present invention will be explained clearly by using a preferred embodiment with pertinent drawings as follows:

FIGS. 1A to 1C are cross-sectional views showing a prior art contact window process; and

FIGS. 2A to 2H are cross-sectional views showing a metal salicide-CMP-metal salicide semiconductor process for etching contact windows according to an embodiment of the invention.

Referring to FIG. 2A, the present invention relates to a metal salicide-CMP-metal salicide semiconductor process, suitable for semiconductor substrate 20 on which gate 23, source (drain) 21, spacers 24, and field oxide 22, are already formed. The process includes the following steps:

Step 1

Still referring to FIG. 2A, first Ti metal layer 25 is formed on gate 23, source (drain) 21, spacers 24, and field oxide 22 by using a chemical vapor deposition (CVD) method.

Step 2

Referring to FIG. 2B, a first high temperature process, for example, a rapid thermal process (RTP), is executed in order to form first Ti metal salicide layer 26 on gate 23 and source (drain) 21, then a first wet etching is executed in order to remove a part of first Ti metal layer 25 that does not join a silicide reaction.

Step 3

Referring to FIG. 2C, first oxide layer 27 is formed over semiconductor substrate 20 by using a chemical vapor deposition (CVD) method wherein the horizontal line of first oxide layer 27 is over first Ti salicide layer 26 located on gate 23. Then a first chemical mechanical polishing (CMP) is performed until first Ti salicide layer 26 on gate 23 is reached by means of first Ti salicide layer 26 having a different polishing rate.

Step 4

Referring to FIG. 2D, second Ti metal layer 28 is formed on first Ti salicide 26 located on gate 23 and on first oxide layer 27 by using a chemical vapor deposition (CVD) method.

Step 5

Referring to FIG. 2E, a second high-temperature process, for example, a rapid thermal process (RTP), is executed in order to form thicker second Ti salicide layer 29 on gate 23. Then a second wet etching is performed to remove a part of second Ti metal layer 28 that does not join a silicide reaction.

Step 6

Referring to FIG. 2F, second oxide layer 30 is formed over semiconductor substrate 20 by using a chemical vapor deposition (CVD) method. Then, a second chemical mechanical polishing (CMP) is executed.

Step 7

Referring to FIG. 2G, photoresistor layer 31 is formed on second oxide layer 30. Then first trench 32 and second trench 33 are formed by using a photolithography technology.

Step 8

Referring to FIG. 2H, shallow contact window 34 and deep contact window 35 are formed for gate 23 and source (drain) 21, respectively, by using an anisotropic plasma etching.

Based on the metal salicide-CMP-metal salicide semiconductor process according to an embodiment of the invention, thicker second Ti salicide layer (TiSi.sub.2) 29 formed can prevent itself, and even gate 23, from over etching resulting in an unacceptable increased gate contact resistance that causes low device efficiencies.

Although the invention has been disclosed in terms of a preferred embodiment, the disclosure is not intended to limit the invention. Those knowledgeable in the art can make modifications within the scope and spirit of the invention that is determined by the claims below.