Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
1998-09-08
2001-01-09
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S290000, C438S299000
Reexamination Certificate
active
06171913
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the general field of field effect transistors having very narrow channel widths with particular reference to the inclusion of an asymmetric single ‘pocket’ implant limited to the drain side.
BACKGROUND OF THE INVENTION
As field effect transistors grow ever smaller, problems begin to arise because of the very short channels that have to be used. In particular, the punchthrough resistance of these devices is often very low. A solution to this problem that has been widely adopted in the industry has been to include an additional implant close to the low density drain. In order to achieve uniformity across the full wafer, it is usual to rotate the wafer during the implantation. As a result, the prior art practice has generally been to simultaneously form a pair of such ‘pocket’ implants, one near the source and one near the drain as this happens automatically. From a performance standpoint only the implant close to the drain is needed so many processes end up with two pocket implants even though the space taken up by the implant close to the source is valuable real estate, particularly as devices continue to grow smaller. It is therefore desirable to form an asymmetric device having a pocket implant only on the drain side. While this is known in the prior art, forming such an asymmetric arrangement has required additional process steps. The method of the present invention allows asymmetric design to be implemented without these additional steps.
In searching the prior art we have been unable to find a process that achieves a single sided pocket implant in the manner taught by the present invention. The following references have, however, been found to be of interest:
Burr et al. (U.S. Pat. No. 5,650,340 July 1997) describes a device having an asymmetric pocket implant under either the source or the drain. Shrivastava (U.S. Pat. No. 5,518,942 May 1996) shows a large angle single sided implant under the source. Kao et al. (U.S. Pat. No. 5,492,847 February 1996) as well as Hsu ( U.S. Pat. No. 5,554,544 September 1996), Pan (U.S. Pat. No. 5,595, 919 January 1997), Huang (U.S. Pat. No. 5,618,740 April 1997), and Tsai et al. (U.S. Pat. No. 5,668,024 September 1997) all show pocket implants using various implant processes including large angle ion implantation.
Han (U.S. Pat. No. 5,409,848 April 1995) and Fratin et al. (U.S. Pat. No. 5,712,814 January 1998) also teach examples of large angle ion implants.
SUMMARY OF THE INVENTION
It has been an object of the present invention to provide a method for forming a‘pocket’ ion implant.
It has been a further object of the present invention to form said‘ pocket’ ion implant on only one side of the device.
Another object of the present invention has been to provide a method whereby said single asymmetric implant may be formed in a single step.
These objects have been achieved by locating the photoresist that is used to protect the source and drain regions during ion implantation at different distances from the gate pedestal. The photoresist on the source side is placed closer to the gate pedestal than it is on the drain side. As a result, when ions arrive at the surface at a sufficiently shallow angle to be able to penetrate the semiconductor regions immediately beneath the gate oxide, photoresist at the source side blocks the beam while the photoresist on the drain side is far enough away from the gate not to interfere with the beam. Thus, a single asymmetrically located pocket is formed in a single step.
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Liu Jing-Meng
Tsai Chaochieh
Wang Jau-Jey
Ackerman Stephen B.
Bowers Charles
Saile George O.
Schillinger Laura M
Taiwan Semiconductor Manufacturing Company
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