Active solid-state devices (e.g. – transistors – solid-state diode – Test or calibration structure
Reexamination Certificate
2002-04-22
2004-09-07
Tran, Thien F (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Test or calibration structure
C027S021100, C438S014000
Reexamination Certificate
active
06787797
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Fields of the Invention
The present invention relates to a semiconductor wafer and an apparatus of the process for fabricating semiconductor devices in terms of improving the productivity of the process for fabricating semiconductor devices and improving the yield of device chips in the process.
2. Description of the Related Art
Concerning the properties of the backside surface of a semiconductor used in the process for fabricating semiconductor devices, it has been said heretofore to be preferable for the good control of the process that the surface is more uniform. As for the surface roughness, it has been considered ideal to be uniform and even. The uniformity within the whole surface of the backside of a wafer has been common even with a backside damaged wafer or a wafer having a polysilicon layer on its backside. In case of more strict condition of fabricating in the process or others, however, the principle heretofore is not always a golden rule.
In the processes fabricating semiconductor devices, many processes such as chemical or physical are carried out on the surface of a wafer. The wafer is held by a holding means such as a vacuum chuck or an electrostatic chuck to hold the wafer stable in the apparatus used in the processes. Under these circumstances, the backside of the wafer comes in contact with the surface of the holding means. The wafer and the holding means influence each other through the contact surfaces with regard to the physical phenomena such as suction or heat transfer. Hence, the state of the contact with each other, or more specifically, backside of the wafer that takes part in the contact varies the mutual “contact surface density”, which is defined hereinafter, to influence the effect of these physical phenomena and affect considerably the efficiency and a result of the process of fabricating the device as well.
Contact surface density is defined herein as an index indicating a ratio of the real area of the mutual physical contact to the apparent area of contact in the state of two objects, such as a wafer and a holding means contacting each other. As shown in the schematic drawing of
FIG. 1
which illustrates microscopically the sectional view of the area of the contact point, the contact surface density is defined as a ratio of the integrated sum of real small contact areas &Dgr;s of the mutual physical contact to the apparent contact area S; hence the form of the equation may be written as follows.
contact surface density(%)=(&Sgr;&Dgr;
s/S
)×100
In case a wafer is processed by heating in a device fabrication process, the contact surface density of the backside surface of the wafer on the surface of the wafer holding means affects the heat transfer behavior between them. As a result, when the contact surface density in the apparent mutual contact area (hereinafter referred to as “contact area” simply) has a distribution, the temperature in the front surface of the wafer becomes non-uniform in accordance with the distribution. It follows therefore that the rate of processing within a wafer surface comes to also have a distribution.
In order to avoid this drawback, the wafer having the state on its backside surface of as small surface roughness and as excellent uniformity as possible have been used heretofore.
As stated above, however, when the device fabricating process is carried out by placing a conventional wafer having a uniform backside surface within a plane on a conventional wafer holder, the temperature in the wafer surface of the peripheral part of the wafer differs from that in other parts. That is nothing but the condition of heat transfer is different whether the place is in the periheral part or in other inner parts. Since the temperature condition of the process which should bring about the proper quality is set appropriately in accordance with the vicinity of the center of nearly flatly distributed temperature, the devices produced from the peripheral part of the wafer whose temperature condition is not always consistent with the proper temperature condition of the process are discarded as failure products.
In technical views obtained international consensus, when the line width of the patterning size on a device was 0.25 &mgr;m, the patterned region within 3 mm in the direction of the diameter from the circumference of a wafer was excluded as out of the specification. When the line width is 0.15 &mgr;m, the region which should be excluded is to be 2 mm from the circumference; when 0.1 &mgr;m, the region is to be 1 mm. Thus, the target of the specification with this regard is set in order to increase the effective area of a wafer. In case of a 200 mm diameter wafer, for instance, if the region is reduced to 2 mm from 3 mm, the effective area increases by 2%; if the region is reduced to 1 mm from 3 mm, the effective area increases by 4%. In accordance with these facts, it is very important to uniformize the distribution of the temperature from the center region to the circumferential end within the wafer surface in a future device fabrication process.
There is another problem in connection with holding means of a wafer. A method of treating with plasma under reduced pressure is often used in the latest process for fabricating devices. It is often the case to hold a wafer with an electrostatic chuck in this process. In this method, the wafer is held on the face of the chuck with an electrostatic attracting force by applying an electrostatic voltage of several hundred volts between an electrode of the chuck and the wafer on the chuck. The attracting force varies with not only the applied voltage but also the state of contacting the backside of the wafer to the face of the chuck i.e. the state of the surface of the both faces. After processing, the wafer is removed off the electrostatic chuck. A series of device fabricating processes has many such same processes or others so that the wafer removing operations are executed repeatedly after completion of the each process.
The wafer removing operations are carried out by releasing the application of voltage. When chucking, if the wafer adheres too tightly to the chuck face, a difficulty arises to remove the wafer from the chuck face, as is frequently the way. If the roughness of the contact surfaces is increased too much, the attracting force of chucking reduces accompanied by disadvantage of the leakage of the helium gas applied to the contact faces.
DISCLOSURE OF INVENTION
The present invention has an object of providing a new semiconductor wafer, the state of the back side surface of which is prepared so as to improve the yield and productivity of device-chip fabricating process, and also has an object of providing a method for processing the same.
Further, the present invention has an object of providing a new apparatus of process for fabricating semiconductor device having a wafer holding means with the holding face prepared so as to improve the yield and productivity of device-chip fabricating processes.
The present invention is directed to a semiconductor wafer having the roughness of the backside surface varied in a direction of a radius, wherein varied sections exist substantially coaxially in the direction of the radius or sections of the different roughness exist at least in a peripheral part of the wafer and in the arbitrary sections inner than the periphery.
According to the present invention, when a conventional wafer of uniform backside roughness is not sufficient for the control of a device fabrication process, the problems that have been difficult to solve by the conventional method can be coped with using the new wafer having the roughness of the backside surface varied in the direction of the radius.
Further according to the present invention, the variation of the roughness of the backside surface in the direction of the radius, that is the roughness variation, may be continuous in the direction of the radius or stepwise so that the roughness varies with each approximately designated annular width in the radius direction, t
Demizu Kiyoshi
Kato Tadahiro
Netsu Shigeyoshi
Shin-Etsu Handotai & Co., Ltd.
Tran Thien F
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