The present invention relates to an ion implanter operating in environments under sub-zero temperature, and more particularly, to an ion implanter that effectively thermally insulates the wafer and the chuck from other portions of the ion implanter.
Generally, an ion implanter includes an ion source placed inside an ion source chamber. The ion source chamber connects to an extraction voltage source for extracting ions for projecting an ion beam to a beam analyzer provided with an analyzer magnet. The beam analyzer adjusts the ion beam and may project the ion beam through a plasma shower for carrying out a beam neutralization process. The ion beam then reaches a target wafer that is moved in and out such that the target wafer can be scanned by the ion beam.
During ion implantation for the making of semiconductor devices, keeping the wafer at a low temperature is advantageous for the reduction of implant damage, avoidance of a self-annealing effect, and so on. Semiconductor devices implanted at low temperatures have features of low current leakage, low parasitic capacitance, and low parasitic resistance, and consequently have high reliability and low distortion output signals even after long-term operation at high temperature.
Generally, a chuck is employed to clamp and hold a wafer during ion implantation. To keep wafers at low temperatures during ion implantation, the chuck typically is connected to a water chiller. Therein, a gas also may be applied to the backside of the wafer to bring heat away.
With the critical dimension of semiconductor devices continuously decreasing, especially as ultra shallow junctions become more important, low temperature ion implantations in which wafer temperature is held below 0° C. during ion implantation becomes more necessary. Such requirements call for a detailed heat transfer analysis of the wafer to chuck assembly, because any heat source may transfer heat to the wafer thus causing the wafer temperature to be raised.
During implantation, ion beams are absorbed in the top surface of the wafer causing its temperature to rise. To maintain the wafer temperature at an acceptable range, heat caused by the ion beams must ultimately be transferred from the surface of the wafer to the coolant circulating within the chuck (the heat also can be transferred to the gas for carrying heat away). The heat transfer follows the following path sequence: 1) through the wafer; 2) across the wafer/chuck 20 interface; and 3) through a portion of the chuck 20 to the water jacket inside the chuck 20.
The largest thermal resistance in the heat transfer path is typically the wafer/chuck interface, this being a reason to use the gas to bring heat away from here. Moreover, the majority of heat is transferred out via the cooling water, but a small amount of heat is also lost to the base 22 and even to other portions of the ion implanter connected to the base 22. Essentially, the slight rise in base temperature that may occur is non-problematical. Therefore, in the prior art, no special precautions are taken to isolate the chuck from the base.
However, for the requirement of maintaining the wafer temperature below 0° C., the above prior art is not without problems. First, the cooling technique described above suffers in that the DI water may freeze at atmospheric pressure. In other words, the conventional technique could not properly operate under 0° C. Second, there is no precaution to thermally isolate the chuck from the base. Hence, heat may be transferred from the base to the wafer through the chuck, causing the wafer temperature to be raised.
Therefore, it would be advantageous to provide an apparatus for sub-zero degree C. ion implantation to improve the deficiencies of the prior art.
The present invention provides an apparatus for sub-zero degree C. temperature maintenance of a chuck assembly or an ion implanter during ion implantation, to overcome the deficiencies of the prior art.
The invention provides a chuck assembly comprising a chuck capable of holding a wafer, a base capable of connecting to a portion of an implanter, and at least one fastener that mounts the chuck on the base, such that the chuck and the base have no direct surface-to-surface contact.
The invention also provides an ion implanter comprising an ion beam generation assembly for generating an ion beam and a chuck assembly, where the chuck assembly comprises a chuck capable of holding a wafer to be implanted by the ion beam, a base capable of connecting to a portion of the ion implanter, and at least one fastener that mounts the chuck on the base, such that the chuck and the base have no direct surface-to-surface contact.
The detailed description of the present invention will be discussed in the following embodiments, which are not intended to limit the scope of the present invention and which can be adapted for other applications. While the drawings are illustrated in detail, it is appreciated that the quantity of the disclosed components may be greater or less than that disclosed except where expressly restricted. Wherever possible, the same or similar reference numbers are used in the drawings and description to refer to the same or like parts. It should be noted that any drawing presented is in simplified form and is not to precise scale. In reference to the disclosure herein, for purposes of convenience and clarity only, directional terms are used with respect to the accompanying drawings and should not be construed to limit the scope of the invention in any manner.
The present invention discloses an ion implanter that comprises an ion beam generation assembly to generate an ion beam and a chuck assembly to clamp, hold, and cool a wafer during implantation. Certain details of the ion beam generation assembly are not included. It is to be understood that any available or commercial product can be used in any combination according to the knowledge of one skilled in the art, and detailed drawings and descriptions of such items are thus omitted. The ion beam generation assembly comprises an ion source, an extraction voltage, and an analyzer magnet.
One approach of the invention is to change the mounting between the chuck and the base to improve the above deficiencies of the prior art. Herein, both the chuck and the base are not obviously changed, and then both the conventional chuck and the conventional base still can be used. The change is focused on how to minimize the direct surface-to-surface contact between the chuck and the base. In other words, it is focused on the interface between the chuck and the base.
According to one embodiment, as exemplified by the chuck assembly 30 shown in
Significantly, when the thermal conduction through the fastener 36 can be remarkably reduced, the efficiency of the approach can be remarkably enhanced. Accordingly, as with the example shown in
Provision of thermal insulation between the chuck 42 and the base 44 can be achieved in several ways. Referring to
Besides the fasteners, the only other mechanical contacts between the platen (e.g., chuck) and the base are the coolant passageways and the wafer cooling gas passageway. Hence, in one embodiment, there is at least one o-ring for sealing the coolant passageway and/or the cooling gas passageway. Therefore, the only physical contact made with the chuck is through a collectively small area of fasteners, insulators and o-rings, corresponding to a conduction heat transfer that is quite small. Moreover, the effect of heat radiation is also quite small, and the heat transfer by radiation from the chuck to the base and other surroundings can be ignored.
In addition to the above, the invention further includes another approach. With reference to
According to an aspect of the present invention, design changes to this new cooling assembly can be provided for operation with this fluid at sub-zero degree C. temperatures. For example, part or all flexible tubing can be changed to be chemically compatible, reliable for sealing using standard fittings and/or extremely flexible at a predetermined temperature or temperature range. For example, all fittings can be changed to compression-type fittings with ferrules or externally clamped barbs. Moreover, as another example, all tubing and fittings can be covered with thermal insulation to prevent condensation on their outside surfaces. As a further example, all cold areas not covered by insulation can be enclosed and purged with dry purge gas, such as dry nitrogen or other very low dew point air, to prevent condensation. In one exemplary embodiment, all o-ring seals subject to contact with the fluid are changed to Ethylene Propylene Diene Monomer (EPDM), and/or all flow sensors are Proteus® flow sensors compatible with the coolant (Fluorinert FC-3283) and temperatures and calibrated specifically for the coolant at the expected operating temperature (sub-zero degree C.). Other features and components of the disclosed cooling assembly 34 can be the same as the SMC HRZ001-L-Z Thermo chiller, so that the detail description is omitted.
Although specific embodiments have been illustrated and described, it can be appreciated by those skilled in the art that various modifications may be made without departing from the scope and spirit of the present invention, which is intended to be limited solely by the appended claims.