Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into...
Utility Patent
2000-01-14
2001-01-02
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
C438S514000, C250S492210
Utility Patent
active
06169015
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to equipment for manufacturing semiconductor devices, and deals more particularly with a method and apparatus for controlling the dosage of ions implanted into a semiconductor wafer during manufacturing of a semiconductor device.
BACKGROUND OF THE INVENTION
Ion implantation has become a standard, commercially accepted technique for introducing conductivity-altering dopants into a workpiece, such as a semiconductor wafer, in a controlled and rapid manner. Conventional ion implantation machines include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. This beam is directed at the surface of the workpiece. Typically, the energetic ions in the beam penetrate into the bulk of the workpiece and are embedded into the crystalline lattice of the material to form a region of desired conductivity. This ion implantation process is typically performed in a high vacuum, gas tight process chamber which encloses a wafer handling assembly and the ion source. This high vacuum environment prevents dispersion of the ion beam by collisions with gas molecules and also minimizes the risk of contamination of the workpiece by air-borne particulates.
The ion beam path in typical ion implantation systems includes an ion source, electrodes, an analyzing magnet arrangement, an optically resolving element, and a wafer processing system. The electrodes extract and accelerate ions generated in the ion source produce a beam that is directed toward the analyzing magnet arrangement. The analyzing magnet arrangement sorts the ions in the ion beam according to their charged-to-mass ratio, and the water processing system adjusts the position of the workpiece along two axes relative to the ion beam path.
In particular, as each individual ion leaves the electrodes and enters the analyzing magnets, its line of flight is bent into a path having a radius proportional to the square root of the mass of the ion. A resolving slit in the analyzing magnet arrangement, in conjunction with the optical resolving element, focus ions having a pre-selected charge-to-mass ratio so that the ions are directed toward the workpiece. Ions not having the selected charge-to-mass ratio are focused either to the left or to the right of the resolving slit and are thereby selected out of the final ion beam striking the target work piece.
The selected ions exiting the analyzing magnet and optical resolving element arrangement are then generally moved across the workpiece in a controlled manner to spread the particles across the workpiece. In doping semiconductor wafers, a common technique is to move the wafers relative to a fixed beam of selected ions along two orthogonal directions. The wafers are supported on a moving surface, which moves them at a high speed along a scanning direction and at a slower speed along an orthogonal direction. Using a slightly different technique, the scanning process is performed by positioning a multiple number of the wafers around the circumference of a rotating disk. The disk is rotated so that all of the semiconductor wafer are scanned at a high speed, with the ion beam being moved at a comparatively low speed in the radial direction of the rotating disk so that the individual semiconductor wafers are scanned at a low speed.
The quantity of ions implanted into a semiconductor wafer is sometimes referred to as the ion “dose” remaining in the wafer after the implantation process. It is extremely important to maintain control over the amount and uniformity of the implanted dose, from wafer-to-wafer, and from batch-to-batch. Because of the relatively large number of ions implanted into the wafer during each scan, even a single extra scan beyond a pre-computed scan number can result in an overdose, causing the wafer to be scrapped. In calculating a prescribed, “correct” dosage level, it is important to determine the current of the ion beam, in addition to determining the scanning velocity and the number of scans. The current level of ion beams are notoriously unstable, particularly at higher levels of energy, e.g. 80 KeV to 150 KeV. At lower operating currents, scanning may be conducted for single wafers for only 10 seconds, whereas at higher current levels where a greater dosage is desired, scanning may be conducted from 1½ minutes up to 40 minutes.
Typical ion beam implantation systems include a dose controller that allows an operator to pre-select a number of parameters that determine the dosage to be applied for a particular application. For example, based on empirical data and control readouts, the operator can use the dose controller to predict the proper amount of ion beam current that needs to be selected for a given scan rate and number of scans to achieve a desired dosage. While scanning speed and the number of scans can be controlled with some degree of precision, the current level of the ion beam is difficult to control, and thus is not always predictable. Much of the instability of ion beam current is related to fluctuations in the level of power supplied to the ion implantation machine. Since this same power source is also used to supply power to the dose controller, error in predicting values sometimes occurs.
Accordingly, there is clear need in the art for an improved method and apparatus for controlling the dose of ions implanted into a semiconductor wafer that eliminates error, and consequent over-dosage resulting from fluctuations in the ion beam current. The present invention is directed towards satisfying this need.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the present invention, a method and apparatus is provided for controlling the dosage of ions implanted into a substrate, such as a semiconductor wafer. An interlock controller device is used in combination with a dose controller to calculate the correct number of scans at a particular ion beam current and to maintain a count-down of the pre-selected scan number. When the count-down reaches 0 the interlock device switches off current to the ion beam, thus terminating ion implantation, even though actual scanning of the wafers continues.
According to one aspect of the invention, apparatus is provided for controlling the dose of ions implanted into a semiconductor wafer using an ion implantation device of the type including mean of generating a beam of ions, means for causing the ion beam to scan the wafer and means for sensing each scan of the beam. The apparatus comprises means for counting the number of times the wafer is scanned by the beam and for outputting a shut-off control signal to the beam generating means when the counted scans reaches a pre-selected number. The beam generating means is responsive to the shut-off signal to shut off electrical current supplied to the beam. The counting means preferably includes a register for storing a pre-calculated count related to the desired number of times the beam is to scan the wafer, and means for decrementing the count stored in the register each time the sensing means senses that the beam has scanned the wafer. The apparatus desirably employs opto-isolators for coupling the interlocked controller device with other elements in the system in order to isolate the interlock device against the adverse effects of the high voltage components of the ion implantation machine. The interlock controller device is preferably powered by an uninterruptable power supply that is separate from and independent of the power supply used to power the ion beam.
According to another aspect of the invention, a method is provided for controlling the dose of ions implanted into a semiconductor wafer by a scanning ion beam, comprising the steps of: determining the number of ion beams scans necessary to implant a desired dose of ions into the wafer; storing a count related to the number of determined scans; enabling the beam to scan the wafer and implant ions into the wafer; sensing each scan of the wafer by the beam; decrementing the stored count each time a scan is
Malsawma Lex. H.
Smith Matthew
Taiwan Semiconductor Manufacturing Company Ltd
Tung & Associates
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