Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-05-24
2003-09-23
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192120, C204S298030, C204S298230, C204S298290
Reexamination Certificate
active
06623606
ABSTRACT:
FIELD OF INVENTION
This invention relates to cathode sputter deposition and more particularly to a method and apparatus for overcoming the adverse effects on deposited thin film uniformity from wafer to wafer due to changes in sputtering target geometry as a result of the utilization of target material.
BACKGROUND OF THE INVENTION
In sputter deposition processes, substrates are placed in a processing chamber adjacent to a sputtering cathode target, which serves as a source of coating material. The pressure in the processing chamber, which is usually filled with an inert gas such as argon, is then reduced to a near vacuum, and a negative voltage is applied to the target. The negatively charged target emits electrons, which strike and ionize atoms of the gas to produce a plasma discharge. Often the plasma is intensified and confined over the target surface by the application of a magnetic field generated by magnets, which are usually placed behind or around the periphery of the target. The large quantities of positive ions from the plasma that are produced in the sparse gas within the chamber are attracted to the negatively charged target, bombarding its surface and thereby dislodging atoms or small particles of the material of which the target is made from the surface of the target. The atoms or particles move across the space in front of the target until they strike the surface such as the surface of a semiconductor wafer or other substrate disposed, for example, in a plane parallel to the surface of the target, where they adhere to the substrate surface and form a thin film or coating layer thereon.
A primary consideration in designing a sputter deposition process has long been to achieve a specified degree of uniformity in the thickness of the resultant film being deposited on the substrate. In semiconductor wafer manufacturing processes, for example, such uniformities in the area of +/−2 to 5 percent or better are currently being demanded. Factors that influence the degree of uniformity achieved in sputter deposition include the relative sizes of the target and substrates, the configurations of field producing magnets and other factors controlling the utilization or erosion profile of the target and the sputtering target to substrate spacing.
In the prior art, the factors of target to substrate size ratio, magnet design to control target erosion profile and target to substrate spacing are designed into the sputtering target and cathode assembly of the sputtering apparatus in an effort to produce the required film thickness uniformity. For a given target material, and with other process conditions being held constant, cathode assembly design has provided an ability to deposit films to some degree of the desired uniformities with targets of limited thickness, where the erosion of the target surface over the life of the target cannot materially alter the target to substrate spacing that was the basis for the system design. With such constant geometries, those skilled in the art of sputtering system design have concentrated on the control of erosion profiles, for example by altering magnet configuration, to fine tune the cathode design to achieve the desired film uniformity.
Typical prior art semiconductor wafer sputter deposition systems have employed targets of, for example ten inches (250 mm) in diameter to apply coatings to six inch (150 mm) diameter wafers. With such applications, uniformity in film thickness was approached by configuring sputtering cathode magnets to produce a greater sputtering rate around the periphery of the target, usually outside of the six inch diameter of the wafer, to simulate the incidence of sputtered material onto the substrate from the remote regions of a sputtering target of infinite diameter, which in theory would produce a equal incidence of sputtered material on every increment of the surface of the substrate. The increased sputtering rate around the periphery of the target compensates for target size limitations and increases the uniformity of the deposited film.
Theoretically also, with the target of infinite diameter, uniformity of the deposited coating is not generally affected by target to substrate spacing, at least not by spacing variations of thirty to fifty percent where other effects, not necessary to consider here, would not be factors. However, with finite targets, increased sputtering around a peripheral area of the target causes a more deeply eroded peripheral area or annular groove to form around the rim of the target. As the target erodes, the target surface recedes from the substrate, and does so faster at the target rim than at the center of the target. The target to substrate spacing change produces substantial changes in the rate at which material is deposited in the vicinity of the rim of the substrate. However, at the center of the substrate, the deposition rate is much less affected by such changes.
With a ten inch target used for coating six inch wafers, targets having thicknesses of from one-sixteenth inch to one and one-half inches are commonly found. Typically, target-to-substrate spacing with such targets may be approximately two inches. With the thin target, the target-to-substrate spacing change experienced over the life of the target will be at most about three percent, which should have a negligible effect on the deposition uniformity on the substrate. With the thicker targets, however, the erosion of a peripheral groove can result in an increase in target-to-substrate spacing, at certain points on the target, by more than seventy percent. Such changes can result in substantial decreases in the deposition rates on the substrate, particularly in the vicinity of the substrate rim. Thus, cathodes designed to produce a desired coating uniformity on wafers early in the life of the target do not coat wafers with sufficiently uniform films late in the life of the target.
Some prior art systems have been proposed in which the deposition rate “roll-off” or decrease over the life of the sputtering target due to the progressive erosion of the target is offset by an increase in sputtering power. Such increases in many such systems have a uniform compensating effect across the surface of the target. Thus, where the erosion rate roll-off is usually greater at the peripheral groove on the target than at the target center, the uniformity of the coating changes along with the reduction in deposition rate as the target erodes, while the loss of uniformity is retained as the sputtering power is increased. Some systems have disproportionately increased sputtering power around the peripheral groove. While such adjustment is possible where stationary electromagnets are used, in those sputtering systems where rotating permanent magnets are desired, magnet field compensation for non-uniform deposition rate roll-off is less practical and less effective. This increases cathode assembly complexity, and is difficult with one piece sputtering targets, and tends to even more greatly increase the erosion rate around the rim of the target in proportion to the area in the target center. Such schemes of compensating for erosion, however, also have effects on voltage levels, component heating and plasma shaping that have other often adverse, undesirable or troublesome effects.
Accordingly, there is a need for an effective and efficient method and apparatus for maintaining high degrees of deposited film uniformity in a sputter coating process, particularly where thick targets are employed which substantially are substantially eroded over their useful lives.
SUMMARY OF THE INVENTION
It is a primary objective of the present invention to maintain a desired sputtered film thickness distribution throughout the life of a sputtering target. A more particular objective of the present invention is to overcome changes in deposited film uniformity caused by changes in the geometry of the surface of a sputtering target as the target erodes. A specific objective of the present invention is to provide a method and apparatus for maintaining the u
Hurwitt Steven
Wagner Israel
McDonald Rodney G.
Tokyo Electron Limited of IBS Broadcast Center
Wood Herron & Evans L.L.P.
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