The invention relates generally to sputtering of materials. In particular, the invention relates to sputtering apparatus and a method capable of producing a high fraction of ionized sputter particles.
Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. In particular, the sputtered metals are used in forming the many layers of electrical interconnects required in advanced integrated circuits. However, advanced integrated circuit structures have via holes connecting two layers of metallization and formed through an intermediate dielectric layer. These via holes tend to be narrow and deep with aspect ratios of 5:1 and greater in advanced circuits. Coating the bottom and sides of these holes by sputtering is difficult because sputtering is fundamentally a ballistic and generally isotropic process in which the bottom of a via hole is shielded from most of an isotropic sputtering flux.
It has been long recognized, however, that if a large fraction of the sputtered particles are ionized, the positively charged sputtered ions can be accelerated towards a negatively charged wafer and reach deep into high aspect-ratio holes.
This approach has long been exploited in high-density plasma sputter reactors in which the ionization density of the sputtering working gas, typically argon, is increased to a high level by, for example, using inductive RF coils to create a remote plasma source. As a result of the high-density plasma, a large fraction of the sputtered metal atoms passing through the argon plasma are ionized and thus can be electrically attracted to the biased wafer support. However, the argon pressure needs to be maintained relatively high, and the argon ions are also attracted to the wafer, resulting in a hot process. The sputtered films produced by this method are not always of the best quality.
A recently developed technology of self-ionized plasma (SIP) sputtering allows plasma sputtering reactors to be only slightly modified but to nonetheless achieve efficient filling of metals into high aspect-ratio holes in a low-pressure, low-temperature process. This technology has been described by Fu et al. in U.S. patent application Ser. No. 09/546,798, filed Apr. 11, 2000, now issued as U.S. Pat. No. 6,306,265, and by Chiang et al. in U.S. patent application Ser. No. 09/414,614, filed Oct. 8, 1999, both incorporated herein by reference in their entireties. An earlier form of the former reference has been published as PCT publication WO 00/48226 on Aug. 17, 2000.
The SIP sputter reactor employs a variety of modifications to a fairly conventional capacitively coupled magnetron sputter reactor to generate a high-density plasma adjacent to the target and to extend the plasma and guide the metal ions toward the wafer. Relatively high amounts of DC power are applied to the target, for example, 20 to 40 kW for a chamber designed for 200 mm wafers. Furthermore, the magnetron has a relatively small area so that the target power is concentrated in the smaller area of the magnetron, thus increasing the power density supplied to the HDP region adjacent the magnetron. The small-area magnetron is disposed to a side of a center of the target and is rotated about the center to provide more uniform sputtering and deposition.
In one type of SIP sputtering, the magnetron has unbalanced poles, usually a strong outer pole of one magnetic polarity surrounding a weaker inner pole. The total magnetic flux integrated over the area of the outer pole is at least 150% of that of the inner pole. The magnetic field lines emanating from the stronger pole may be decomposed into not only a conventional horizontal magnetic field adjacent the target face but also a vertical magnetic field extending toward the wafer. The vertical field lines extend the plasma closer toward the wafer and also guide the metal ions toward the wafer. Furthermore, the vertical magnetic lines close to the chamber walls act to block the diffusion of electrons from the plasma to the grounded shields. The reduced electron loss is particularly effective at increasing the plasma density and extending the plasma across the processing space.
Gopalraja et al. disclose another type of SIP sputtering, called SIP+ sputtering, in U.S. patent application Ser. No. 09/518,180, filed Mar. 2, 2000now U.S. Pat. No. 6,277,249, also incorporated herein by reference in its entirety. SIP+ sputtering relies upon a target having a shape with an annular vault facing the wafer. Magnets of opposed polarities disposed behind the facing sidewalls of the vault produce a high-density plasma in the vault. The magnets usually have a small circumferential extent along the vault sidewalls and are rotated about the target center to provide uniform sputtering. Although some of the designs use asymmetrically sized magnets, the magnetic field is mostly confined to the volume of the vault.
SIP sputtering may be accomplished without the use of RF inductive coils. The small HDP region produced by a small-area SIP magnetron is sufficient to ionize a substantial fraction of metal ions, estimated to be between 10 and 25%, which is sufficient to reach into deep holes. Particularly at the high ionization fraction, the ionized sputtered metal atoms are attracted back to the targets and sputter yet further metal atoms. As a result, the argon working pressure may be reduced without the plasma collapsing. Therefore, argon heating of the wafer is less of a problem, and there is reduced likelihood of the metal ions colliding with argon atoms, which would both reduce the ion density and randomize the metal ion sputtering pattern.
However, SIP sputtering could still be improved. The ionization fraction is only moderately high. The remaining 75 to 90% of the sputtered metal atoms are neutral and not subject to acceleration toward the biased wafer. This generally isotropic neutral flux does not easily enter high-aspect ratio holes. Furthermore, the neutral flux produces a non-uniform thickness between the center and the edge of the wafer since the center is subjected to deposition from a larger area of the target than does the edge when accounting for the wider neutral flux pattern. Further increases in target power would increase the ionization levels. However, large power supplies become increasingly costly, and this problem will be exacerbated for 300 mm wafers. Also, increases in power applied to the target requires increased target cooling if the target is not to melt. For these reasons, it is desired to limit the average power applied to sputtering targets.
Short-pulse sputtering is an alternative approach to producing a high metal ionization fraction in a low-pressure chamber, as described by Kouznetsov et al. in xe2x80x9cA novel pulsed magnetron sputter technique utilizing very high target power densities,xe2x80x9d Surface and Coating Technology, vol. 122, 1999, pp. 290-293. This techniques apparently uses a stationary magnetron with 50 to 100 xcexcs pulses of DC power applied to the target with a repetition rate of about 50 Hz, that is, a target power duty cycle of less than 1%. As a result, a relatively modestly sized pulsed DC power supply having an average power capability of the order of tens of kilowatts can deliver peak power of up to 2.4 MW. Kouznetsov et al. have shown effective hole filling with a peak power density of 2.8 kW/cm2. However, the favorable results shown by Kouznetsov et al. have apparently been accomplished with a target having a diameter of 150 mm. Such a target size is adequate for 100 mm wafers, but considerably smaller than the size required for 200 mm or 300 mm wafers. When the power supplies are scaled up for the larger area targets required for the larger wafers now of commercial interest, again the size of the power supply becomes an issue. Switching of large amounts of power is both costly and operationally disadvantageous.
A pulsed magnetron sputter reactor in which a small magnetron is rotated about the back of a target and DC power is delivered to the target in short pulses having duty cycles of less than 10%, preferably less than 1%. Thereby, a high plasma density is achieved adjacent to the magnetron during the pulse. The rotation waveform and the pulse waveform should be desynchronized.
In one variation, the pulses rise from a DC level sufficient to maintain the plasma in the reactor between pulses. The pulses preferably have a power level at least 10 times the DC level, more preferably 100 times, and most preferably 1000 times for the greatest effect of the invention.
The level of metal ionization can be controlled by varying the peak pulse power. In the case that pulsed power supply is limited by the total pulse energy, the peak pulse power can be controlled by varying the peak pulse width. In a multi-step sputtering process, the pulse width is changed between the steps.