Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2000-03-02
2001-08-21
Huff, Mark F. (Department: 1756)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298190, C204S298200, C204S298210, C204S298220, C204S298160, C204S298170, C204S298180, C204S298120, C204S192170, C204S192150
Reexamination Certificate
active
06277249
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to a plasma sputter reactor. In particular, the invention relates to the sputter target and associated magnetron used in a sputter reactor and to an integrated via filling process using sputtering.
BACKGROUND ART
A semiconductor integrated circuit contains many layers of different materials usually classified according to whether the layer is a semiconductor, a dielectric (electrical insulator) or metal. However, some materials such as barrier materials, for example, TiN, are not so easily classified. The two principal current means of depositing metals and barrier materials are sputtering, also referred to as physical vapor deposition (PVD), and chemical vapor deposition (CVD). Of the two, sputtering has the inherent advantages of low cost source material and high deposition rates. However, sputtering has an inherent disadvantage when a material needs to filled into a deep narrow hole, that is, one having a high aspect ratio, or coated onto the sides of the hole, which is often required for barrier materials. Aspect ratios of 3:1 present challenges, 5:1 becomes difficult, 8:1 is becoming a requirement, and 10:1 and greater are expected in the future. Sputtering itself is fundamentally a nearly isotropic process producing ballistic sputter particles which do not easily reach the bottom of deep narrow holes. On the other hand, CVD tends to be a conformal process equally effective at the bottom of holes and on exposed top planar surfaces.
Up until the recent past, aluminum has been the metal of choice for the metallization used in horizontal interconnects. Vias extending between two levels of copper can also be formed of copper. Contacts to the underlying silicon present a larger problem, but may still be accomplished with copper. Copper interconnects are used to reduce signal delay in advanced ULSI circuits. Due to continued downward scaling of the critical dimensions of microcircuits, critical electrical parameters of integrated circuits, such as contact and via resistances, have become more difficult to achieve. In addition, due to the smaller dimensions, the aspect ratios of inter-metal features such as contacts and vias are also increasing. An advantage of copper is that it may be quickly and inexpensively deposited by electrochemical processes, such as electroplating. However, sputtering or possibly CVD of thin copper layers onto the walls of via holes is still considered necessary to act as an electrode for electroplating and as a seed layer for the electroplated copper. The discussion of copper processes will be delayed until later.
The conventional sputter reactor has a planar target in parallel opposition to the wafer being sputter deposited. A negative DC voltage is applied to the target sufficient to ionize the argon working gas into a plasma. The positive argon ions are attracted to the negatively charged target with sufficient energy to sputter atoms of the target material. Some of the sputtered atoms strike the wafer and form a sputter coating thereon. Most usually a magnetron is positioned in back of the target to create a magnetic field adjacent to the target. The magnetic field traps electrons, and, to maintain charge neutrality in the plasma, the ion density also increases. As a result, the plasma density and sputter rate are increased. The conventional magnetron generates a magnetic field principally lying parallel to the target.
Much effort has been expended to allow sputtering to effectively coat metals and barrier materials deep into narrow holes. High-density plasma (HDP) sputtering has been developed in which the argon working gas is excited into a high-density plasma, which is defined as a plasma having an ionization density of at least 10
11
cm
−3
across the entire space the plasma fills except the plasma sheath. Typically, an HDP sputter reactor uses an RF power source connected to an inductive coil adjacent to the plasma region to generate the high-density plasma. The high argon ion density causes a significant fraction of sputtered atoms to be ionized. If the pedestal electrode supporting the wafer being sputter coated is negatively electrically biased, the ionized sputter particles are accelerated toward the wafer to form a directional beam that reaches deeply into narrow holes.
HDP sputter reactors, however, have disadvantages. They involve a relatively new technology and are relatively expensive. Furthermore, the quality of the sputtered films they produce is often not the best, typically having an undulatory surface. Also, high-energy ions, possibly the argon ions attracted as well to the wafer, tend to damage the material already deposited.
Another sputtering technology, referred to as self-ionized plasma (SIP) sputtering, has been developed to fill deep holes. See, for example, U.S. patent application Ser. No. 09/373,097 filed Aug. 12, 1999 by Fu and U.S. patent application Ser. No. 09/414,614 filed Oct. 8, 1999 by Chiang et al. Both of these patent applications are incorporated by reference in their entireties. In its original implementations, SIP relies upon a somewhat standard capacitively coupled plasma sputter reactor having a planar target in parallel opposition to the wafer being sputter coated and a magnetron positioned in back of the target to increase the plasma density and hence the sputtering rate. The SIP technology, however, is characterized by a high target power density, a small magnetron, and a magnetron having an outer magnetic pole piece enclosing an inner magnetic pole piece with the outer pole piece having a significantly higher total magnetic flux than the inner pole piece. In some implementations, the target is separated from the wafer by a large distance to effect long-throw sputtering, which enhances collimated sputtering. The asymmetric magnetic pole pieces causes the magnetic field to have a significant vertical component extending far towards the wafer, thus enhancing and extending the high-density plasma volume and promoting transport of ionized sputter particles.
The SIP technology was originally developed for sustained self-sputtering (SSS) in which a sufficiently high number of sputter particles are ionized that they may be used to further sputter the target and no argon working gas is required. Of the metals commonly used in semiconductor fabrication, only copper is susceptible to SSS resulting from its high self-sputtering yield.
The extremely low pressures and relatively high ionization fractions associated with SSS are advantageous for filling deep holes with copper. However, it was quickly realized that the SIP technology could be advantageously applied to the sputtering of aluminum and other metals and even to copper sputtering at moderate pressures. SIP sputtering produces high quality films exhibiting high hole filling factors regardless of the material being sputtered.
Nonetheless, SIP has some disadvantages. The small area of the magnetron requires circumferential scanning of the magnetron in a rotary motion at the back of the target. Even with rotary scanning, radial uniformity is difficult to achieve. Furthermore, very high target powers have been required in the previously known versions of SIP. High-capacity power supplies are expensive and necessitate complicated target cooling. Lastly, known versions of SIP tend to produce a relatively low ionization fraction of sputter particles, for example, 20%. The non-ionized fraction has a relatively isotropic distribution rather than forming a forward directed beam as the ionized particles are accelerated toward a biased wafer. Also, the target diameter is typically only slightly greater than the wafer diameter. As a result, those holes being coated located at the edge of the target have radially outer sidewalls which see a larger fraction of the wafer and are more heavily coated than the radially inner sidewalls. Therefore, the sidewalls of the edge holes are asymmetrically coated.
Other sputter geometries have been developed which increase the ionization density. One example is a multi-pole h
Athreya Sankaram
Chen Fusen
Dixit Girish
Fu Jianming
Gopalraja Praburam
Applied Materials Inc.
Chacko-Davis Daborah
Guenzer Charles S.
Huff Mark F.
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