Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2002-10-15
2004-03-30
Pert, Evan (Department: 2829)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C438S678000, C118S058000, C118S603000, C134S028000
Reexamination Certificate
active
06713122
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods and apparatus for electroless plating on substrates. More specifically, the invention relates to improved methods for controlling heat load and airflow during electroless plating in order to better control bath chemistry and plating uniformity.
BACKGROUND OF THE INVENTION
The Damascene process provides inlaid copper lines in dielectric layers of integrated circuits. The copper lines provide electrical routing (metal interconnects) between circuit elements in the integrated circuit. Damascene copper lines are rapidly replacing traditional aluminum etched lines in high-performance integrated circuitry.
Currently, a preferred method of metal-interconnect layer deposition is electroplating. This is in part due to the success of “bottom-up” copper filling methods for damascene features. The process typically involves formation of a barrier layer (typically composed of Ta, TiN or TiSiN) and a seed layer (typically copper) over the wafer, followed by plating the wafer to fill embedded structures from the bottom-up. A number of problems occur when trying to accomplish this task. Such problems include: corrosion of the seed layer and associated reactions in the plating bath, poor structures (morphology) of PVD-deposited seed layers, non-uniform deposition of the metal over and into features, and shrinking of feature volume (and associated increase in aspect ratio) when seeded.
As features become smaller, the seed layer must become thinner (otherwise the feature will be closed off by the generally non-conformal PVD seeding process). However, most useful electroplating tool designs require supplying current to the wafer from the wafer's outer edge via the seed layer. When attempting to electroplate using ever thinner seed layers for supplying plating current, the current distribution becomes increasingly dominated by the resistance in the seed layer. This phenomenon is commonly referred to as the “terminal effect.”
Thus, the need exists to find methods of depositing seed layers in a more conformal manner. This is because conformal seed layers reduce resistance by providing a greater average thickness in comparison non-conformal layers deposited by PVD, for example. As a result, the terminal effect is mitigated during electroplating.
Electroless plating can provide highly conformal seed layers. And in some cases, electroless plating can replace not only PVD seed deposition, but electroplating as well, thereby dispensing with the need for a plating current and a seed layer. This, of course, circumvents the problems of the terminal effect and poor seed layer step coverage.
There are however problems associated with conventional electroless plating methods and apparatus. Electroless plating baths are typically unstable, undergoing varying degrees of homogeneous and heterogeneous plating (depending on such things as bath composition, purity, and temperature). Additionally, decomposition of electroless plating bath reagents can contaminate the plating process. This decomposition is related to the unstable nature of electroless bath reagents when exposed to heat and air, for example.
The problems of bath instability due to heating are well known. For example, Ang (U.S. Pat. Nos. 6,093,451 and 5,938,845) describe an electroless bath heating apparatus designed to uniformly heat an electroless bath for improved plating uniformity. The problem of bath decomposition onto vessel walls was discussed by Cardin et al. (U.S. Pat. No. 4,674,440). They describe a bath design that allows the introduction of a plating bath poison near the bath wall.
Bath degradation and plating uniformity problems are also described in U.S. Pat. No. 6,065,424 by Shaeham-Diamond et al. They describe a spray electroless apparatus and process. In an early patent by the same authors (U.S. Pat. No. 5,830,805) a single sealed chamber is used to perform a number of electroless deposition spray related steps. In both of these patents, mixing the unstable chemicals just prior to use is said to mitigate instability (point of use mixing). However, problems associated with mixing unstable chemicals and spraying them on a wafer are: 1) high capital cost of precision flow mixing and in line (high rate) heating and control, 2) large volume of expensive chemical used in the spraying process, 3) inability of the thermal capacity of the fluid in the spraying process to rapidly or efficiently heat the wafer (which is initially at ambient temperature) to the electroless plating temperature, and 4) metal, formed from the electroless deposition or sprayed solution on the walls of the chamber, produces numerable sources of in film defects. It is necessary to keep the wafer constantly wet during the spinning and/or spraying process. This requires an excess of plating fluid per wafer that is not recovered, and such methods are therefore wasteful. Also, the wafer itself has a substantial thermal heat capacity. Though the wafer surface is being heated by hot spraying chemical, heat is being removed from the surface and being absorbed by the wafer. The process therefore undergoes a temperature transient that is difficult to control, and slows the overall plating rate and thereby reduces throughput.
Along with heat load on an electroless plating formulation, there is decomposition due to air exposure. This problem has two aspects. First, exposure of electroless plating formulations to air can degrade certain components by oxidation. Second, evaporation of bath components cools the bath and requires additional heating, which can accelerate decomposition. Evaporation is a particular problem because most processes will heat their electroless plating fluids to drive plating reactions. Exposure to air cools the electroless plating fluid, due to evaporation, and thus the fluid needs to be heated above desirable temperatures in order to compensate for the evaporative cooling. This additional heating only speeds up the decomposition pathways.
General process requirements for wafer plating include global and local plating uniformity, defect free process, and high throughput. In order realize high-volume manufacturing (e.g. damascene copper processing) meeting these requirements, electroless plating processes must overcome the bath instability issues. In this respect, electroless bath stability can impact both cost of ownership and defect formation. Therefore methods and apparatus that minimize heat and air exposure to the bath are needed. Methods and hardware design that allow improved bath stability, independent of bath composition, are required.
What is therefore needed are improved apparatus and methods for controlling heat load and airflow to which electroless plating fluids are exposed during electroless plating in order to better control bath chemistry, particularly decomposition pathways, and thereby improve plating uniformity and throughput.
SUMMARY OF THE INVENTION
Methods and apparatus for reducing the heat load and air exposure to an electroless plating fluid during a plating process are presented. An electroless plating apparatus, including an electroless plating vessel and recirculation systems, is presented. The electroless plating vessel minimizes air exposure (and thus evaporative cooling and degradation) of the electroless plating fluid while the recirculation systems minimize heat load of the electroless plating fluid.
One aspect of the invention is an electroless plating apparatus for reducing loss of electroless plating components from an electroless plating fluid used during an electroless plating process and/or reducing the total heat load imparted to the electroless plating fluid. Such apparatus may be characterized by the following features: a heat exchange recirculation loop, the heat exchange recirculation loop including a heat exchanger and a fluid pump, an inlet and an outlet of the heat exchange recirculation loop configured in fluid communication with: an electroless plating vessel recirculation loop, said electroless plating vessel recirculation loop comprising an elect
Alexy John B.
Feng Jingbin
Mayer Steven T.
Beyer Weaver & Thomas LLP
Novellus Systems Inc.
Pert Evan
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