Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-12-10
2002-03-12
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204SDIG007, C204S284000
Reexamination Certificate
active
06355147
ABSTRACT:
BACKGROUND
The field of the present invention relates generally to electroplating processes used to produce metal deposits having precise microscopic features. In particular, the field of the invention relates to the fabrication of LIGA microstructures and metallization of semiconductor devices, or to any process wherein metal is electrochemically deposited into lithographically produced molds containing a multiplicity of microscopic cavities such as trenches and holes.
The present invention is particularly beneficial in producing uniform voidfree deposits under adverse circumstances where there are large variations in the size and areal density of mold cavities or when these cavities are much deeper than their lateral dimensions.
The primary target technologies of the present invention are: (1) fabrication of microdevices by the LIGA process and (2) metallization of semiconductor devices. Each of these processes is separately described below, followed by an explanation of manufacturing difficulties that are remedied by the present invention. A common aspect of both target technologies is the need for uniform void-free plating of microscopic features having high aspect ratios.
LIGA Technology
LIGA, an acronym from the German words for lithography, electroforming and molding, is a promising new process for producing metal microdevices having micron to millimeter features. In the first step of LIGA, a photoresist (photosensitive or photo-definable) material is exposed to high-energy radiation through a patterned mask. This photoresist is then developed in a chemical bath which preferentially dissolves the exposed regions. The developed photoresist, bonded or clamped to a conducting substrate, subsequently serves as a mold to be filled with metal by electrodeposition. After filling of the mold cavities is complete, the mold is usually dissolved by chemical etching, leaving only the desired metal structure or a collection of finely detailed metal parts still attached to the substrate. In some cases, the resulting metal structure is not the final product but rather serves as a mold or die in the final step of mass production by injection molding or embossing.
Currently under worldwide development, this process offers a means to manufacture high resolution, high aspect-ratio metal devices including microscale valves, motors, solenoid actuators, and gear trains. Such devices cannot be fabricated either by silicon micromachining or by precision machine tool operations. The depth dimension of a typical LIGA structure ranges from about 10 microns to a few millimeters. Since the mold is generally fabricated from a flat sheet of photoresist, the depth of all mold cavities and that of all electroformed features is generally the same in a single-layer device. However, the lateral dimensions and geometries of individual features may be widely variable and highly complex. Features having aspect ratios (depth to width) greater than 10 pose a challenge to conventional electroplating, particularly for feature depths greater than 100 microns.
The LIGA process is applicable to a broad range of enabling technologies which require high aspect ratio microstructures such as microscale pumps, motors, valves, micro actuators, switches, positioners, or the like. As such, the LIGA process is ideal for miniaturizing mechanical components used in sensing and process control, computer peripherals, automotive, medical, and aerospace and defense applications.
The total market for microdevices is growing very rapidly and is estimated to be between approximately $3 billion and $14 billion per year by the year 2000 The potential for high aspect ratio metal devices in the total microdevice market depends strongly on exploiting the unique capability of LIGA-like processes to produce high aspect ratio microstructures, and on reducing product costs through improved manufacturing methods. The aspects of the invention described herein provide a solution to several outstanding problems in the LIGA manufacturing process and so should make possible an increase in the future market share for metal devices or microstructures produced by this and related electrodeposition processes.
LIGA Deficiencies
One difficult problem in the LIGA process is nonuniform deposition of metal within the mold. In all electroplating processes, geometric irregularities give rise to nonuniform electric current densities. Since electric currents drive the electrodeposition process, nonuniform currents give rise to nonuniform metal deposition rates. For example, the corners of a rectangular region will always have a local current density that exceeds the mean value for the surface by a significant factor. As a result, deposition rates at these corners will be greater than the average rate. Similarly, a hole in an otherwise uniform surface, sharp bends in a linear feature, or parallel linear features of irregular spacing will also give rise to nonuniform deposition rates near the geometric irregularity.
In conventional electroplating practice, robbers and shields are employed to improve metal deposition uniformity on surfaces of irregular geometries. Robbers are electrically conducting elements placed near the deposition surface with the intent of locally altering the electric potential to produce a more uniform current flux over the surface. The shape, position and electric potential of the robber must be carefully selected to produce the desired effect. In contrast, shields are electrically insulating elements usually placed between the bath electrode and the deposition surface. Their purpose, however, is the same as that of a robber to locally alter the current density to obtain a more uniform deposition rate. Like robbers, shields must be carefully designed and placed to produce the desired benefit.
Because of the very small feature sizes of LIGA molds, shields and robbers are not practical for micro molding of complex three dimensional structures . In principle, robbers could be designed as part of the LIGA mold, but this would require many iterations of the robber design to effectively achieve uniform deposition over the many features present in a typical LIGA part. Further, such integral robbers would likely limit the range of possible designs for the LIGA device. Special shields also could be fabricated using lithographic methods, but again these would require many trial-and-error iterations to be highly effective. Since shields and robbers are not very practical for the LIGA process, other techniques must be pursued to ensure uniform deposition in the LIGA mold.
Current practice in LIGA manufacturing is largely to tolerate nonuniform deposition or to attempt to correct it in an iterative fashion. The mold is periodically removed from the plating bath and inspected. Areas experiencing excessive deposition rates are coated with an insulating paint to inhibit further deposition in those areas, and the mold is then returned to the bath for an additional period of plating. This cycle is repeated until all cavities in the mold are filled. This is a costly and time-consuming practice that is not well suited to the mass production of LIGA parts. Therefore, to successfully develop the LIGA process as a flexible and cost-effective manufacturing method, a method is needed for solving the problem of nonuniform deposition in the mold. What is also needed is a method for providing uniform deposition of metal for complex three-dimensional microstructures. It also would be advantageous to provide a method for fabricating a uniform metal layer which is scalable and applicable to nanometer through micron size microstructures and devices.
Metallization of IC Devices
As Integrated Circuit devices continue to shrink in size, metal interconnects become the dominant limitation in circuit speed. Interconnect areal dimensions must shrink in proportion to the semiconductor devices, which reduces their conductivity. In order to compensate for this loss in conductivity, interconnects must be made taller, requiring a large aspect ratio. Achieving this large aspect rat
Griffiths Stewart K.
Hruby Jill M.
Nilson Robert H.
Evans Timothy P.
Gorgos Kathryn
Hetherington Michael
Nicolas Wesley A.
Sandia Corporation
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