Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Involving measuring – analyzing – or testing
Reexamination Certificate
1999-06-25
2001-05-29
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Involving measuring, analyzing, or testing
C204S228700
Reexamination Certificate
active
06238539
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for controlling the evolution of stress during an electroplating process.
2. Description of Related Art
In an electroplating process, a particular phenomenon occurs in that all electroplated metals tend to shrink or expand relative to their substrate during or after the plating process. Electroplated metals that are under tensile or compressive stresses may: peel and crack, and create non-uniform plated sections causing dimensional instability of electroformed sections and increase vulnerability to corrosive attack. Thus, in general, stress in electroplating is undesirable.
Stress is of especially a great concern in micro-electro-mechanical systems such as micro sensors and microelectronics. Examples of micro sensors are accelerometers and glyoscopes which, are used in applications including but not limited to aerospace and automotive. Due to the high precision required in these systems, any stress at the electroplated metal will have a pronounced effect.
In 1958, Joseph B. Kushner, a professor of Engineering at Evansvile College, Indiana, conducted research of the principal factors affecting plating stresses including plating temperature, film thickness, plating current density, and the influence of contaminants. Related to his research, Joseph B. Kushner published an article entitled Stress in Electroplated Metals in a trade journal called Metal Progress, on Feb. 22, 1962. His research results showed that all electroplated metals shrink or expand relative to their substrate during or after the plating process. This, in fact, is due to tensile or compressive stresses. In his case study of rhodium plating, the tensile stress developed ran as high as 100,000 psi. Experimenting with deposit thicknesses, he found that with the exception of the initial stage of deposition, tensile stress decreases as the deposition thickness increases.
A complete description on the subject of metal stresses is beyond the scope of the specification. For details, and for an extensive bibliography of references on metal stresses, see J. W. Deni,
Stress
, published in a book entitled Electrodeposition by Noyce Publications of New Jersey in 1993.
A commonly known equation used in the electroplating industry is the Stoney Equation. The Stoney Equation calculates the average stress in an electroplated metal. The equation is as follows:
Average
Stress
=
1
E
T
s
h
3
(
1
-
V
)
r
2
T
f
where
E is the Young's modules of the substrate,
V is the Poisson's ratio of the substrate,
T, is the thickness of the substrate,
r is the radius of the wafer,
h is the displacement of the wafer at the center, and
T
f
is the thickness of the film.
A positive stress represents the tensile stress while negative stress implies the compressive stress in the electroplated metals. A further explanation of the Stoney equation can be found in the following publications: C. M. A. Ashruf, P. J. French, C. de Boer and P. M. Sarro, “Strain Effects in Multi-Layers,” SPIE Vol. 3223, 1997, pp. 149-159; J. A. Cairns, C-H. Liu, A. C. Hourd, R. P. Keatch and B. Lawrenson, “Potential Limitations of Conventional Photomask to Inherent Internal Stress
The Need for an Alternative Opaque Layer,” Mat. Res. Soc. Symp. Proc., Vol. 356, 1995, pp. 239-244; and A. Brenner and s. Senderoff, “Calculation of Stress in Electrodeposits for the Curvature of a Plated Stip,” U.S. Department of Commerce, National Bureau of Standards, Research Paper RP1954, Vol. 42, February 1949.
A method for controlling stress induced by electroplating is known in the prior art, being disclosed in U.S. Pat. No. 4,648,944 to Ronald George, et al. Specifically disclosed is a monitoring system consisting of a strain gauge, a strain gauge monitor, several DC current regulated programmable power supplies, and a computer controlling the power supplies. The method of the prior art has disadvantages, including the following:
1. A dummy part and a second setup are being used for measuring and data gathering purposes instead of using the actual part being electroplated. Thus, an actual part that uses a different shape or a different material from the dummy part will cause errors.
2. A strain gauge is needed to be glued onto the substrate being measured.
3. The strain gauge glued onto the substrate will destroy the substrate being measured;
4. The strain gauge has low sensitivity and is inherently imprecise due to its mechanical nature;
5. The cathode on the dummy part and the second setup needs to be replaced after each run. Thus, the material cost is higher.
6. High part content because an additional cathode and an additional power supply is needed for the dummy part and second setup; and
7. High system cost due to high part content.
Somewhat related to this application is Kubona et al., U.S. Pat. No. 5,666,253, Method of Manufacturing Single Wafer Tunneling Sensor. The patent discloses a method of photo lithographically fabricating a unitary structure sensor on a semiconductor substrate. A cantilever beam is formed on the substrate, while the centilever beam has a nickel plating. It is through the process of electroplating nickel on the cantilever beam that the problem of metal stress was investigated.
Thus, there is a need for a method of in-situ displacement/stress control in electroplating that avoids the disadvantages of the prior art. The specific need is to have a more accurate measurement of the displacement of the substrate instead of the usage of a dummy part. In addition, the need to have a lower system cost by reducing unnecessary or redundant components.
SUMMARY OF THE INVENTION
The present invention provides a method for controlling electroplated metal stresses occurring in electroplating. It employs a closed-loop current and temperature control so a near-zero stress state in the electroplated material can be achieved. In one aspect of the invention, the method includes the operation of an apparatus containing a substrate for electroplating, a plating material, a displacement sensor system, a closed-loop control system, a fountain plating system, a power supply, a temperature control system, displacement data signals, a feedback input, current density control signals, power supply control signals, and temperature control signals.
The closed-loop control system has 2 portions: a feedback portion and a control portion. The fountain plating system can include a thermometer, apparatus for placing the substrate for electroplating, the plating material, and plating solution. The substrate for electroplating is placed in the fountain plating system. A cathode is attached to the substrate for electroplating. A plating material is also placed in the fountain plating system at a fixed distance from the substrate for electroplating. An anode is attached to the plating material. A displacement sensor of the displacement measurement system is positioned at a fixed distance from the substrate located within the fountain plating system.
The displacement sensor generates displacement data signals. The closed-loop control system receives the displacement data signals. The displacement data signals constitute the feedback portion of the closed-loop control system. The closed-loop control system generates at least one control signal comprising one or two of the following signals: a current density control signal and/or a temperature control system control signal.
A power supply is coupled between the closed-loop control system and the fountain plating system. The closed-loop control system generates current density control signals and controls the current density output of the power supply. The power supply is coupled between the cathode and the anode. A temperature control system is coupled between the fountain plating system and the closed-loop control system. The closed-loop control system generates temperature control signals and controls the temperature output of the temperature control system to the fountain plating system.
In processing the data from the displacem
Doty Robert E.
Joyce Richard J.
Kubena Randall L.
Wei Ronghua
Duraiswamy V. D.
Gorgos Kathryn
Hughes Electronics Corporation
Nicolas Wesley A.
Sales M. W.
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