Electrostatic chuck using A.C. field excitation

Details Electrostatic chuck using A.C. field excitation Electrostatic chuck using A.C. field excitation

1990-01-03

1992-04-07

Pellinen, A. D.

Electricity: electrical systems and devices

Electric charge generating or conducting means

Use of forces of electric charge or field

269 8, 269903, 361235, 361145, H01F 1300

Patent

active

051033674

DESCRIPTION:

BRIEF SUMMARY
The present invention relates generally to the field of materials handling and in particular the invention provides an improved electrostatic chuck for the holding of semiconductor wafers and other materials in operations such as vacuum sputter etching.
The semiconductor industry requires clamping and transport of crystalline wafers inside various processing machines. The wafers are intricately patterned on one face and can be damaged by contact on that face. In addition, contact on the wafer sides can result in chipping which not only could result in dirt particles on the patterned surface, but also in wafer breakage at a later time. Thus many rear-contact wafer transport methods have been developed (e.g., see P. H. Singer, "The Role of Wafer Transport in Front-End Automation", Semiconductor International, August 1983, pp. 60-65) but none of them allow motion of the wafer away from a horizontal plane due to their reliance on gravity and friction forces.
Applications which require wafer clamping in vacua, such as ion implantation or dry etching at high rates, have relied on mechanical clamps or electrostatic forces to counteract the cooling gas pressure applied at the wafer rear surface.
An example of a mechanical clamp is given in the U.S. Pat. No. 4,603,466 (M. J. Morley) where an outer ring presses down on the front face of a wafer, while the domed back surface is pressurised. However, wafer stress (though small) and possible damage to the wafer surface are occasioned by such prior art arrangements. In addition the clamping device of necessity obscures the outer ring which becomes useless for electronic device purposes.
Electrostatic attraction is in theory able to replace the above domed electrode shape with a flat one, and should also allow wafer retention without an outer mechanical clamping ring. Electrostatic chucks are described by G. A. Wardly, Reviews of Scientific Instruments Vol. 44, No. 10, October 1973 pp. 1506-1509; W. A. Eckes et al, U.S. Pat. No. 4,412,133; and in these chucks the electrostatic forces are used in addition to that of an outer mechanical clamping ring. A method of enhanced electrostatic attraction is described in U.S. Pat. No. 4,554,611 (I. H. Lewin) by utilising the charge retention, often observed in typical chuck insulating media, by mechanically jiggling the wafer then disconnecting the applied voltage. Wardly found that this dielectric retentive property decays at a rate related to atmospheric conditions, and employed a square-wave excitation voltage on his chuck to maintain long-term clamping action. Presumably the force described by Lewin decays according to Wardly's findings. Other electrostatic holding methods are described in U.S. Pat. Nos. 4,551,192 (V. Di Milia); 4,502,094 (I. H. Lewin et al); 4,480,284 (T. Tojo et al); and U.K. Patent Applications GB 2,050,064A (U. Berger); GB 2,149,697A (A. P. Neukermans et al).
These previous methods have in addition to the charge retention problem which makes both wafer retention and removal difficult, a need to directly contact the surface of the wafer in order to provide a voltage difference across the dielectric insulating layer. This direct contact, though advantageous in ensuring that the potential of the wafer surface is at a known fixed value, has the problems of wafer scratching mentioned earlier as well as of reliability in the contact itself. The U.S. Pat. No. 4,384,918 (N. Abe) overcomes this problem with application of a voltage between two rear electrodes; the wafer potential is intermediate and equal electrode areas result in nominally equal voltages and forces between each electrode and the wafer. The exact wafter potential is, however, not controlled.
Thus a need exists for an electrostatic transporting and chucking means which will indefinitely retain a wafer, if desired; release it quickly when required; and maintain a wafer at a known voltage level to prevent undesirable interactions with low-voltage plasma discharges. Another benefit of holding the wafer potential at known levels is that large voltage gra

REFERENCES:
patent: 3197682 (1965-07-01), Klass et al.
patent: 3253200 (1966-05-01), Klass et al.
patent: 4480284 (1984-10-01), Tojo et al.
patent: 4502094 (1985-02-01), Lewin et al.
patent: 4667110 (1987-05-01), Kariya
patent: 4766515 (1988-08-01), Bollen et al.
Derwent Abstract Accession No. 83172 K/04 class 502. SU/A, 915192 (Gelikman) 23 Mar. 1982 (23.03.82).

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