Electrostatic chuck

Abstract: An electrostatic chuck for electrostatically attracting and holding an object such as a semiconductor wafer includes a base plate, at least two layers each including an electrically insulating film and an electrode attached to a lower surface thereof, the at least two layers being disposed as attractive layers on the base plate, and a voltage applying assembly for selectively applying a voltage to at least one of the electrodes to electrostatically attract the object to the insulating film of an uppermost one of the layers. The insulating films may have different insulation resistances to allow for a wider range of temper­ ature changes.

Description: The present invention relates to an electrostatic chuck for electrostatically attracting and securing holding an object such as a semiconductor wafer. Recent years have seen the use of an electrostatic chuck as a jig for firmly holding a semiconductor wafer while the semiconductor wafer is being processed at very small dimensions, e.g., it is being patterned. For example, Japanese Patent Publication No. 60(1985)-59104 discloses an electrostatic chuck including an electrode plate having one surface on which an electrically insulating dielectric layer of alumina or the like is deposited by flame spraying. An object such as a semiconductor wafer is placed on the dielectric layer, and a voltage is applied across the dielectric layer between the wafer and the electrode plate for holding the wafer on the dielectric layer. With the conventional electrostatic chuck, the dielectric layer has a large resistivity ρ of about 10 14 Ω·cm, and almost no electric charges move in the dielectric layer. The wafer is attracted by an electrostatic force developed between the electrode plate to which the voltage is applied and the object itself. However, the electrostatic force thus produced is small and does not sufficiently attract the object. In view of the foregoing shortcoming of the conventional electrostatic chuck, it is an object of the present invention to provide an electrostatic chuck which is capable of attracting and holding an object such as a semiconductor wafer under a large electrostatic force that remains stable at all times. According to the present invention, the resistivities ρ of electrically insulating films are selected to be about 10 11 Ω.cm to permit electric charges to move in the electrically insulating films. By selectively applying a voltage to the electrically insulating films, volume charges are produced in the surface of the electrically insulating film which is positioned adjacent to an object to be attracted, so that a large electrostatic force can effectively be generated between the volume charges and the object. If the insulating films are made of ceramic, then since the ceramic has its insulation resistance or resistivity decreasing as the temperature thereof increases, the resistivity thereof tends to be lower than the above value, resulting in a larger leakage current in excess of a power supply capability. When this happens, a suitable voltage may not be applied, and the electrostatic chuck can operate effectively only in a very small temperature range. To eliminate the above drawback, the electrostatic chuck according to the present invention is arranged to produce a large stable electrostatic force in a wide temperature range. Therefore, in accordance with the present invention there is provided an electrostatic chuck for electrostatically attracting and holding an object, comprising a base plate; at least two layers each including an electrically insulating film and an electrode attached to a lower surface thereof, said at least two layers being disposed as attractive layers on said base plate; one of the said layers comprising alumina with 1.0 wt % titanium oxide having a resistivity range of 10 11 -10 12 ohm cm. in the temperature range 300-473 K and the second of the said layers comprising alumina with 0.3 wt % titanium oxide having a resistivity of 10 11 -10 12 ohm cm. in the temperature range 473-673 K and voltage applying means for selectivity applying a voltage to at least one of said electrodes to electrostatically attract the object to the insulating film of an uppermost one of said layers. The invention will now be described in greater detail by way of example, with reference to the drawings, in which :- FIG 1 is a vertical cross-sectional view of an electrostatic chuck according to an embodiment of the present invention, the view showing the layer structure of the electrostatic chuck; FIG 2 is a cross-sectional view taken along line II-II of the electrostatic chuck shown in FIG 1; FIG 3 is a graph showing the relationship between the resistivity as it varies depending on the temperature of electrically insulating films in the electrostatic chuck and the amount of TiO 2 added; FIG 4 is a vertical cross-sectional view of an electrostatic chuck according to a modification of the embodiment shown in figure 1; FIG 5 is a cross-sectional view taken along live V - V of FIG 4; and FIG 6 is a vertical cross-sectional view of an electrostatic chuck according to another modification of the embodiment shown in figure 1. As shown in FIGS 1 and 2, an electrostatic chuck according to an embodiment of the present invention comprises a circular base plate 1, and first and second layers 2 and 3. The second layer 3 is disposed on the circular base plate 1, while the first layer 2 is disposed on the second layer 3. The first layer 2 comprises a first electrode 4 and a first electrically insulating film 5, and the second layer 3 comprises a second electrode 6 and a second electrically insulating film 7. The first and second electrodes 4, 6 comprise electrode films attached to the lower surfaces of the first and second electrically insulating films 5, 7, respectively, in substantially fully covering relation thereto. The base plate 1 is made of Al 2 O 3 , Si 3 N 4 , AlN, SiC, or the like. The first and second electrodes 4, 6 are made of Ag/Pd or the like, and the first and second insulating films 5, 7 are made of Al 2 O 3 with TiO 2 added. The electrostatic chuck is fabricated by pressing a plurality of green sheets (two green sheets in this embodiment) in the form of pastes with the electrode films printed thereon, against a ceramic green sheet serving as the base plate 1, and then firing the laminated body. Each of the first and second insulating films 5, 7 has a thickness of about 300µm, but they have different electric resistances corresponding to the amounts of Ti0 2 added. More specifically, as shown in FIG 3, to obtain the resistivity ρ of the first insulating film 5 in the range from 10 11 to 10 12 Ω.cm at any given temperature between normal temperature and 200°C (300-473K) 1.0 wt. % of TiO 2 is added. Likewise to obtain the resistivity ρ of the second insulating film 7 in the range of 10 11 to 10 12 Ω.cm in a temperature range from 200°C to 400°C (473-673K), 0.3 wt. % of TiO 2 is added. The electric resistance R and the resistivity ρ are related to each other as follows : R = ρ.ℓ/S where ℓ is the distance to be insulated and S is the area to be insulated. Thus, it is apparent that the electric resistance of an insulating film varies depending on the resistivity ρ of the insulating film, the distance ℓ to be insulated and the area S to be insulated. The resistivity ρ of the insulating film can be changed by adjusting the amount of TiO 2 . The first and second electrodes 4, 6 are electrically connected to a power supply 12 through respective terminals 8, 9 and the respective switches 10, 11. The power supply 12, the terminals 8, 9 and the switches 10, 11 jointly serve as a voltage applying means. To attract a semiconductor wafer W to the electrostatic chuck as a flat support at a low temperature (ranging from normal temperature (27°C) to 200°C), the switch 10 is turned on and the switch 11 is turned off to apply a voltage to the first electrode 4 so that only the first insulating film 5 is used as a dielectric film or an attracting layer. Since the resistivity ρ of the first insulating film 5 at this time is in the range of from 10 11 to 10 12 Ω.cm, as described above, a large attractive force is developed by volume charges which exist on the upper surface of the insulating film 5. When the temperature ranges from 200°C to 400°C, the switch 10 is turned off and the switch 11 is turned on to apply the voltage to the second electrode 6 so that the first and second insulating films 5, 7 are used as dielectric films or attractive layers. At this time, a sufficient attractive force is developed because the resistivity ρ of the principal dielectric film (ie. the second insulating film 7) ranges from 10 11 to 10 12 Ω.cm. Even if the first and second insulating films 5, 7 have the same volume resistivity ρ, the combined volume resistivity of the dielectric films may be kept in a constant range by selectively switching on and off the electrodes even when the temperature changes. However, the different volume resistivities of the insulating films 5,7 allow for a wider range of temperature changes. FIGS 4 and 5 illustrate a modification of the electrostatic chuck of figures 1 and 2. The modified electrostatic chuck includes a pair of semicircular electrodes 4,4' spaced from each other, a pair of semicircular electrodes 6,6',and first and second insulating films 5, 7 having different resistivities. The semicircular electrodes 4,4' and 6,6' are attached to and extend in substantially fully covering relation to the lower surfaces of the first and second insulating films 5, 7, respectively. As shown in FIG 4, it is possible to apply voltages of opposite polarities to paired electrodes (eg. a positive voltage to the electrode 4 or 6 and a negative voltage to the electrode 4' and 6') With this modification, the resistivity of the dielectric films can be kept in a substantially constant range by selectively turning on and off switches 10,10',11,11' regardless of whether the temperature is low or high. The electrodes are not limited to the semicircular shape, but may be of any of various other shapes depending on the intended application of the electrostatic device. In the above embodiment and modification, the two layers each comprising an insulating film and an electrode are disposed on the base plate 1. However three or more layers may be disposed on the base plate 1. Moreover, the electrodes and insulating films may be fabricated by printing, plasma flame spraying, etching, evaporation, or the like, rather than the green sheet lamination. To process a semiconductor wafter electrostatically attracted and firmly held by the electrostatic chuck in a plasma etching process, it is possible to apply a DC voltage to one of the embedded electrodes and to apply a high-frequency AC voltage to the other embedded electrode for highly accurately controlling the ion energy for increased etching controllability. Such a procedure may be achieved by still another modification shown in FIG 6. As shown in FIG 6, a second electrode 6a is a single electrode as is the case with the embodiment shown in FIG 2, rather than the two electrodes 6,6'illustrated in FIGS 4 and 5, and a high-frequency voltage is applied to the second electrode 6a. Three or more layers each comprising an insulating film and an electrode may be disposed on the base plate by adding a set of electrodes to which a high-frequency voltage is applicable. With the present invention, as described above, the insulating films having respective resistivities are successively disposed on the base plate, and a voltage may be selectively applied to electrodes attached to the insulating films depending on the temperature at which the electrostatic chuck is used. The electrostatic chuck can develop a high attractive force which is stable at all times within a wide temperature range without being subjected to current leakage. Although there have been described what are at present considered to be the preferred embodiments of the present invention, it will be understood that the invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments are therefore to be considered in all aspects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description.

Claims: An electrostatic chuck for electrostatically attracting and holding an object, comprising: a base plate; at least two layers each including an electrically insulating film and an electrode attached to a lower surface thereof, said at least two layers being disposed as attractive layers on said base plate; one of the said layers comprising alumina with 1.0 wt % titanium oxide having a resistivity range of 10 11 -10 12 ohm cm. in the temperature range 300-473 K and the second of the said layers comprising alumina with 0.3 wt % titanium oxide having a resistivity of 10 11 -10 12 ohm cm. in the temperature range 473-673 K and voltage applying means for selectively applying a voltage to at least one of said electrodes to electrostatically attract an object to the insulating film of the uppermost one of said layers. An electrostatic chuck according to claim 1, wherein each of said electrodes comprises a single electrode film covering substantially the entire lower surface of each one of said electrically insulating films. An electrostatic chuck according to claim 1, wherein each of said electrodes comprises a plurality of electrode films of a desired shape covering substantially the entire lower surface of each one of said electrically insulating films. An electrostatic chuck according to claim 1, wherein said voltage applying means comprises means for applying a DC voltage to one of said electrodes and applying an AC voltage to the other of said electrodes when an object which is electrostatically attracted and held is processed by plasma etching. An electrostatic chuck according to claim 1, wherein the upper insulating film is of a lower resistance than the lower insulating film.

Inventors: KITABAYASHI TETSUO, WATANABE TOSHIYA, KITABAYASHI, TETSUO, WATANABE, TOSHIYA, KITABAYASHI, TETSUO C/O TOTO LIMITED, WATANABE, TOSHIYA C/O TOTO LIMITED
Applicants: TOTO LTD, TOTO LTD.
Application year: 1989
Main classification: H01L21
IPC codes: B23Q3, B25J15, H01L21