Plasma etching methods

A plasma etching method includes forming a polymer comprising carbon and a halogen over at least some internal surfaces of a plasma etch chamber. After forming the polymer, plasma etching is conducted using a gas which is effective to etch polymer from chamber internal surfaces. In one implementation, the gas has a hydrogen component effective to form a gaseous hydrogen halide from halogen liberated from the polymer. In one implementation, the gas comprises a carbon component effective to getter the halogen from the etched polymer. In another implementation, a plasma etching method includes positioning a semiconductor wafer on a wafer receiver within a plasma etch chamber. First plasma etching of material on the semiconductor wafer occurs with a gas comprising carbon and a halogen. A polymer comprising carbon and the halogen forms over at least some internal surfaces of the plasma etch chamber during the first plasma etching. After the first plasma etching and with the wafer on the wafer receiver, second plasma etching is conducted using a gas effective to etch polymer from chamber internal surfaces and getter halogen liberated from the polymer to restrict further etching of the material on the semiconductor wafer during the second plasma etching. The first and second plasma etchings are ideally conducted at subatmospheric pressure with the wafer remaining in situ on the receiver intermediate the first and second etchings, and with the chamber maintained at some subatmospheric pressure at all time intermediate the first and second plasma etchings.

TECHNICAL FIELD
 This invention relates to plasma etching methods.
 BACKGROUND OF THE INVENTION
 Plasma etchers are commonly used in semiconductor wafer processing for
 fabrication of contact openings through insulating layers. A photoresist
 layer having contact opening patterns formed therethrough is typically
 formed over an insulative oxide layer, such as SiO.sub.2 and doped
 SiO.sub.2. An oxide etching gas, for example CF.sub.4, is provided within
 the etcher and a plasma generated therefrom over the wafer or wafers being
 processed. The etching gas chemistry in combination with the plasma is
 ideally chosen to be highly selective to etch the insulating material
 through the photoresist openings in a highly anisotropic manner without
 appreciably etching the photoresist itself. A greater degree of anisotropy
 is typically obtained with such dry plasma etchings of contact openings
 than would otherwise occur with wet etching techniques.
 One type of plasma etcher includes inductively coupled etching reactors.
 Such typically include an inductive plasma generating source coiled about
 or at the top of the reactor chamber and an electrostatic chuck within the
 chamber atop which one or more wafers being processed lies. The
 electrostatic chuck can be selectively biased as determined by the
 operator. Unfortunately when utilizing etching components having both
 carbon and fluorine, particularly in inductively coupled etching reactors,
 a halocarbon polymer develops over much of the internal reactor sidewall
 surfaces. This polymer continually grows in thickness with successive
 processing. Due to instabilities in the polymer film, the films are prone
 to flaking causing particulate contamination. In addition, the build-up of
 these films can produce process instabilities which are desirably avoided.
 The typical prior art process for cleaning this polymer material from the
 reactor employs a plasma etch utilizing O.sub.2 as the etching gas. It is
 desirable that this clean occur at the conclusion of etching of the wafer
 while the wafer or wafers remain in situ within the reactor chamber. This
 both protects the electrostatic chuck (which is sensitive to particulate
 contamination) during the clean etch, and also maximizes throughput of the
 wafers being processed. An added benefit is obtained in that the oxygen
 plasma generated during the clean also has the effect of stripping the
 photoresist from the over the previously etched wafer.
 However in the process of doing this reactor clean etch, there is an
 approximate 0.025 micron or greater loss in the lateral direction of the
 contact. In otherwords, the contact openings within the insulating layer
 are effectively widened from the opening dimensions as initially formed.
 This results in an inherent increase in the critical dimension of the
 circuitry design. As contact openings become smaller, it is not expected
 that the photolithography processing will be able to adjust in further
 increments of size to compensate for this critical dimension loss.
 Accordingly, it would be desirable to develop plasma etching methods which
 can be used to minimize critical dimension loss of contact openings,
 and/or achieve suitable reactor cleaning to remove the polymer from the
 internal surfaces of the etching chamber. Although the invention was
 motivated from this perspective, the artisan will appreciate other
 possible uses with the invention only be limited by the accompanying
 claims appropriately interpreted in accordance with the Doctrine of
 Equivalents.

SUMMARY OF THE INVENTION
 In but one aspect of the invention, a plasma etching method includes
 forming a polymer comprising carbon and a halogen over at least some
 internal surfaces of a plasma etch chamber. After forming the polymer,
 plasma etching is conducted using a gas which is effective to etch polymer
 from chamber internal surfaces. In one implementation, the gas has a
 hydrogen component effective to form a gaseous hydrogen halide from
 halogen liberated from the polymer. The hydrogen component is preferably
 one or more of H.sub.2, NH.sub.3 and CH.sub.4. The conversion of the
 halogen, released from the clean into a hydrogen halide, renders it
 substantially ineffective in etching the substrate and thus reduces the
 critical dimension loss. In one implementation, the gas comprises a carbon
 component effective to getter the halogen from the etched polymer.
 In another implementation, a plasma etching method includes positioning a
 semiconductor wafer on a wafer receiver within a plasma etch chamber.
 First plasma etching of material on the semiconductor wafer occurs with a
 gas comprising carbon and a halogen. A polymer comprising carbon and the
 halogen forms over at least some internal surfaces of the plasma etch
 chamber during the first plasma etching. After the first plasma etching
 and with the wafer on the wafer receiver, second plasma etching is
 conducted using a gas effective to etch polymer from chamber internal
 surfaces and getter halogen liberated from the polymer to restrict further
 etching of the material on the semiconductor wafer during the second
 plasma etching. The first and second plasma etchings are ideally conducted
 at subatmospheric pressure with the wafer remaining in situ on the
 receiver intermediate the first and second etchings, and with the chamber
 maintained at some subatmospheric pressure at all time intermediate the
 first and second plasma etchings.
 The halogen preferably comprises fluorine, chlorine or mixtures thereof.
 The gas at least during the second etching preferably includes oxygen,
 such as O.sub.2 ;
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 This disclosure of the invention is submitted in furtherance of the
 constitutional purposes of the U.S. Patent Laws "to promote the progress
 of science and useful arts" (Article 1, Section 8).
 It has been discovered that the polymer deposited on the internal walls of
 the etching chamber includes a significant concentration of fluorine. It
 is believed that the oxygen during the clean etching under plasma
 condition combines with the carbon and fluorine of the polymer liberated
 from the internal walls and forms carbon monoxide and carbon dioxide plus
 an activated or reactive fluorine species. Unfortunately, this liberated
 fluorine species is also apparently reactive with the silicon dioxide
 material on the wafer, which results in more etching of such material and
 the widening of the contact openings.
 Referring to FIG. 1, a plasma etching reactor is indicated generally with
 reference numeral 10. Such includes sidewalls 12 having internal surfaces
 14. One or more gas inlets 16 and one or more gas outlets 18 are provided
 relative to etching chamber 12. A pump 20 is associated with outlet 18 for
 exhausting and establishing desired subatmospheric pressure conditions
 within chamber 12 during processing.
 Plasma etching reactor 10 in the described embodiment is configured as an
 inductively coupled plasma etcher having a wafer receiver 22 within
 chamber 12 in the form of an electrostatic chuck. A biasing source 24 is
 electrically coupled with receiver 22. An inductive plasma inducing source
 26 is diagrammatically shown externally at the top of chamber 10.
 In accordance with the preferred embodiment, a semiconductor wafer 30 is
 positioned upon wafer receiver 22 within chamber 12. Wafer 30 has
 previously been processed to have a photoresist layer 32 formed on an
 insulative oxide layer (not specifically shown) formed on the outer
 surface of wafer 30. Photoresist layer 32 has contact opening patterns
 (not specifically shown) formed therethrough which ideally outwardly
 expose selected portions of the underlying insulative oxide layer.
 A desired vacuum pressure is established and maintained within chamber 12
 utilizing vacuum pump 20. An example chamber pressure is from about 30
 mTorr to about 5 Torr. Inductively coupled source 26 and chuck 22 are
 appropriately biased to enable establishment of a desired plasma within
 and immediately over wafer 30. An example power range for inductively
 coupled source 26 is from 100 watts to about 2,000 watts, with wafer
 receiver 22 being negatively biased to an example of 100-400 volts.
 Receiver 22 can have a temperature which is allowed to float, or otherwise
 be established and maintained at some range, for example from about
 -10.degree. C. to about 40.degree. C.
 Desired etching gases are injected to within chamber 12 through inlet 16,
 or other inlets, to provide a desired etching gas from which an etching
 plasma is formed immediately over wafer 30. Such gas can comprise, for
 example, carbon and a halogen. An exemplary gas would be CF.sub.4. Etching
 is conducted for a selected time to etch contact openings within the
 insulative oxide material on semiconductor wafer 30 through the contact
 opening patterns formed within photoresist layer 32. Unfortunately, a
 polymer layer 40 comprising carbon and the halogen, in this example
 fluorine, forms over some of internal surfaces 14 of plasma etch chamber
 12 during such etching. Such polymer can also form over photoresist layer
 32 (not specifically shown). Such provides but one example of forming a
 polymer comprising carbon and a halogen over at least some internal
 surfaces of a plasma etch chamber.
 Referring to FIG. 2, and at the conclusion of the first plasma etching and
 with wafer 30 on electrostatic chuck 22, chuck 22 is ideally provided at
 ground or floating potential and second plasma etching is conducted using
 a gas effective to etch polymer from chamber internal surfaces 14. The gas
 ideally has one or more components effective to etch photoresist layer 32
 from substrate 30 and polymer from chamber internal surfaces 14 (both
 being shown as removed in FIG. 2). Further, such one or more components of
 the gas are selected to be effective to getter halogen liberated from the
 polymer to restrict further etching of the insulative oxide or other
 previously etched material on the semiconductor wafer during the second
 plasma etching.
 In one example, the gettering component comprises hydrogen which combines
 with the halogen during the second plasma etching to form a gaseous
 hydrogen halide which has a low reactivity with material of the
 semiconductor wafer, and accordingly is withdrawn from the reactor through
 outlet 18. Example hydrogen atom containing gases include NH.sub.3,
 H.sub.2, and CH.sub.4. One example gas for providing the hydrogen
 component to the chamber is forming gas which consists essentially of
 N.sub.2 at about 96% or greater and H.sub.2 at about 4% or less, by
 volume.
 In another example, the gettering component comprises a carbon compound.
 Examples include hydrocarbons, aldehydes (i.e., formaldehyde) and ketones
 (i.e., methyl ketone). Hydrocarbons will typically getter the halogen as a
 hydrogen halide. Where the carbon compound comprises a C--O bond which
 survives the processing, the halogen will typically be gettered as
 COA.sub.x, where A is the etched halogen. One example carbon containing
 gettering compound having a C--O bond is CO, produced for example within
 the plasma from injecting CO.sub.2 to within the reactor.
 The gas also ideally comprises an additional oxygen component, such as
 O.sub.2 or other material. Such facilitates etching of both polymer and
 photoresist over the substrate. Where the gas components comprise O.sub.2
 and a hydrogen atom containing component, the O.sub.2 component and
 hydrogen atom containing component are preferably provided in the chamber
 during the second plasma etching at a volumetric ratio of at least 0.1:1
 of O.sub.2 to the hydrogen atom containing component. One reduction to
 practice example in a thirty-five liter high density plasma etcher
 included a feed for the second plasma etching of 60 sccm NH.sub.3 and
 1,000 sccm per minute of O.sub.2. For a carbon containing compound, such
 is preferably provided at from about 5% to about 80% by volume of the
 oxygen/carbon compound mixture.
 Plasma conditions within the chamber with respect to pressure and
 temperature and biasing power on induction source 26 can be the same as in
 the first etching, or different. Regardless, such first and second plasma
 etchings are ideally conducted at subatmospheric pressure where the wafer
 remains in situ on the electrostatic chuck intermediate the first and
 second etchings with the chamber being maintained at some subatmospheric
 pressure at all time intermediate the first and second plasma etchings.
 In compliance with the statute, the invention has been described in
 language more or less specific as to structural and methodical features.
 It is to be understood, however, that the invention is not limited to the
 specific features shown and described, since the means herein disclosed
 comprise preferred forms of putting the invention into effect. The
 invention is, therefore, claimed in any of its forms or modifications
 within the proper scope of the appended claims appropriately interpreted
 in accordance with the doctrine of equivalents.