METHOD OF HIGH DENSITY PLASMA
ETCHING FOR SEMICONDUCTOR
CROSS-REFERENCE TO RELATED
This application is a continuation of application Ser. No. 08/213.556 filed Mar. 16. 1994. now abandoned.
FILED OF THE INVENTION
This invention relates generally to high density plasma etching and more particularly to an improved method for controlling the ratio of ions to neutrals in a plasma during high density plasma etching of semiconductor structures.
BACKGROUND OF THE INVENTION
Dry etching is frequently employed during semiconductor manufacture for forming semiconductor structures with small feature sizes. In dry etching, gases are the primary etch medium and a substrate such as a semiconductor wafer is etched without wet chemicals or rinsing. During the etching process the material to be etched is converted to gaseous byproducts which are removed by a vacuum pumping system.
One example of dry etching is plasma etching. Plasma etching systems utilize a plasma as the etching medium. As an example in a parallel plate plasma reactor, a process chamber is evacuated and a gas mixture is energized to a plasma state using a radio frequency (rf) source. The rf source is capacitively coupled to the substrate and to one electrode while the other electrode is grounded. Cathode voltages are typically on the order of several hundred volts. The gas mixture provides a medium in which a glow discharge can be initiated and maintained. In addition, the gas mixture contains one or more chemical etchants that react with and form volatile byproducts with the solid material being etched. Under these conditions the substrate is bombarded by energetic particles which arrive at a normal incidence to the substrate and produce an anisotropic etch.
Another example of dry etching is ECR (electron cyclotron resonance) etching. ECR etching systems utilize microwaves and a magnetic field to generate plasmas at low pressures. ECR systems are one particular type of high density plasma systems.
Other high density plasma etching systems utilize one energy source to generate the plasma and another energy source to accelerate ions towards the substrate. This type of system is referred to as a high density plasma system because a high-electron-density plasma is produced. In (his type of high density plasma etching system, the plasma is typically generated in a separate chamber using a source of electromagnetic radiation (e.g.. microwave or low frequency whistler waves). Antennae initiate wave coupling between the source and a gas injected into the chamber. Magnetic fields are often used to control the flow direction of the ions and to provide a uniform plasma density over a large circular area. The substrate is capacitively coupled to an rf bias source which accelerates etching ions contained in the plasma towards the substrate. In addition to containing the etching ions, the high density plasma also contains reactive neutrals. Both the ions and the neutrals have an effect on the etching process. The direction of the etch however, is mostly effected by the ions.
In a high density plasma etching system, the plasma current density and bias power are used to control the energy
of the ions that strike the substrate. These systems are generally operated with a very high plasma density (e.g.. 10lo-1012 charges/cm3) and at a very low pressure (e.g., 0.1 mtorr-200 mtorr). Commercially available high density
5 plasma etching systems, also known as "Mori Source Etchers" are manufactured by Plasma & Materials Technologies of Burbank, Calif. U.S. Pat. NOS. 4.990.229; 5.091.049 and 5,122,251 to Campbell et al.. which are incorporated herein by reference, describe high density plasma etching systems.
io During high density plasma etching, particularly during the etching of polysilicon over oxide, one problem that frequently occurs is referred to as "notching". FIGS. 1A-1C illustrate the notching phenomena during plasma etching of a simplified semiconductor structure. In FIG. 1A, a poly
15 silicon layer 10 has been deposited on a substrate oxide 12 (e.g., TEOS, nitride, thermal oxide). A patterned layer of photoresist 14 is formed on the polysilicon layer 10 as an etch mask. As shown in FIG. IB. during a plasma etching process, the polysilicon layer 10 is etched to the endpoint of
20 the substrate oxide 10. As the etching process continues, however, notches 16, 18 (FIG. 1C) may also form in the polysilicon layer 10, at the interface with the substrate oxide 12. The notches 16. 18 physically alter the semiconductor structure and may adversely effect the electrical character
25 istics of the completed semiconductor devices.
This notching problem is particularly acute when the plasma is formed in a pure halogen gas such as chlorine (Cl2). Chlorine is favored for etching polysilicon deposited on a vertical or retrograde topography, because isotropic
30 stringer removal steps are known to be sucessful after an isotropic Cl2 etch. Stringers comprise a residual material that is not removed during the etch process. FIG. 2 illustrates a polysilicon layer 20 that has been deposited on an oxide layer 22 having a retrograde topography (i.e., an overhang
35 occurs). During plasma etching the polysilicon material located in an area protected by an overhanging portion 26 of the oxide layer 22 may not eteh completely. This forms the stringers 24 which are shown in FIG. 2. Stringers formed of a conductive material, such as polysilicon, may cause short
40 circuiting between adjacent areas in a semiconductor structure.
Although etching with pure chlorine gas helps to prevent stringers 24, these stringers are not always removed during
45 an anisotropic Cl2 etch. A separate step may be required to remove them. One such step uses a mixture of Cl2 and NF3 (or SF6) to provide a controlled lateral etching.
In addition, in order to achieve a higher etch selectivity of polysilicon to oxide during pure chlorine etching, it is
5Q preferable to use high plasma powers for generating the plasma and relatively low bias power at the substrate. As the plasma power increases the ion current increases. The general trend appears to be that notching increases in intensity as the ion current increases at constant ion energy. Lowering
55 the plasma source power and thus the ion current can reduce notching but may also reduce the etch selectivity of the polysilicon to oxide.
One prior art approach to the problem of notching is to add oxygen, nitrogen or hydrogen additives to the chlorine
60 gas. Pure Cl2 is preferred over Cl2—02 or Cl2—HBr mixtures however, because these mixtures prevent notching by forming a deposit on the feature sidewalls. This deposit prevents the controlled lateral etching of the stringers because it protects the stringers from the lateral etchants.
65 The technical article entitled "ECR Plasma Etching of N+ Polysilicon Gate" by Nozawa et al., (Electrochemical Society Meeting, San Francisco, Calif. (1993). abstract no. 245)