Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
1999-08-10
2001-05-22
Powell, William (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C156S345420, C438S724000, C438S725000, C438S738000
Reexamination Certificate
active
06235643
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a method for etching a trench in a silicon substrate. In particular, the present invention pertains to particular plasma etch chemistries and process conditions which may be used to produce a rounded top trench corner, a rounded bottom trench corner, or both.
2. Brief Description of the Background Art
Trenches formed in silicon using traditional etching methods typically have sharp, squared-off top corners. These sharp, squared-off corners lead to high field stress in film layers subsequently deposited thereon during further processing steps. The high field stress can potentially lead to the electrical breakdown of the overlying deposited film layers. Further, the sharp, squared-off corners are a point of charge accumulation, which can cause edge leakage. Rounding of the top corners in trench structures can be critical for device performance. However, rounding of the corners in a manner which results in a loss of device active area is undesirable.
Various methods for obtaining a rounded top corner on a trench formed in a silicon substrate are known in the art. For example, U.S. Pat. No. 5,843,846, issued Dec. 1, 1998 to Nguyen et al., discloses a method for rounding the top corners of a sub-micron trench in a semiconductor device after trench formation. The method comprises exposing the previously formed trench to a gas comprising a carbon-fluorine gas, argon, and nitrogen directly after trench formation. The combination of the carbon-fluorine and nitrogen gases etch back the silicon nitride and stress relief oxide layers in order to expose the top corners of the trench. As the top corners of the substrate are exposed, nitrogen and argon gases are said to sputter the top corners, rounding them as the etch process completes the trench. The method is preferably performed using a low density parallel plate etch reactor.
Commonly assigned, copending U.S. application Ser. No.09/042,249, filed Mar. 13, 1998, discloses a method of obtaining a rounded top corner on a trench formed in a semiconductor substrate comprising the following steps: (a) providing a film stack comprising the following layers, from the upper surface of the film stack toward the underlying substrate, (I) a first layer of patterned material (typically, a patterned photoresist) which is resistant to a wet etch solution used to etch an underlying second layer and which is resistant to dry etch components used to etch the semiconductor substrate (typically, silicon), and (ii) a second layer of material (typically, silicon dioxide) which can be preferentially etched using a wet etch solution, wherein the second layer of material is deposited directly on top of the semiconductor substrate; (b) wet etching the second layer by immersing the film stack in a wet etch solution for a period of time sufficient to form an undercut beneath the first layer and to expose the underlying semiconductor substrate; and isotropically dry etching the exposed semi conductor substrate so as to form a trench in the semiconductor substrate.
A sharp corner at the bottom of a trench can also be a source stress, causing problems of the kind described with reference to the top corners of the trench. In addition, a rounded corner facilitates filling of the trench with a reduced possibility of trapping voids within the fill material. It is desirable to provide an etch process which provides a rounded bottom corner while maintaining a desired trench sidewall angle, for example about 80°to 90°.
SUMMARY OF THE INVENTION
We have discovered two methods for creating a rounded top corner on a trench etched in a silicon substrate. We have also discovered a straight forward bottom corner rounding method which may be used in combination with either of the top corner rounding methods.
A typical etch stack for etching a silicon trench comprises, from top to bottom: a patterned photoresist layer, a layer of silicon nitride, and a layer of silicon oxide (which are deposited upon the silicon substrate into which the trench is to be etched).
A first method for creating a rounded top corner on the etched silicon trench comprises etching both the silicon oxide layer and an upper portion of the underlying silicon substrate during a “break-through” step which immediately precedes the step in which the silicon trench is etched. The break-through step is used for removal of the silicon oxide layer overlying silicon substrate areas in which a trench is to be etched. The plasma feed gas for the break-through step comprises carbon and fluorine. The atomic ratio of fluorine to bromine in the plasma feed gas is preferably within the range of about 10:1 to about 50:1.
In this method, the photoresist layer is preferably not removed prior to the break-through etching step. Subsequent to the break-through step, a trench is etched to a desired depth in the silicon substrate using a different plasma feed gas composition. Both the silicon oxide etch and silicon trench etch may be performed in a single processing chamber, reducing processing time, providing increased throughput, and providing a decrease in processing costs. In some instances, other etch stack layers, such as the photoresist layer and the silicon nitride layer, can be removed in the same process chamber, using a plasma which produces byproducts compatible with the plasmas used in subsequent device processing steps. The principal etchant of the break-through etch plasma is generated from a feed gas containing fluorine which may be, by way of example and not by way of limitation, selected from the group consisting of CF
4
, CHF
3
, CH
2
F
2
, CH
3
F, and combinations thereof. The principal etchant is preferably selected from the group consisting of CF
4
, CHF
3
, and combinations thereof; and CF
4
has been shown to work very well. The plasma feed gas may further include CH
4
. In some instances the presence of CH
4
may be required, for example, when the amount of photoresist material available during the silicon oxide layer etch is minimal. The plasma feed gas preferably further includes a nonreactive, diluent gas selected from the group consisting of argon, helium, xenon, krypton, and combinations thereof. The nonreactive, diluent gas is most preferably argon. The plasma feed gas may further include a controlled amount of oxygen, which may be used to improve the top corner rounding effect. The radius of the rounded corner depends on the feature size of the structure being etched. For a feature size of about 0.35 &mgr;m, a typical radius for a rounded top corner ranges from about 15 nm to about 25 nm.
A second method for creating a rounded top corner on the etched silicon trench comprises formation of a built-up extension on the sidewall of the patterned silicon nitride layer during etch (break-through) of the underlying silicon oxide adhesion layer. The built-up extension upon the silicon nitride sidewall acts as a sacrificial masking material during etch of the silicon trench, delaying etching of the silicon at the outer edges of the top of the trench. This permits completion of trench etching with delayed etching of the top corner of the trench and provides a more gentle rounding (increased radius) at the top corners of the trench. For example, for a feature size of about 0.16&mgr;, a typical radius for a rounded top corner ranges from about 25 nm to about 40 nm.
In this second method for creating a rounded top corner, the photoresist used to pattern the silicon nitride hard mask is removed prior to the silicon oxide break-through step. The plasma feed gas used during the break-through step must provide a source of hydrogen, a source of fluorine, a source of carbon, and a source of surface bombardment atoms. Typically the surface bombardment atom source is an inert plasma feed gas such as argon, helium, krypton, nitrogen, xenon, or a combination thereof. Argon works particularly well. A single compound may be used to provide the hydrogen, carbon, and fluorine. Typically more than one gaseous compound is used. It is important
Chinn Jeff
Kim Nam-Hun
Lee Gene
Liu Wei
Mui David
Applied Materials Inc.
Church Shirley L.
Powell William
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