Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2000-04-19
2001-10-23
Picardat, Kevin M. (Department: 2823)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S700000, C438S713000, C438S735000
Reexamination Certificate
active
06306772
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method for making sub-micron-sized semiconductor devices containing at least one deep-trench type capacitor. More specifically, the present invention relates to a method for fabricating into a semiconductor substrate one or more bottle-shaped deep trenches with an enlarged diameter, or more generally speaking, with enlarged circumference or cross-sectional area, at the lower portion thereof, with minimum polymer deposition problems on the plasma chamber wall. The method disclosed in the present invention increases the surface area and thus the capacitance of the capacitor that is formed around the side wall of the deep trench. However, unlike prior art techniques, the method disclosed in the present invention does not require the formation of a collar oxide nor the additional thermal oxidation step in order to form an oxide layer laterally into the substrate. However, as a further improvement over the prior method, the present invention also eliminates or at least minimizes the amount of polymer deposition that may disadvantageously occur inside the plasma chamber due, which could be caused by the use of certain etchants in order to enlarge the trench diameter.
BACKGROUND OF THE INVENTION
There are two basic types of capacitors provided in a semiconductor device, the crown-type capacitors and the deep-trench type capacitors. A capacitor comprises a dielectric layer sandwiched by a pair of spaced conducting plates. As the trend in the fabrication of semiconductor devices is toward ever-increasing density of circuit components that can be tightly packed per unit area, there are great demands to develop technologies that can reduce the surface area to be taken by individual circuit components. As a result, deep trench technologies have been developed which result in structures, particularly large area capacitors, that are vertically oriented with respect to the plane of the substrate surface.
A deep trench capacitor typically comprises a dielectric layer formed on the sidewalls of a deep trench, which is formed into and surrounded by a highly doped buried plate (which constitutes the first conducting plate), and a highly doped poly fill (which constitutes the second conducting plate), which fills the deep trench. The capacitance of the deep trench capacitor is determined by the total sidewall surface of the trench, which, in turn, is determined by the diameter, or more specifically the circumference, of the deep trench. As the semiconductor fabricating technology moves into the sub-micron or even deep sub-micron range, it is increasingly recognized that the present technology for making deep trench capacitors may be inadequate. For deep sub-micron semiconductor devices, a deep trench can have a length-to-diameter aspect ratio of 35:1 or even greater. With current technology, the diameter (or width or circumference) of the trench generally decreases with depth. Such a tapered cross-sectional area causes a significant decrease in the overall sidewall surface of the trench, and, consequently, the capacitance provided by the deep trench capacitor. This problem is expected to become even more profound as we move into the next generation of ULSI fabrication technologies that are characterized with critical dimensions of 0.15-micron or even finer.
To increase the capacitance of a semiconductor deep-trench capacitor, the so-called bottle-shaped deep trench has been proposed. In an article entitled “0.228 &mgr;m Trench Cell Technologies with Bottle-Shaped Capacitor for 1 Gbit DRAMs”, by T. Ozaki, et al, IEDM, 95, PP661-664 (1995), the authors disclosed a method to increase the diameter of a deep trench. The method disclosed therein includes the steps of: (1) forming an 80 nm collar oxide at the upper portion of the trench by the selective oxidation; (2) performing a capacitor process which includes oxidation mask removal, native oxide removal, etc., during which process the collar oxide thickness reduces to 50 nm; and (3) in-situ phosphorous doped polysilicon is deposited and phosphorous doping into the trench side wall at the capacitor portion (plate electrode) is performed by the furnace annealing technology. The collar oxide prevents phosphorous doping at the upper portion of the trench; it also makes the electrical isolation between the plate electrode and the transfer transistor. The poly-silicon is removed by chemical dry etching and the diameter of the trench under the collar oxide is enlarged at the same time.
Since the method disclosed in Ozaki et al requires the additional steps of first forming a collar oxide followed by thermal oxidation of the substrate in the lower portion of the deep trench, it can substantially increase the manufacturing cost. In a co-pending application App. Ser. No. 09/399,825, the content thereof is incorporated herein by reference, it is disclosed an improved method for fabricating bottle-shaped deep trenches; however, if the amount (i.e., flow rate) of the HBr gas so used is too high, it may also undesirably cause polymeric material to be formed which is then deposited on chamber wall during the plasma etching process, causing cleaning up problems.
SUMMARY OF THE INVENTION
The primary object of the present invention is to develop a process for fabricating bottle-shaped deep trenches, which can be used in deep sub-micron deep trench type capacitors with an enhanced sidewall surface so that a capacitance of 40 pF or more can be attained, without substantially increasing the manufacturing cost. More specifically, the primary object of the present invention is to develop a method for enlarging the sidewall surface of a deep trench capacitor, by forming a bottle-shaped deep trench in the substrate, without substantially deviating from the conventional process, so as to obtain the maximum benefit under a controlled manufacturing cost. The present invention also relates to the semiconductors that are made from a process incorporating this method. The main difference between the present invention and the '825 invention is that the present invention prevents formation and accumulation of polymer deposits in the plasma chamber wall, thus eliminating the need for subsequent cleanup procedure.
Conventionally, deep trenches are formed into a substrate by an anisotropic plasma etching process using a plasma gas composition that comprises hydrogen bromide (HBr), nitrogen fluoride (NF
3
), helium, and oxygen, at a predetermined ratio. In order to minimize the disparity in the trench width (i.e., diameter) from top to bottom, as well as not to substantially increase the width in the upper portion of the trench, the pressure of the plasma composition is increased midway during the anisotropic etching process, while maintaining the concentration of HBr and NF
3
constant. The underlying consideration for the conventional approach is to minimize the width degradation in forming a submicron deep trench; it was not considered to be possible to the use the same approach, even in a modified form, to fabricated bottle-shaped deep trenches.
In the '825 invention, it was discovered that the conventional approach can indeed be modified so that a bottle-shaped trench can be formed. The method disclosed in the present invention involves two substitute steps. First, the trench being formed is subject to a “shock” treatment at substantially increased concentrations of HBr and NF
3
(as opposed to constant HBr and NF
3
concentrations in the conventional process), but at about the same plasma pressure for a short duration. Then the concentrations of HBr and NF
3
are cut back, but the plasma pressure is substantially reduced (as opposed to substantially increased plasma gas pressure), in a subsequent substitute step. The second substitute step continues until the etching process is completed. One of the main advantages of the present invention is that a bottle-shaped deep trench can be formed with the same equipment and plasma etching components as the conventional method, thus eliminating the need for cap
Lee Ray
Lin Ming-Horng
Tsai Nien-Yu
Collins D. M.
Liauh W. Wayne
Picardat Kevin M.
ProMos Technology, Inc
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