Deep trench bottle-shaped etching using Cl2 gas

A method to fabricate bottle-shaped deep trench into a semiconductor substrate. After a neck profile is formed, the chlorine gas at a predetermined flow rate is added to the etching plasma gas composition, while the flow rates of the plasma gases are increased by about 30% by volume, to create an enlarged lower portion of the deep trench. Preferably, the neck portion is etched using an etching composition which contains HBr, NF.sub.3, and (He/O.sub.2) provided at flow rates of about 87:13:35 sccm. The enlarged lower portion is etched using an etching composition which contains HBr, NF.sub.3, and (He/O.sub.2) provided at flow rates of about 113.+-.12:17.+-.2:46.+-.5 sccm, and Cl.sub.2 provided at a flow rate between 10 and 40 sccm. It was found that the width of the lower portion of the deep trench can be increased by 100% with minimum side effects such as polymer deposition in the plasma chamber, which could occur as result of substantially increased flow rate of HBr and/or NF.sub.3.

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 .mu.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.sub.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.sub.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.sub.3 (as opposed to
 constant HBr and NF.sub.3 concentrations in the conventional process), but
 at about the same plasma pressure for a short duration. Then the
 concentrations of HBr and NF.sub.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 capital investments as
 well as other extra operational expenses that may be otherwise required.
 However, it was discovered by the inventors of the present invention that
 the use of HBr at high concentrations (or actually high flow rates)
 without compensations from other co-etchants can cause polymers to build
 up and which are deposited on the plasma chamber wall. This becomes more
 profound if the process calls for a very high concentration of HBr than
 those used in the '825 invention.
 The inventors unexpectedly discovered that, by adding Cl.sub.2 gas in the
 traditional deep trench etching recipe which contains NF.sub.3, HBr and
 He/O.sub.2, a bottle-shaped deep trench profile can be obtained without
 the spikes in the HBr concentration relative to other etching components.
 One of the main advantages of the present invention is that, unlike the
 '825 process, the polymer deposition on the plasma chamber was
 substantially prevented, thus, allowing a clean chamber to be maintained.
 And the trench diameter can be increased almost 100% compared to those
 made from the conventional process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 The present invention discloses a method for making deep sub-micron deep
 trench type capacitors with a bottle-shaped enhanced sidewall surface so
 that a capacitance of 40 pF or more can be attained. One of the main
 advantages of the present invention is that the bottle-shaped deep trench
 can be formed with the same equipment, essentially the same plasma
 components, and similar procedure, as the conventional method, thus
 eliminating the need for capital investments that may be otherwise
 required. Another main advantage of the present invention is that, while a
 high concentration of HBr was utilized during such plasma etching
 treatment no polymer deposition was found on the plasma chamber, and a
 clean chamber was maintained. This eliminates the need to have to
 regularly clean the chamber, and thus substantially improving the
 cost-effectiveness of this novel process for fabricating bottle-shaped
 deep trenches.
 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.sub.3), helium, and oxygen,
 at a predetermined composition. 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 the concentrations of the HBr, NF.sub.3
 and (He/O.sub.2) components are maintained constant. A high plasma
 pressure increases the horizontal (or radial) etching rate relative to
 vertical etching rate, while the effect of the plasma etch on the already
 formed sidewall is minimized by maintaining the concentrations of HBr,
 NF.sub.3 and (He/O.sub.2) relatively constant. The main design
 consideration of the conventional approach is to contain the width
 degradation in the deep trench, it was not considered to be possible to
 form a bottle-shaped deep trench.
 FIGS. 1 through 4 are schematic drawings showing the main steps in forming
 a deep trench according to an approach that is similar to the conventional
 approach. In FIG. 1, it is shown that a pad stacked layer 15 is formed on
 a substrate 10. The pad stacked layer 15 typically consists of a pad oxide
 layer 11, a silicon nitride layer 12, and a dielectric boron silicate
 glass layer 14. The pad oxide layer 11 is provided mainly to improve the
 adhesion between the nitride layer and the silicon substrate, and to
 reduce thermal and mechanical stresses. FIG. 1 also shows a photoresist
 pattern 16 which is formed by a photolithography process.
 FIG. 2 shows that an opening 20 is formed through the pad stacked layer 15,
 by reactive ion etching or plasma etching techniques, utilizing the
 photoresist layer 16. After the photoresist layer is removed, the
 substrate is subject to a first plasma etching to remove a native oxide
 layer which may be formed due to the exposure of the silicon substrate to
 the outside environment. The first plasma etching, which is often called a
 "breakthrough" step, is conducted at a plasma gas pressure of about 20 to
 50 mtorr, preferably 25 mtorr; an RF power of about 500 to 900 W,
 preferably at 600 W; and a magnetic field of about 10 to 40 Gauss,
 preferably at 15 Gauss. The plasma gas composition consists of HBr and
 NF.sub.3 a at a ratio of about 20:5, expressed in terms of volumetric flow
 rate, sccm (standard cubic centimeters). The etching time is about 20 to
 40 seconds, preferably 25 seconds. This step is described as the first
 plasma etching.
 FIG. 3 shows that a neck profile is formed in the substrate by subjecting
 the substrate to a subsequent plasma etching process. The second plasma
 etching is conducted at a plasma gas pressure of about 80 to 110 mtorr,
 preferably 100 mtorr; an RF power of about 700 to 900 W, preferably at 800
 W: and a magnetic field of about 80 to 110 Gauss, preferably at 100 Gauss.
 The plasma gas composition consists of HBr, NF.sub.3, and (He/O.sub.2) at
 a flow rate (sccm) ratio of about 87:13:35. The ratio between He and
 O.sub.2 is about 70%: 30% in the (He/O.sub.2) mixture. The etching time is
 about 90 to 110 seconds, preferably 95 seconds. FIG. 3 shows a tapered
 neck profile, which constitutes the upper portion of the deep trench 21.
 This step is described as the second plasma etching.
 In order to arrest the sharp degradation in the width of the trench as the
 plasma etching further progresses, the prior art approach calls for an
 increase in the plasma gas pressure from between 80 to 110 mtorr, to about
 110 to 130 mtorr, preferably at 125 mtorr, while maintaining other
 conditions, including the plasma etching gas composition, substantially
 the same. The etching time is about 450 to 500 seconds, preferably 485
 seconds. The result is shown in FIG. 4, which indicates that the slope of
 trench width decrease is substantially ameliorated. However, the width of
 the entire lower portion 22 of the trench is narrower than the narrowest
 width in the upper portion. This step is described as the third plasma
 etching.
 With the method disclosed in the present invention, which involves a
 different process to replace the third plasma etching step of the
 conventional process, the trench is first subject to the first and second
 substitute plasma etching steps as in the conventional approach.
 Thereafter, the flow rates of HBr, NF.sub.3, and (He/O.sub.2) are
 increased by 20 to 40%, preferably 30%, to about 113.+-.12
 :17.+-.2:46.+-.5 (in sccm). The ratio between He and O.sub.2 is the same
 at about 70% :30% in the (He/O.sub.2) mixture. Increasing the flow rates
 of HBr and NF.sub.3 increases the etching rate in the radial direction;
 however, this effect is suppressed by the increased flow rate of the
 (He/O.sub.2) mixture. Increasing the flow rates of HBr and NF.sub.3 alone
 could cause polymer deposition problems. In the present invention,
 unexpected results were observed that, when chlorine gas was further added
 to the etching gas stream at the increased flow rate, a bottle-shaped deep
 trench can be obtained. This is achieved with minimum polymer deposition
 in the plasma chamber.
 The third plasma etching step is conducted at a plasma gas pressure of
 about 110 to 130 mtorr, preferably 125 mtorr; an RF power of about 600 to
 1,000 W, preferably at 1,000 W; and a magnetic field of about 40 to 55
 Gauss, preferably at 55 Gauss. Unlike the process disclosed in the '825
 invention, no spikes in the HBr flow rate is needed relative to other
 etching components. This prevents polymer deposition on the plasma chamber
 wall.
 The present invention will now be described more specifically with
 reference to the following examples. It is to be noted that the following
 descriptions of examples, including the preferred embodiment of this
 invention, are presented herein for purposes of illustration and
 description, and are not intended to be exhaustive or to limit the
 invention to the precise form disclosed.
 EXAMPLE 1
 A pad stacked layer 15, which consists of a pad oxide layer 11, a silicon
 nitride layer 12, and a dielectric boron silicate glass layer 14 is formed
 on a substrate 10, as shown in FIG. 1, via chemical vapor deposition. A
 photoresist pattern 16 is then formed by a photolithography process on the
 pad stacked layer 15.
 An opening 20 is formed through the pad stacked layer 15, by plasma etching
 technique, utilizing the photoresist layer 16, as shown in FIG. 2. After
 the photoresist layer is removed, the substrate is subject to a first (or
 breakthrough) plasma etching to remove a native oxide layer which may be
 formed due to the exposure of the silicon substrate to the outside
 environment. The first plasma etching process is conducted at a plasma gas
 pressure of 25 mtorr, an RF power of 600 W, and a magnetic field of 15
 Gauss. The plasma gas composition consists of HBr and NF.sub.3 a at a
 ratio of about 20:5. The numbers in the ratio indicate the gas flow rates,
 in sccm. The etching time is 25 seconds.
 The substrate is then subjected to a second plasma etching step to form a
 neck profile 21 as shown in FIG. 3. The second plasma etching is conducted
 at a plasma gas pressure of 100 mtorr, an RF power of 800 W, and a
 magnetic field of 100 Gauss. The plasma gas composition consists of HBr,
 NF.sub.3, and (He/O.sub.2) a at a ratio of about 87:13:35. The ratio
 between He and O.sub.2 is about 70% :30%. The etching time is 95 seconds.
 In order to increase the width of the deep trench to form a bottle shape,
 the substrate is now subjected to a third plasma etching step wherein the
 flow rates of HBr, NF.sub.3, and (He/O.sub.2) were respectively increased
 by 30% to 113:17:46 (in sccm), and a chlorine gas (Cl.sub.2) was added to
 the etching gas stream at a rate of 10 sccm. The ratio between He and
 O.sub.2 was maintained the same at about 70%:30% in the (He/O.sub.2)
 mixture. The third plasma etching step was conducted at a plasma gas
 pressure of 120 mtorr, an RF power of 1000 W, and a magnetic field of 50
 Gauss.
 By increasing the flow rates of HBr, NF.sub.3, and (He/O.sub.2) and
 including the chlorine gas (Cl.sub.2) at 10 sccm, the width of the deep
 trench was increased to 213 .mu.m. No observable polymer deposit was found
 in the plasma etching chamber. The trench depth was measured at a location
 about 1.5 .mu.m from the trench bottom. This appeared to correspond to the
 location with the largest trench width.
 EXAMPLES 2-3
 The procedures in Examples 2-3 were identical to that in Example 1, except
 that the flow rate of the chlorine gas (Cl.sub.2) was increased to 25 sccm
 and 40 sccm, respectively. The widths of the deep trench was measured to
 be 296 .mu.m and 333 .mu.m, respectively.
 Comparative Example 1
 The procedures in Comparative Example 1 was identical to that in Example 1,
 except that no chlorine gas (Cl.sub.2) added to the plasma etching gas
 stream. The width of the deep trench was measured to be 160 .mu.m.
 The foregoing description of the preferred embodiments of this invention
 has been presented for purposes of illustration and description. Obvious
 modifications or variations are possible in light of the above teaching.
 The embodiments were chosen and described to provide the best illustration
 of the principles of this invention and its practical application to
 thereby enable those skilled in the art to utilize the invention in
 various embodiments and with various modifications as are suited to the
 particular use contemplated. All such modifications and variations are
 within the scope of the present invention as determined by the appended
 claims when interpreted in accordance with the breadth to which they are
 fairly, legally, and equitably entitled.