Situ stringer removal during polysilicon capacitor cell plate delineation

Capacitors such as storage cells for Dynamic Random Access Memories are formed in a process for etching a polycrystalline silicon layer to form a storage cell during the manufacture of a semiconductor device. The etch results in a cell having reduced undercutting of the poly cell, and eliminates the formation of poly stringers. The inventive etch comprises the use of NF.sub.3 and/or SF.sub.6 during a magnetically enhanced low pressure reactive ion etch using a carbon-free etch gas of Cl.sub.2.

FIELD OF THE INVENTION 
This invention relates to the field of semiconductor manufacture. More 
specifically a method of removing polycrystalline silicon stringers from 
between storage plates is described. 
BACKGROUND OF THE INVENTION 
In the manufacture of semiconductors, several process steps are required to 
produce a functional die. A wafer of a starting material such as silicon 
or gallium arsenide is layered with oxide, poly, nitride, photoresist, and 
other materials in various configurations, depending on the type and 
design of the device which is being produced. Each step may require the 
local deposition, growth, or other formation of one of the above listed 
materials (patterning), or a blanket layer of the material may be laid 
down and a pattern etched away with chemicals or abraded away by 
particles. The etch step described may be a single etch step, or a series 
of etches. 
In dynamic semiconductor memory storage devices it is essential that 
storage node capacitor cell plates be large enough to retain an adequate 
voltage level in spite of parasitic capacitances, noise, and leakage that 
may be present during circuit operation. As is the case for most 
semiconductor integrated circuitry, circuit density is continuing to 
increase at a fairly constant rate. The issue of maintaining storage node 
capacitance is also important as the density of DRAM arrays continue to 
increase for future generations of memory devices. The ability to densely 
pack storage cells while maintaining required storage capabilities is a 
crucial requirement of semiconductor manufacturing technologies if future 
generations of expanded memory array devices are to be successfully 
manufactured. One method of maintaining, as well as increasing, storage 
node size in densely packed memory devices is through the use of "stacked 
storage cell" design. With this technology, planar layers of a conductive 
material such as polycrystalline silicon (polysilicon or poly) are 
deposited over an access device on a silicon wafer with dielectric layers 
sandwiched between each poly layer. A cell constructed in this manner is 
known as a stacked capacitor cell (STC). Such a cell utilizes the space 
over the access device for capacitor plates, has a low soft error rate 
(SER) and may be used in conjunction with interplate insulative layers 
having a high dielectric constant. 
As shown in FIG. 1, the stacked capacitor design includes a substrate 10, 
source 12 and drain 14 regions, field 16 and gate 18 oxide, word lines 20 
or "runners" (manufactured from pillars of poly 22, tungsten silicide 24, 
and oxide 26, for example), a layer of dielectric 28 such as tetraethyl 
orthosilicate overlying the word lines, a capacitor storage cell plate 30 
interposed between every other pair of word lines 20, a top plate of the 
capacitor 32, digit (bit) lines 34, and various other dielectric layers 
36. Other features which are not shown, such as P and N wells, may be 
necessary for proper functioning of the device and are easily determined 
by one of skill in the art. 
As shown in FIG. 2 the cell plate is ideally formed by first laying down a 
patterned layer of photoresist 44 over a layer of poly 42. As shown in 
FIG. 3 the poly layer (as well as other exposed layers) is isotropically 
etched to isolate the cell plates 30. 
As used herein, "anisotropic" etch refers to a directional etch in which 
the etch rate in one direction, usually vertically, greatly exceeds the 
etch rate in other directions. Directional etching is normally achieved by 
placing the wafer or substrate to be etched on a biased electrode. The 
applied bias acts to focus charged plasma particles down to the electrode 
in a substantially perpendicular direction. Advantages of anisotropic 
etching include reduced sidewall erosion and reduced undercutting. This 
contrasts with isotropic etching, wherein the removal of material is 
achieved at a more uniform rate over all exposed surfaces. The etch 
conventionally used to define the cell plates is a reactive ion etch (a 
"dry" etch). The speed and direction of the etch is affected by the energy 
(or pressure) imparted to the particles which bombard the exposed 
surfaces. 
FIG. 3 shows the results of an ideal cell formation process using an 
anisotropic etch. Typically, however, the structure appears as shown in 
FIG. 4 after formation of the word lines 20, a layer of dielectric 28 over 
the word lines 20, the cell poly layer 42, and after depositing a layer of 
photoresist 40. Process etch steps leading to the FIG. 4 structure result 
in a retrograde sloping of the dielectric 28 as shown, especially with the 
use of tetraethyl orthosilicate (TEOS) which is commonly used. 
A subsequent high pressure anisotropic plasma etch with a material such as 
chlorine (Cl.sub.2) forms the structure of FIG. 5. The high pressure etch 
undercuts 50 the poly storage plate 30 as shown and decreases its size and 
therefore its storage capacity. In addition to reducing the storage 
capacity of the cell, undercutting the poly can result in sharp points of 
poly which can shear off and cause unwanted shorts on the die surface. 
To reduce the undercutting of the storage plate a low pressure plasma etch 
can be substituted for the high pressure etch. This, however, would leave 
the poly "stringers" 52 as shown in FIG. 5. The conductive poly stringers 
52 which result from the low pressure etch can cause shorts between 
subsequently formed conductive layers, and are therefore undesirable. In 
addition to forming stringers, a low pressure etch requires a longer etch 
time which can reduce output. 
Note that FIG. 5 is for description only, and shows the disadvantages of a 
high pressure etch (undercutting of the storage cell) and of a low 
pressure etch (the incomplete removal of the exposed poly to form 
stringers). A high pressure etch does not typically result in the unwanted 
poly stringers because the high pressure etch has relatively high 
isotropic properties due to the high kinetic energy imparted onto the etch 
particles. A low pressure etch would not typically result in the 
undercutting as shown because of the high anisotropic properties of a low 
pressure etch. Also, in a typically formed cell one of the word lines of 
FIGS. 2-5 would be formed over a layer of field oxide as can be determined 
from the structure of FIG. 1. An actual cell design which uses the 
invention can easily be determined from the description and Figures 
herein. 
An etch process which maintains a high etch rate without undercutting and 
which removes the poly stringers between the word lines would be a 
desirable process. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a method of etching a layer of 
material which results in minimal undercutting of the capacitor storage 
plate. Another object of the invention is to provide an etch process which 
removes the poly stringers between the word lines. A third object of the 
invention is to provide an etch process which has an etch rate which is 
sufficient for maintaining production throughput. 
These and other objects of the invention are realized by providing a 
reactive ion etch (RIE) comprising a chlorine plasma etch with added 
particles of SF.sub.6 or NF.sub.3 at low pressure under magnetic 
influences. The magnetic force increases the etch rate and isotropic 
properties of the etch to allow for sufficient stringer removal. A 
sufficient anisotropic etch property is maintained, however, so that the 
storage plate poly is not undercut.

DETAILED DESCRIPTION OF THE INVENTION 
The inventive process uses NF.sub.3 or SF.sub.6 in conjunction with a 
plasma etch under controlled magnetic influences and a low pressure to 
allow an etch having little undercutting of the material which is being 
etched and with sufficient isotropic properties to allow for the removal 
of stringers. The inventive process was carried out on a reactive ion 
etcher, specifically an Applied Materials 5000although any etcher which is 
capable of controlling the pressure, magnetism, and plasma composition as 
described herein would function sufficiently. 
The inventive process was used to manufacture a storage plate of a 
capacitor in the manufacture of a dynamic random access memory (DRAM) die, 
although the process is applicable to any type of poly processing where 
the increased resistivity (reduced conductivity) from cell to cell by the 
removal of polycrystalline silicon is desired. 
A cell structure was manufactured as shown in FIG. 4 to have word lines 20, 
a layer of dielectric 28 (TEOS in the instant case), a layer of 
polycrystalline silicon 42, and a patterned layer of photoresist 40 over 
the poly layer 42. 
The structure of FIG. 4 was subjected to a chlorine RIE, the chlorine being 
present in the plasma at a concentration of from 20 standard cubic 
centimeters (sccm) to 70 sccm. However, the addition of carbon to the etch 
gas, for example in the form of hydrocarbons or halocarbons is to be 
avoided. It was found that carbon adversely affected the etch by 
increasing the amount of polymerization which occurs on the feature 
sidewall. These halogen or hydrocarbon compounds deposited on the 
sidewalls act to block the etch of silicon/conductive residuals which for 
the undesirable shorts. By avoiding carbonated feed gasses, this mechanism 
is substantially reduced. Some carbon may remain in the etch chamber from 
previous steps, but the amount of carbon in the chamber should be 
minimized. Thus, a carbon-free etch gas is provided, although some carbon 
may remain in the chamber. In addition to the chlorine, nitrogen 
trifluoride (NF.sub.3) was added to the plasma in a concentration ranging 
from 2 to 20 sccm. The addition of fluorinated gas was found to remove any 
native oxide on the poly surface which may inhibit the stringer removal. 
This acts to increase the isotropic nature of the etch. Concentrations of 
NF.sub.3 above 20 sccm increased the likelihood of etching the material 
underlying the poly layer, such as the gate oxide. At under 2 sccm the 
etch was essentially an RIE of pure chlorine which has the disadvantages 
listed above. 
The pressure of the etch was controlled to be below within the range of 
about 50 millitorr (mt) to about 10 mt, with about 20 mt being preferable. 
At higher pressures, plasma particle trajectories are randomized due to 
the reduction of mean free path, and thus results in a high number of 
particle collisions. The randomized trajectories of plasma particles 
produce isotropic etching, the extent being directly controllable by 
pressure. Above 50 mt the isotropic etch cell loss becomes unacceptable. 
At low pressure the mean free path of plasma particles becomes large so 
that particles travel greater distances between collisions which allows 
etching to take place in a substantially vertical direction. Pressures 
below those described herein were found to produce stringers. 
The Applied Materials 5000 is capable of controlling the magnetic force 
under which the etch takes place. It was found that a force of between 
about 75 to 150 gauss produced sufficient results, with about 125 gauss 
being preferable. The magnetic force influencing the plasma was found to 
increase the etch rate, which was otherwise insufficient for production 
due to the low pressure of the etch. The magnetic force was also found to 
be proportional with the isotropic properties of the etch. At lower gauss 
ranges it was found that stringer removal was difficult due to the high 
degree of anisotropic etch properties. Above 130 gauss, however, it was 
difficult to control the capacitor plate size due the increased 
undercutting. 
It was found that the etch as described herein was similar and sufficient 
when sulfur pentafluoride (SF.sub.6) was substituted in similar 
concentrations for NF.sub.3. A mixture of the two gasses would also be 
adequate. With either material, stringers were eliminated and undercutting 
of the poly storage plate was minimized. 
While this invention has been described with reference to illustrative 
embodiments, this description is not meant to be construed in a limiting 
sense. Various modifications of the illustrative embodiments, as well as 
additional embodiments of the invention, will be apparent to persons 
skilled in the art upon reference to this description. It is therefore 
contemplated that the appended claims will cover any such modifications or 
embodiments as fall within the true scope of the invention.