Chemical Mechanical Polishing by mikesanye

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Chemical Mechanical
Polishing

Kenneth C. Cadien



1.0   INTRODUCTION

      Polishing of materials has been known to many disciplines for many
decades. For example, in astronomy, glass mirrors and lenses have been
polished since at least the seventeenth century. In metallurgy, metal
samples are routinely polished prior to metallographic examination. In the
semiconductor field, lapping and polishing have been used for the prepara-
tion of single crystal substrates (wafers) during the last 40 years. The
lapping process is used after sawing and prior to polishing. Lapping is
performed on both sides of the wafer to insure flatness and parallelism.
Polishing is used to remove surface damage and haze. All polishing
processes described above refer to the surface finishing of bulk materials.
During the past decade, a new application of polishing has occurred in
integrated circuit manufacturing. This application involves the removal of
thin metal, insulator, and semiconductor films. This new field is referred to
as chemical mechanical polishing (CMP).
      The primary driving force for CMP was the realization during the
1980s that shrinking feature size and the increased number of devices on a
chip required additional layers of metallization (separated by interlayer
dielectrics, ILD). Since the metal layers were etched into metal lines, and

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the ILD layers were deposited by conformal CVD processes, additional
metal layers meant that there would be a dramatic increase in topography
within the die. This effect is illustrated in Fig. 1. This increase in topogra-
phy was in direct opposition to trends in lithography where the printing of
ever-smaller feature sizes meant that less topography was required, not
more. The reason for this was that small features require improved
resolution. Increasing resolution meant reduction in depth of focus. This
trend is summarized in Fig. 2. The need for both additional metal layers and
increased resolution lead to the development of many planarization strate-
gies. The two predominant ones were spin-on glass (SOG) with or
without etchback, and CMP. Following the development of CMP for
planarization, it has also been used for tungsten polish and shallow trench
polish, to name a few applications. This chapter will focus on these
applications of CMP as well as a brief survey of polishing tools that are
available; special tool features: endpoint, pad conditioning, retaining rings,
and consumables.




Figure 1. Photograph showing topography due to two metal layers.
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Figure 2. The effect of shrinking minimum device feature size on depth of focus.



2.0    PROCESSING

       During CMP the wafer environment consists of the slurry and
polishing pad, and the backing material behind the wafer. The wafer is
pressed against the polishing pad with a known pressure. Typical polishing
variables are polishing pressure, pad rotation rate, wafer rotation rate,
slurry type and flow rate, backing pad and curvature, and polishing pad
material. A schematic of the polishing process is shown in Fig. 3.
       The polishing pad is mounted on a rigid base plate that is rotated about
its center axis. Slurry is pumped onto the pad near the center and centripetal
force spreads the slurry over the pads surface. The wafer is placed face down
on the pad with a given pressure. The wafer is also rotated about its own axis.
The wafer can also move in a radial motion on the pad in order to widen the
track of the wafer over the pad and thereby improve the life of the pad.
       In this section we discuss three different CMP processes: global
planarization, shallow trench isolation polish, and tungsten polish.
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Figure 3. Schematic diagram of a polishing apparatus.



2.1    Oxide Polish

       As mentioned earlier, during IC fabrication thick ILD layers are
deposited over metal topography. The ILD layer must be thick enough so
that during CMP enough oxide can be removed so that the topography can
be eliminated, and enough oxide left behind to adequately isolate metal
layers from each other. The planarization process sequence is summarized
schematically in Fig. 4.
       During polishing, elevated surfaces of the substrate are polished
more rapidly than the lower regions, leading to a flattening of the surface.
In addition there are pattern density effects during polishing. Large areas
polish more slowly than small areas. Polish processes are optimized to
minimize pattern density effects as well as polish rate and uniformity.
Process conditions that are typically used are summarized below in
Table 1.
       In oxide polish, the slurries that are used typically consist of a fumed
silica abrasive suspended in an ammonia or potassium hydroxide environ-
ment that is buffered to prevent pH drift. At high pH’s, SiO2 is softened
and easier to remove.
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Figure 4. Schematic diagram of the global planarization process.



Table 1. Typical Process Conditions for Global Planarization

      PROCESS PARAMETER                               RANGE OF VALUES
 Polish pressure, psi                      7–9
 Relative velocity (pad-wafer), ft/min 125–300
 Slurry and flow rate, sccm                Silica abrasive, pH = 10–11, 100–200
 Polishing pad                             Hard pad on soft pad
 Backing pad                               Soft




       The biggest challenge for global planarization using CMP is the fact
that in this process there is no endpoint. The process is complete when the
surface is planarized and a specified thickness of oxide is remaining. This
means that either an in situ oxide measurement tool is required, or the oxide
CMP process must be very stable. Another strategy that is used is to target
the process so that out-of-control (OOC) events leave the ILD too thick so
that rework is possible. Then a post polish thickness measurement must be
made of the polished wafers on the polishers (or an external tool) to verify
that the ILD thickness is within specification. Commercial equipment is
now available that is capable of making within-die thickness measurements
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under water and communicating with the polisher so that OOC wafers can
be sent back to the tool for rework.

2.2   STI Polish

       Shallow trench isolation is a process that uses trenches in the silicon
substrate filled with undoped polysilicon or silicon dioxide to isolate active
regions. STI replaces the LOCOS process. During STI polish, the fill material
is polished off to leave trenches filled with the fill material. Silicon nitride is
often used on the planar silicon surfaces to act as a polish stop. While STI
polish uses conditions similar to the oxide polish described above, the process
is fundamentally different. Oxide polish is designed to stop in the middle of
the oxide layer, while STI polish clears the fill off the stopping layer.
       The thickness of the stopping layer, within die, within wafer, and
wafer-to-wafer, is often used as the measure of success of this process. The
process is very sensitive to pattern density variations within the die. Areas of
the die with a high density of trenches tend to polish faster and the stopping
layer is thinner than low density regions. Techniques such as dummification
have been used to even out pattern fluctuations.
       It is interesting to note that the pattern sensitivity noted for STI polish
is also found at tungsten polish. A simple mechanical model proposed by
Rutten, et al.,[1] explains both phenomena.

2.3   Tungsten Polish

      Tungsten studs are used to connect different metal layers, or metal 1
to the diffusion layer. Vias or contacts are etched in the ILD, an
adhesion layer is sputter deposited, and W is then deposited by CVD. The
W/adhesion layer is then removed by a blanket polish process leaving
behind tungsten studs.
      Unlike global planarization, during W polish there is a clear stopping
layer, the ILD. Several authors have reported endpoint systems and
processes for W polish related to the different friction between W, the
adhesion layer, and ILD. There is relatively good understanding of the
phenomena that occur during W polish which is based on the seminal work
published by Kaufman, et al., during 1991.[2] The role of the slurry during
W polish is to have a redox potential such that in the presence of the
chemistry, W is oxidized while the slurry chemistry is reduced. The reactions
are summarized below using iron (Fe) as an example.
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Reaction (1)          W0 → W+6 + 6e-1                 Oxidation

Reaction (2)          Fe+3 + e-1 → Fe+2               Reduction

       In reaction (1), W is oxidized and forms a stable oxide film which is
removed by the abrasive during the polish process. Areas of the wafer that
are not in contact with the polish pad are protected from the chemistry by
the oxide film.
       The conditions under which these reactions are thermodynamically
favorable are shown in Pourbaix diagrams.[3] These diagrams are calcu-
lated from thermodynamic data. Since there is usually only complete data
for metal/water systems, Pourbaix diagrams typically do not exist for
complex nonaqueous systems. The Pourbaix diagram for W in water at
25°C indicates that pH and electrochemical potential determine the condi-
tion under which the passive W oxide layer forms. For the W water
system, the optimum pH range is below 4.


3.0   POLISH EQUIPMENT

      There are three basic steps to CMP:
         i) down force is applied to the wafer pressing it against
              the polishing pad
         ii) slurry is pumped onto the pad
         iii) the wafer and pad are rotated
       This simplified view of CMP is shown schematically in Fig. 3.
Polishing does not occur unless pressure forces the wafer against the
polishing pad, and there is relative motion between the wafer and the polish
pad. The uniformity of the pressure of the wafer against the pad is critical to
obtaining polish rate uniformity. In fact, controlled polish non-uniformity can
be used to correct systematic non-uniformity due to the pad/wafer interac-
tion. Examples of this are curved carriers and back pressure. During
polishing, both the pad and the wafer are rotated in the same or opposite
directions. This applies large shear stresses to the wafers that tend to cause
the wafer to slip out of the wafer carrier. A retaining ring made out of a wear-
resistant material is used to hold the wafer in place. Traditionally, the ring was
fixed to the carrier. Pumping of slurries is difficult. Slurries tend to be
corrosive and contain very fine particles that destroy moving parts such as
bearings. It has been found that the optimal method of pumping slurry is to
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use a system that completely isolates the slurry from moving parts of the
pump. Pumps such as peristaltic and dual diaphragm pumps work well.
Polishers tend to have multiple pumps per pad which allows for two or more
components to be pumped on to the pad for in situ mixing. This is especially
useful when premixed slurry components are unstable, have short shelf life,
or have some other difficulty. Since peristaltic pumps are positive displace-
ment pumps they have difficulty pumping compressible fluids. Liquids are
not compressible. However, liquids that entrap lots of gas such as foams or
froth are compressible and cannot be pumped with peristaltic pumps. Most
polishers deliver slurry onto the top surface of the pad where centripetal force
spreads the slurry over the pad surface. There are new tools on the market
today that deliver slurry through the pad right to the wafer surface. This leads
to very effective consumption of the slurry. It may also have some positive
impact on polish uniformity. Slurry is normally delivered to semiconductor
fabs in 55 gallon drums or totes that have volumes in excess of 300 gallons.
The fab slurry delivery system mixes the totes, dilutes the slurry and/or
makes additions to the slurry, then distributes the slurry to the polishers. A
detailed discussion of slurry delivery systems is beyond the scope of this
chapter.


4.0   HISTORY

       Although polishing has been done on blanket silicon wafers for many
years, the first fabrication (fab) compatible polisher did not appear until the
late 1980s. The key characteristic that made this polisher fab compatible was
robotic cassette-to-cassette capability (C-to-C). The first production tool was
delivered in 1989. It had a single polish platen and a single polishing head.
Wafers were held in the head by a fixed retaining ring. The tool had inherent
problems related to the large size of the machine and the relatively low
throughput rate for long polish steps. As the number of polish steps have
increased in submicron IC processes, the large number of polishers required
for manufacturing has become a major issue. Other competitive machines
began to arrive on the market in 1990. The first of these consisted of two
polish heads on a single large polish platen. The system had inherently higher
throughput but also had a larger foot print.
       Today, most polishers are distinguishable by the number of polishing
heads and the number of primary polish platens that each tool has. These
features are summarized in Table 2 for several tools that are available
                     Chapter 12: Chemical Mechanical Polishing            509


today. The number of polish heads tells us the number of wafers that can
be in process simultaneously. It is also an indication of increasing through-
put. Machines in the middle rows represent compromises between high
throughput, size, and complexity.


Table 2. Examples of Polish Tools Available Today Based on Number of
Polish Heads


                  Number of      Number of polish
                                                       Tool
                 polish heads       platens
                       1                 1              A
                       2                 1              B
                       3                 3              C
                       4                 4              D
                       5                 1              E
                       6                 1              F




5.0   INNOVATIONS

       One innovation that these early tool vendors brought to CMP market
was the use of pad conditioners. It had been discovered that during
polishing the rate decreases due to pad glazing. Mechanically abrading the
surface of the pad, either during polishing or between polishes, reduces the
rate at which the polishing rate drops. There are several novel designs for
pad conditioners exemplified by the design disclosed by Intel.[4]
       There are several innovations on the current generation of tools. For
example, a method has been invented[5] for applying differential pressure
to the retaining ring to improve uniformity. Systems have been developed
that deliver slurry through the pad instead of on top of the pad, and move
the wafer in an orbital motion.[6] There are also polish systems where the
polish table is not rigid, and an equal air pressure needs to be applied behind
the pad and the wafer.[6] In addition, in situ endpoint systems have been
developed that directly measure optical film thickness during polishing.
Motor current techniques that measure friction at the wafer surface have
also been used to endpoint polish.
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6.0   AUTOMATION

      There are different approaches to automation with respect to wafer
motion within the tool. The major approaches are to use a load cup with
single and multiple heads, and pick-and-place. In the load cup example, a
robotic system delivers wafers from the load cassette to the load cup. The
polish head either swings over the load cup or indexes in a rotary motion
over the load cup. This approach means that the polishing head, in addition
to having up and down motion, must also be able to move in the XY plane.
With pick-and-place systems, a robot picks the wafer from a cassette and
places it directly in the polishing head. The polishing head only has to have
vertical motion.


7.0   WAFER/PAD RELATIVE MOTION

      In the simplified view of CMP that was shown earlier in Fig. 3, the
wafer was rotated with respect to a large polish pad that was also rotated
producing relative motion. Other methods of relative motion have been
developed. In one instance, the wafer is subjected to an off-axis rotation of
about 1 inch in diameter to produce a buffing type of motion. The pad is kept
very small (2–4 inches larger than the wafer size). Slurry has to be delivered
through the pad in order to reach the wafer surface.[7] In another case, a
polishing pad is moved linearly in front of a rotating wafer.[8] This has a
similar motion to a belt sander.


8.0   FUTURE CHALLENGES

       There are several major challenges for the future generation of tools
that will be emerging within the next year. These include automation, 300
mm wafers, and in-situ metrology. On the automation front, fully integrated
dry-in/dry-out polishing systems that improve throughput and reduce foot-
prints are being developed. Figure 5 shows schematically linked and inte-
grated systems. In linked systems, discrete polishers, cleaners, and an
input/output interface are linked by an external robot. The external robot
moves cassettes of wafers between tools. While this system is large, it
does reduce the number of people required to run the polish area, and
eliminates possible ergonomic issues with polish and clean tools. In the
                        Chapter 12: Chemical Mechanical Polishing                511


integrated system, a supplier delivers a total solution to their customers.
Within the tool, either cassettes or single wafers are moved between the
polish and clean modules.




                             (a)                                  (b)

Figure 5. Schematic figure of linked (a) and integrated (b) polishing systems.



      The transition from 150 to 200 mm wafers was fairly transparent to
CMP tool suppliers, since existing tools could easily accommodate the
larger wafer size. With 300 mm wafers, most machines will need to
increase the frame of the polisher. Of particular concern will be wafer
handling. Three-hundred (300) mm wafers are about four times heavier than
200 mm wafers.
      One of the factors that impacts CMP throughput and floor space is
the need for metrology tools. The drive in future tools is to incorporate in-
situ metrology for device wafers into integrated tools with closed loop
control. This will also limit process excursions to the wafers in process.


CONCLUSION

       CMP was developed to address a major roadblock to future IC
manufacturing. It is now finding widespread acceptance. CMP tools are
undergoing a rapid evolution from fairly simple machines, to very complex
high throughput tools with enhanced capability. These capabilities include
in situ clean and metrology capabilities.
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REFERENCES

  1. Rutten, M., Feeney, P., Cheek, R., and Landers, W., Pattern Density
     Effects in Tungsten CMP, Semiconductor International, p. 123 (Sept.
     1995)
  2. Kaufman, F. B., Thompson, D. B., Broadie, R. E., Jaso, M. A., Guthrie, W.
     L., Pearson, D. J., and Small, M. B., Chemical Mechanical Polishing for
     Fabricating Patterned W Metal Features as Chip Interconnects, J.
     Electrochem. Soc., 138(11):3460 (Nov. 1991)
  3. Pourbaix, M., Lectures on Electromechanical Corrosion, Plenum Press,
     New York (1973)
  4. Breivogel, J. R., Blanchard, L. R., and Prince, M. J., U.S. Patent #5,216,843,
     Polishing Pad Conditioning Apparatus for Wafer Planarization Process
     (June 8, 1993)
  5. Shendon, N., Struven, K. C., and Kolenkow, R. J., U.S. Patent #5,205,082,
     Wafer Polisher Head Having Floating Retainer Ring (Apr. 27, 1993)
  6. Breivogel, J. R., Louke, S. F., Oliver, M. R., Yau, L. D., Barns, C. E., U.S.
     Patent #5,554,064, Orbital Motion Chemical-Mechanical Polishing
     Apparatus and Method of Fabrication (Sept. 10, 1996)
  7. Cleary, T., and Barns, C., Orbital Polish Techniques for CMP, Proc. 13th
     International VLSI Multilevel Interconnection Conference, p. 443 (June
     1996)
  8. Jairath, R., Pant, A., Mallon, T., Withers, B., and Krusell, W., Linear
     Planarization for CMP, Solid State Technology, p. 107 (Oct. 1996)

								
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