06NANO107CMOSfabrication
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Transcript of 06NANO107CMOSfabrication
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CMOS FABRICATION
•Lithography
•Etching
•Oxidation
•Doping
•Deposition
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An Overview of Microfabrication Processes
• Photolithography: Process by which the surface of a substrate can be patterned with micron size features. Photolithography requires a photoresist layer to be put on the substrate, a photomask that has the required pattern and a UV light source to expose the photoresist.
• The photoresist is a photographic emulsion that when exposed to UV light changes its chemical structure. For example region exposed may be cross linked and hardened. Developers exist for removing the non-hardened regions of photoresist. Photoresist is chemically resistant to chemicals that etch inorganic materials (especially buffered HF) and can be removed easily by organic removers such as acetone.
• Photomask is a piece of glass with a photographic emulsion on it. When exposed to light or to e-beam the photographic emulsion turns black and absorbs the UV light. Photomasks are typically generated today by e-beam lithography machines with feature sizes down to 0.1mm.
• Mask Aligner is a special instrument used to align different mask layers corresponding to different photolithographic steps on the substrate such that complex devices can be fabricated
• Oxide growth is process which results in the formation of a SiO2 layer on the top of the silicon circuit. This oxide layer can be used as a mask for subsequent processing steps or can be used as an electrical component within a transistor such as the gate oxide of a MOSFET. When used as a mask its chemical properties are critical and it can be deposited by chemical vapor deposition, that is by decomposing silane gas in a reactor in an oxygen rich environment and depositing the formed SiO2 molecules on silicon typically at lower temperatures e.g. 350oC. When used as gate oxide or as an electrical insulator, the electrical properties of the SiO2 layer are critical. In that case the SiO2 layer is grown by oxidizing the silicon wafer surface by bringing it in contact with oxygen in a high temperature furnace e.g. @1100oC.
• Oxide etching processes involve the removal of SiO2 layers from the surface of the silicon substrate usually using a photoresist mask to pattern the SiO2 layer. This can be done by wet etching, (by dipping the photoresist patterned substrate into buffered HF, where the acid will attack the exposed regions of SiO2 ) or by dry etching (where a chemical gas plasma will attack and etch the exposed SiO2 regions).
• Doping is the process of introducing dopant atoms into silicon for creating n and p type regions. Doping can be realized by diffusion or by ion implantation.
• The diffusion process involves bringing the dopant sources such as Phosphorous or Boron in close contact with the silicon substrate in a diffusion furnace at temperatures above 9000C. The dopant atoms will then diffuse into the exposed silicon regions whereas the SiO2 protected regions will not be doped. Typically the diffusion process is followed by a high temperature (1100oC) oxidation process to drive in the dopant atoms further into the substrate.
• The ion implantation process uses an ion implanter that accelerates the dopant atoms in vacuum such that when they encounter the silicon substrate they have enough energy to penetrate the unprotected regions of the silicon substrate. Subsequently a higher temperature annealing (600oC) step is required to activate the dopant atoms and restore the crystal quality of the silicon surface.
• Metallization is generally carried out by evaporating aluminum and depositing it on a silicon wafer. The process of evaporation is carried out in a vacuum chamber where an aluminum source is heated. The evaporating Aluminum atoms gain enough energy in vacuum to freely move in the chamber and attach themselves to cooler regions including the silicon substrates. The thin Aluminum layer can then be patterned using a photolithographic step.
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PHOTOLITHOGRAPHY
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PHOTOLITHOGRAPHY
The process of selectively removing
un-exposed (or exposed)
photoresist is called development.
The photoresist is patterned
During etching of the desired
material, patterned photoresist acts
as a mask to protect desired regions
of the material being etched
The pattern of the original mask is
transferred to the desired material
by means of photolithography
Using photolithography to pattern polysilicon with positive photoresist
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PHOTOLITHOGRAPHY
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6
Etching
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OXIDATION
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8
Doping by Diffusion or Ion Implantation
Diffusion is done at high temperatures 800-1200C while ion implantaion
can be done at low temperature 500-800C
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DEPOSITION
Metal deposition by evaporation
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EPITAXY
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IC processing loop
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Strip Field oxide Diffuse, oxidize open gate
Grow gate oxide, open contacts Metallize and pattern
STANDARD METAL GATE MOS TRANSISTOR
LAYOUT AND PROCESS
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Strip Field oxide Diffuse, oxidize open gate region
Grow gate oxide, open contacts Metallize and pattern
1 2
3 4
STANDARD METAL GATE MOS TRANSISTOR LAYOUT AND PROCESS
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The PMOS fabrication
• Around 1970, pMOS circuits with aluminum gate metal and wiring were dominant.
• The primary problem at the time was threshold voltage control. Positively charged ions in the
oxide decreased the threshold voltage of the devices. p-type MOSFETs were therefore the device
of choice despite the lower hole mobility, since they would still be enhancement-type devices even
when charge was present.
• Thermal oxidation of the silicon in an oxygen or water vapor atmosphere provided a quality gate
oxide with easily controlled thickness. The same process was also used to provide a high-
temperature mask for the diffusion process and a passivation and isolation layer. Some people
claim that the quality and versatility of silicon’s oxide made silicon the preferred semiconductor
over germanium.
• The oxide was easily removed in hydrofluoric acid (HF), without removing the underlying silicon,
thanks to the high selectivity if the etch
• Aluminum was evaporated over the whole wafer and then etched yielding both the gate metal and
the metal wiring connecting the devices. A small amount of copper (~2%) was added to make the
aluminum more resistant to electromigration. Electromigration is the movement of atoms due to
the impact with the electrons carrying the current through the wire. This effect can cause open
circuits and is therefore a well-known reliability problem.
• Annealing the metal in a nitrogen/hydrogen (N2/H2) ambient was used to improve the metal-
semiconductor contact and to reduce the surface state density at the semiconductor/gate-oxide
interface.
UCSD Esener 5”. 14
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SELF ALIGNED GATE MOS TRANSISTOR LAYOUT
AND PROCESS
Strip Field oxide Regrow gate oxide Deposit gate
Oxidize and open contact holes Metallize and pattern
Pattern gate and oxide
Diffuse
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SELF ALIGNED GATE MOS TRANSISTOR LAYOUT
AND PROCESS
Strip Field oxide & Regrow gate oxide Deposit gate & pattern , diffuse
Oxidize and open contact holes Metallize and pattern
1 2
3 4
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INVERTER LAYOUT
METAL GATE, POLY-GATE,
•Provides better noise margins
•Higher speed (small Cgs and NL pull up)
•More levels of interconnects-smaller area
•But requires one additional mask layer and poly deposition
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CMOS INVERTER LAYOUT
OutIn
VDD
PMOS
NMOS
CMOS circuits have a lower power dissipation and larger operating margin. It
was only when the number of transistors per chip became much larger that the
inherent advantages of CMOS circuits became clear.
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Silicide Silicide n+ Poly Gate oxide p+ Poly
Shallow Trench Isolation
n-well
STI
Source-drain
STI n + n+
p-doping
Source-drain extensions
p+ p+
n-doping
extensions
MODERN HP CMOS INVERTER LAYOUT
NMOS
PMOS POLY GATE
VDD GND
n-tub
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SIMPLIFIED CMOS FABRICATION
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Most changes were introduced to provide superior performance, better reliability and
higher yield.
Increasing the number of transistors:
• Reduction of the gate length. A gate length reduction provides a shorter transit time
and hence a faster device. This reduction is linked to a reduction of the minimum
feature size and therefore yields smaller transistors as well as a larger number of
transistors on a chip with a given size. I
• Making larger chips, so that the number of transistors per chip increased even faster.
• Increasing wafer size to accommodate the larger chips while reducing the loss due to
partial chips at the wafer periphery. Larger wafers further reduce the cost per chip as
more chips can be accommodated on a single wafer
Circuit Improvements
• Early on, the pMOS devices were replaced with nMOS transistors because of the
better electron mobility. Enhancement-mode loads were replaced by depletion-mode
loads yielding faster logic circuits with larger operating margins.
Manufacturing and circuit Improvements
UCSD Esener 5”. 21
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Process improvements can be split into those aimed at improving the circuit
performance and those improving the manufacturability and reliability.
• Self-aligned poly-silicon gate process was introduced before CMOS and marked
the beginning of modern day MOSFETs. The self-aligned structure, is obtained by
using the gate as the mask for the source-drain implant. Since the crystal damage
caused by the high-energy ions must be annealed at high temperature (~800 C), an
aluminum gate could no longer be used. Doped poly-silicon was found to be a very
convenient gate material since it withstands the high anneal temperature and can be
oxidized just like silicon.
• The self-aligned process lowers the parasitic capacitance between gate and drain
and therefore improves the high-frequency performance and switching time.
• Addition of a silicide layer on top of the gate reduces the gate resistance while still
providing a quality implant mask. The self-aligned process also reduced the transistor
size and hence increased the density.
• Local oxidation isolation structure (LOCOS), replaced the field oxide. A Si3N4 layer
is used to prevent the oxidation in the MOSFET region. This oxide provides an implant
mask and contact hole mask yielding an even more compact device.
Process Improvements aiming at
circuit performance I
UCSD Esener 5”. 22
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• Chemical vapor deposition (CVD) of insulating layers replaced thermal
oxidation since it does not consume the underlying silicon andbecause there is
no limit to the obtainable thickness since materials other than SiO2 (for instance
Si3N4) can be deposited.
• Ion implantation replaced diffusion because of its superior control and
uniformity.
• Dry etching including plasma etching, reactive ion etching (RIE) and ion beam
etching has replaced wet chemical etching. These etch processes provide better
etch rate uniformity and control as well as very pronounced anisotropic etching.
• Sputtering of metals has completely replaced evaporation. Sputtering typically
provides better adhesion and thickness control.
• Deuterium anneal is a recent modification of the standard hydrogen anneal,
which passivates the surface states. The use of deuterium therefore reduces the
increase of the surface state density due to hot-electron impact.
Process Improvements aimed at
manufacturability and reliability:
UCSD Esener 5”. 23
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TEM Cross-section
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N-well CMOS fab. process
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Typ. CMOS Fabrication steps I
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CMOS Fab. Steps II
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CMOS Fab. Steps III
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CMOS Fab. Steps IV
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Summary of CMOS Fabrication