Chap'3 Representative Processes

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Chap.3 Representative Processes

• 3.1 Standard Bipolar

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1. Essential Features

• Optimized for NPN, at the expense of PNP
• But, PNPs were still built from existing steps
  does the job, even though relatively poor
  performance.
• NPN
• JI Junction isolation isolate current flow
  between this and other devices P isolation
• JI-P touches underlying substrate also defines
  N Tank
• JI P side is lower potential than N side
  (reverse bias)

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2. Fab Sequence

• Starting Material
• Lightly doped (111) Si P-type
• Off-cut a few degrees to minimize NBL shadow
  distortion
• (111) suppresses parasitic PMOS (N-epi is the
  Backgate of this PMOS, metal-1 over the FOX is
  Gate, P-base is the Source, and P Iso is the
  Drain. Vt thick field threshold)
• (111) has high Thick-Field Threshold due to
  positive Qit, at Oxide-Si interface.

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2. Fab Sequence, contd

• N-Buried Layer
• NBL mask on oxide
• NBL implant (As or Sb)
• NBL drive (anneal thin oxide on top later
  cause NBL shadow)
• Epi Growth
• N-type epi, 10-25 mm thick
• NBL shadow propagates at 45 degree angle (makes
  lateral shift epi thickness)

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2. Fab Sequence, contd

• Isolation Diffusion
• Mask aligned at offset from NBL shadow
• Heavy B deposition
• High Temp drive (also causes oxidation), which
  stops before Iso junction touches substrate

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2. Fab Sequence, contd

• Deep N sinker
• Deep-N mask
• Heavy Phosphorus deposition
• High Temp drive, causes deep N to meet NBL (25
  overdrive), forms thick F.Oxide

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2. Fab Sequence, contd

• Base Implant
• Base mask light Boron implant forms p-type Base
• Implant precise doping level minimizes
  process derived Beta variation !
• Base drive anneal decides junction depth
• Base implant covers Isolation regions too (BOI
  base-over-isolation) gt increases Thick field Vt

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2. Fab Sequence, contd

• Emitter Diffusion
• Emitter mask concentrated P source (POCl3)
  brief drive
• Contact
• Contact mask oxide removal contact OR
• Metallization
• Evaporate or sputter Al-Cu-Si alloy over entire
  wafer, at least 1 mm thick.
• 2 Si suppress the Emitter punchthrough
• 0.5 Cu improves Electromigration resistance
• metal mask ? interconnection wires

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2. Fab Sequence, contd

• Protective Overcoat (PO)
• Compressed Nitride or phophosilicate-doped glass
  (PSG) PO over entire wafer
• PO mask to open bonding areas

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3. Available Devices

• NPN
• CN-epi tank BPbase, counter doping EN
  diffusion
• Base width set by diffusion depths lt min.
  feature size
• NBL and deep-N low Collector resistance
  (minimum NPN lt 100W, power NPN lt 1W)
• Distance from bottom of Base to top of NBL sets
  max. op. voltage (50-80V typical). Epi thickness
  sets Vceo 10V to 100V

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3. Available Devices, contd

• NPN , contd
• Typical Parameter values of minimum-emitter NPN
• Drawn emitter area 100 mm2
• Peak Beta 150
• VA, Early volt. 120V
• Collector resistance (sat) 100W
• IC range for max. Beta 5mA 2 mA
• Vebo 7 V
• Vcbo 60V
• Vceo 45V

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3. Available Devices, contd

• NPN , contd
• NPN as Diode
• CB-shorted Diode or diode-connected transistor
• Breakdown voltage Vebo 7V
• Fast switching speed
• Can be used as Zener Diode also (allow - .3V Vz
  tolerance)

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3. Available Devices, contd

• PNP
• Substrate PNP
• Non-isolated vertical PNP
• Vc Vsubstrate ! usually negative supply rail
• Base N-tank Emitter Pbase diffusion
• Ic exits from Substrate gt substrate debiasing !
• Base width Epi thickness Emitter (Pbase)
  diffusion
• Do not use NBL

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3. Available Devices, contd

• Typical parameters in 40V Stand.Bipolar
• Device Lat_PNP Sub_PNP
• Drawn emitter area 100mm2 100mm2
• Drawn base width 10mm N/A
• Peak Beta 50 100
• VA 100V 120V
• IC for max. Beta 5-100mA 5-200mA
• Vebo 60V 60V
• Vcbo 60V 60V
• Vceo 45V 45V

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3. Available Devices, contd

• Lateral PNP
• Better isolation, worse performance than
  Substrate PNP
• BN-tank
• E, C Pbase diffusion into N-tank single mask
• Base width separation of E C (Pbase)
  diffusions
• Effective Base width ltlt drawn width due to
  outdiffusion
• Narrow-base lateral PNP low VA and low
  punchthrough V.
• Parasitic substrate_PNP
• Much current near surface where large Recomb.
  Centers reside
• Very slow due to large parasitic junction cap at
  B terminal
• PNP supporting role in analog ICs

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3. Available Devices, contd

• Resistors (several types)
• Ohms per square, Sheet resistance. Typical values
  5 to 5000 W/sq.
• Base resistor N-tank, typical 150-250 W/sq.
• Emitter resistor in Emitter diffusion, isolated
  by Pbase diffusion. Typical lt 10 W/sq.
  differential voltage lt 6V
• Pinched Base resistor R body in Pbase, but under
  the Emitter diffusion. Can exceed 5000W/sq
  (notoriously variable). Severe voltage
  modulation. Differential voltage lt 7V.

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3. Available Devices, contd

• Resistor Parameters
• Type Emitter Base Pinch
• Rs (Sheet R) 5 W/sq. 150W/sq. 3kW/s
• Min.dr.Width 8mm 8mm 8mm
• Vbr 7V 50V 7V
• Variability(15mm) 20 20 50 or more

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3. Available Devices, contd

• Capacitors (one type)
• Base-Emitter Junction Capacitance, 0.8 fF/mm2
• Base diff. overlap Emitter diffusion, both in
  common tank
• Vbr 7V
• Depends on bias, varies - 50 or more
• Used in compensating feedback loops where high
  Cap is needed.

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4. Process Extension

• Up-Down Isolation
• Double-level Metal
• Schottky Diode
• High Sheet Resistors