Lecture 2 ic fabrication processing & wafer preparation


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Lecture 2 ic fabrication processing & wafer preparation

  1. 1. ECE614: Device Modelling and Circuit Simulation Unit 1 IC Fabrication Processing & Wafer preparation By Dr. Ghanshyam Singh
  2. 2. Objectives After studying the material in this unit, you will be able to: 1. Describe how raw silicon is refined into semiconductor grade silicon. 2. Explain the wafer fabrication method for producing monocrystal silicon. 3. Discuss the basic transistor behaviour. 4. Outline and describe the basic process steps for wafer preparation, starting from a silicon ingot and finishing with a wafer. 6. Explain what is Latch-up and how to avoid it in fabrication.
  3. 3. Topics • Basic fabrication steps. • Transistor structures. • Basic transistor behavior. • Latch up.
  4. 4. Wafers • A wafer is a thin slice of semiconducting material, such as a silicon crystal, upon which microcircuits are constructed by doping (for example, diffusion or ion implantation, etching, and deposition of various materials. • Wafers are cut out of silicon boules • A boule is a single crystal silicone from which wafers are cut using diamond saws.
  5. 5. Fabrication Process • Once the wafers are prepared, many process steps are necessary to produce the desired semiconductor integrated circuit. In general, the steps can be grouped into four areas: • •Front end processing (formation of transistors on silicon wafers) • •Back end processing (interconnection of transistors by metal wires) • •Test • •Packaging • In semiconductor device fabrication, the various processing steps fall into four general categories: deposition, removal, patterning, and modification of electrical properties.
  6. 6. Deposition • Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Available technologies consist of physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among others.
  7. 7. Removal or Etching Process • Removal processes are any that remove material from the wafer either in bulk or selective form and consist primarily of etch processes, both wet etching and dry etching such as reactive ion etch (RIE). Chemical mechanical planarization (CMP) is also a removal process used between levels.
  8. 8. Masking and Patterning • Patterning covers the series of processes that shape or alter the existing shape of the deposited materials and is generally referred to as lithography. For example, in conventional lithography, the wafer is coated with a chemical called a photoresist. The photoresist is exposed by a stepper, a machine that focuses, aligns, and moves the mask, exposing select portions of the wafer to short wavelength light. The unexposed regions are washed away by a developer solution. After etching or other processing, the remaining photoresist is removed by plasma ashing. • Many modern chips have eight or more levels produced in over 300 sequenced processing steps.
  9. 9. Fabrication processes • IC built on silicon substrate (mono crystal silicone): – some structures diffused into substrate; – other structures built on top of substrate. • Substrate regions are doped with n-type and p-type impurities. (n+,p+ = heavily doped) • When silicon is doped, n-type impurities (5-valence electron elements such as arsenic) charge silicon atoms with electrons, p-type impurities (3- valence electrons such as boron) charge them with holes • Wires made of polycrystalline silicon (poly), and/or multiple layers of aluminum (metal). • Silicon dioxide (SiO2) is insulator. (is grown over Si by heating Si in a pure oxygen or water vapor atmosphere)
  10. 10. Simple cross section substrate n+ n+p+ substrate metal1 poly SiO2(insulator) metal2 metal3 transistor via
  11. 11. Photolithography Mask patterns are put on wafer using photo- sensitive material: A typical wafer is made out of extremely pure silicon that is grown into mono-crystalline cylindrical ingots (boules) up to 12 in (300 mm) in diameter using the Czochralski process. These ingots are then sliced into wafers about 0.75 mm thick and polished to obtain a very regular and flat surface.
  12. 12. Process steps First place tubs to provide properly-doped substrate for n-type, p-type transistors: (Front- end processing) p-tub n-tub substrate
  13. 13. Process steps, cont’d. Pattern polysilicon before diffusion regions: p-tub p-tub poly polygate oxide
  14. 14. Process steps, cont’d Add diffusions, performing self-masking: p-tub p-tub poly poly n+n+ p+ p+
  15. 15. Process steps, cont’d Start adding metal layers: (Backend processing) p-tub n-tub poly poly n+n+ p+ p+ metal 1 metal 1 vias
  16. 16. Transistor structure n-type transistor:
  17. 17. 0.25 micron transistor (Bell Labs) poly silicide source/drain gate oxide
  18. 18. Transistor layout n-type (tubs may vary): w L
  19. 19. Electrical Transistor Model • Vgs: gate to source voltage • Vds: drain to source voltage • Ids: current flowing between drain and source • k’: transconductance > 0 • Vt: threshold voltage > 0 for n-type <0 for p- type transistor. • W/L: width to length ratio
  20. 20. Drain current characteristics
  21. 21. Drain current • Linear region (Vds < Vgs - Vt): – Id = k’ (W/L)[(Vgs - Vt)Vds - 0.5Vds 2] – Not quite a linear relation between Id and Vds but the quadratic term becomes more negligible than the linear term as Vds approaches 0. This is typically the case with the absolute value of the threshold Vt voltage remaining close to 0. • Saturation region (Vds >= Vgs - Vt): – Id = 0.5k’ (W/L)(Vgs - Vt) 2 – Id remains constant over changes in Vds – Increases with transconductance, channel width, and decreases with channel length.
  22. 22. 0.5 µm transconductances From a MOSIS process: • n-type: – kn’ = 73 µA/V2 – Vtn = 0.7 V • p-type: – kp’ = 21 µA/V2 – Vtp = -0.8 V
  23. 23. Current through a transistor (At saturation) Example: Using 0.5 µm transconductance parameter of 73 µA/V2, threshold voltage of 0.7 volts, and SCMOS rules (http://www.mosis.com/Technical/Designrules/scmos/scmos- main.html) with W 3λ, L = 2 λ: • Saturation current at Vgs = 2V: Id = 0.5k’(W/L)(Vgs-Vt)2= 93 µA • Saturation current at Vgs = 5V: Id = 1012 µA ~ 1 mA
  24. 24. Basic transistor parasitics • There are myriad parasitics and parasitics models. The ones considered here are the most widely-encountered parasitics. • Gate to substrate, also gate to source/drain. • Source/drain capacitance, resistance. Cg substrate
  25. 25. Basic transistor parasitics, cont’d • Gate capacitance Cg. Determined by active area. • Source/drain overlap capacitances Cgs, Cgd. Determined by source/gate and drain/gate overlaps. Independent of transistor L. – Cgs = Col W (Col is the unit overlap capacitance per µm2, For small channel length, Col might indirectly depend on L.) – Gate/bulk overlap capacitance.
  26. 26. Latch-up • CMOS ICs have built-in undesirable parasitic silicon-controlled rectifiers (SCRs). • When powered up, SCRs can turn on, creating low-resistance path from power to ground. Current can destroy chip. • Early CMOS problem. Can be solved with proper circuit/layout structures.
  27. 27. Silicon Controlled Rectifier(SCR) Tyristor Circuit • In normal mode, no current flows over the pnpn path when the middle pn junction is reverse-biased. With the help of a gate pulse voltage, this pn junction can be forced into its breakdown region, making it conduct current. At that point, there will be a path of current from the anode to the cathode with no resistance even after the gate voltage is withdrawn. This is the basis for a high current from VDD to the ground (substrate) in MOS transistors, called the latch up. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. p p n n anode cathode gate FB FB RB
  28. 28. Parasitic SCR circuit I-V behavior V Reverse voltage breakdown Rs and Rw control the bias voltage on the green diodes FB Breakdown FB
  29. 29. Parasitic SCR structure n p p n n p Solution: connect the n-tub to the VDD When transistor on the right conducts, it turns on the transistor on the left, and this in turn forces the first transistor to draw more current, establishing a positive feedback loop.
  30. 30. Solution to latch-up Use tub ties to connect tub to power rail. Use enough to create low-voltage connection. Doping the tub at the point of contact reduces the resistance of contact, and this makes it more difficult for bipolar transistor to turn on.
  31. 31. Tub tie layout metal (VDD) n-tub n+ You can learn more about latch up by downloading the article at http://www.fairchildsemi.com/an/AN/AN-339.pdf#search=%22latch%20up%20problem%22