DIGITAL ELECTRONICS SUBJECT CODE (66834)

1. DIGITAL CONTENT SUBJECT: DIGITAL ELECTRONICS SUBJECT CODE (66834)IQBAL HOSSAIN JUNIOR INSTRUCTOR ELECTRONICS DEPARTMENTMUNSHIGONJ POLYTECHNIC INSTITUTE

2. DIGITAL ELECTRONICS SUBJECT CODE (66834) • Fabrication, and Elementary Logic Design Fabrication and Layout

3. Introduction • VLSI - this is an acronym • Means Very Large Scale Integration or Very Large Scale Integrated Circuits Comes a question in mind- • What is a circuit ? • what is an integrated circuit? VLSI: Lecture 1

4. Introduction Contd. When you say circuit, what comes to your mind? • A printed circuit board with different components soldered on to that - capacitors, transistors, diodes, resistors, connecting wires, yes? • This is a discrete circuit; a discrete circuit that is where I have discrete components, right, discrete components - capacitors resistors, transistors, like that. • In contrast, in an integrated circuit, the entire circuitry that is the active and passive elements, everything is housed on the same substrate. • active elements I mean the transistors and passive elements means resistors, capacitors etc VLSI: Lecture 1

5. Supplementary Info. VLSI: Lecture 1 The elements within a circuit will either control the flow of electric energy or respond to it. Those elements which control the flow of electric energy are known as active elements and those which dissipate or store the electric energy are passive elements.""The three linear passive elements are the Resistor, the Capacitor and the Inductor. Examples of non-linear passive devices would be diodes, switches and spark gaps. Examples of active devices are Transistors, Triacs, Varistors, Vacuum Tubes, relays, solenoids and piezo electric devices."

6. Introduction Contd. VLSI: Lecture 1 Example : 4 mega bit dynamic RAM chip, 4 mega bit that is there are 4 million transistors housed in this small bit Depending on the complexity of the integrated circuit IC can be classified as- small scale integrated circuit or SSI (10 to 100 transistors ) medium scale integrated circuit or MSI (Hundreds to thousands ) large scale integrated circuit or LSI (more than ten thousand LSI ) very large scale integrated circuit or VLSI (Million of transistors)

7. Introduction • Integrated circuits: many transistors on one chip. • Very Large Scale Integration (VLSI) • Metal Oxide Semiconductor (MOS) transistor • Fast, cheap, low-power transistors • Complementary: mixture of n- and p-type leads to less power • Today: How to build your own simple CMOS chip • CMOS transistors • Building logic gates from transistors • Transistor layout and fabrication Fabrication and Layout

8. Silicon Lattice • Transistors are built on a silicon substrate • Silicon is a Group IV material • Forms crystal lattice with bonds to four neighbors Fabrication and Layout

9. Dopants • Silicon is a semiconductor • Pure silicon has no free carriers and conducts poorly • Adding dopants increases the conductivity • Group V: extra electron (n-type) • Group III: missing electron, called hole (p-type) Fabrication and Layout

10. p-n Junctions • A junction between p-type and n-type semiconductor forms a diode. • Current flows only in one direction Fabrication and Layout

11. nMOS Transistor • Four terminals: gate, source, drain, body • Gate – oxide – body stack looks like a capacitor • Gate and body are conductors • SiO2 (oxide) is a very good insulator • Called metal – oxide – semiconductor (MOS) capacitor • Even though gate is no longer made of metal Fabrication and Layout

12. nMOS Operation • Body is commonly tied to ground (0 V) • When the gate is at a low voltage: • P-type body is at low voltage • Source-body and drain-body diodes are OFF • No current flows, transistor is OFF Fabrication and Layout

13. nMOS Operation • When the gate is at a high voltage: • Positive charge on gate of MOS capacitor • Negative charge attracted to body • Inverts a channel under gate to n-type • Now current can flow through n-type silicon from source through channel to drain, transistor is ON Fabrication and Layout

14. pMOS Transistor • Similar, but doping and voltages reversed • Body tied to high voltage (VDD) • Gate low: transistor ON • Gate high: transistor OFF • Bubble indicates inverted behavior Fabrication and Layout

15. Power Supply Voltage • GND = 0 V • In 1980’s, VDD = 5V • VDD has decreased in modern processes • High VDD would damage modern tiny transistors • Lower VDD saves power • VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, … Fabrication and Layout

16. Transistors as Switches • We can view MOS transistors as electrically controlled switches • Voltage at gate controls path from source to drain Fabrication and Layout

17. CMOS Inverter Fabrication and Layout

18. CMOS Inverter Fabrication and Layout

19. CMOS Inverter Fabrication and Layout

20. CMOS NAND Gate Fabrication and Layout

21. CMOS NAND Gate Fabrication and Layout

22. CMOS NAND Gate Fabrication and Layout

23. CMOS NAND Gate Fabrication and Layout

24. CMOS NAND Gate Fabrication and Layout

25. CMOS NOR Gate Fabrication and Layout

26. 3-input NAND Gate • Y pulls low if ALL inputs are 1 • Y pulls high if ANY input is 0 Fabrication and Layout

27. 3-input NAND Gate • Y pulls low if ALL inputs are 1 • Y pulls high if ANY input is 0 Fabrication and Layout

28. CMOS Fabrication • CMOS transistors are fabricated on silicon wafer • Lithography process similar to printing press • On each step, different materials are deposited or etched • Easiest to understand by viewing both top and cross-section of wafer in a simplified manufacturing process Fabrication and Layout

29. Inverter Cross-section • Typically use p-type substrate for nMOS transistor • Requires n-well for body of pMOS transistors • Several alternatives: SOI, twin-tub, etc. Fabrication and Layout

30. Well and Substrate Taps • Substrate must be tied to GND and n-well to VDD • Metal to lightly-doped semiconductor forms poor connection called Shottky Diode • Use heavily doped well and substrate contacts / taps Fabrication and Layout

31. Inverter Mask Set • Transistors and wires are defined by masks • Cross-section taken along dashed line Fabrication and Layout

32. Detailed Mask Views • Six masks • n-well • Polysilicon • n+ diffusion • p+ diffusion • Contact • Metal Fabrication and Layout

33. Fabrication Steps • Start with blank wafer • Build inverter from the bottom up • First step will be to form the n-well • Cover wafer with protective layer of SiO2 (oxide) • Remove layer where n-well should be built • Implant or diffuse n dopants into exposed wafer • Strip off SiO2 Fabrication and Layout

34. Oxidation • Grow SiO2 on top of Si wafer • 900 – 1200 C with H2O or O2 in oxidation furnace Fabrication and Layout

35. Photoresist • Spin on photoresist • Photoresist is a light-sensitive organic polymer • Softens where exposed to light Fabrication and Layout

36. Positive and Negative Photoresist • There are two types of photoresist: positive and negative. For positive resists, the resist is exposed with UV light wherever the underlying material is to be removed. In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. In other words, "whatever shows, goes." The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer. Fabrication and Layout

37. Positive and Negative Photoresist • Negative resists behave in just the opposite manner. Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. Masks used for negative photoresists, therefore, contain the inverse (or photographic "negative") of the pattern to be transferred. The figure below shows the pattern differences generated from the use of positive and negative resist. Fabrication and Layout

38. Positive and Negative Photoresist Fabrication and Layout

39. Lithography • Expose photoresist through n-well mask • Strip off exposed photoresist Fabrication and Layout

40. Etch • Etch oxide with hydrofluoric acid (HF) • Seeps through skin and eats bone; nasty stuff!!! • Only attacks oxide where resist has been exposed Fabrication and Layout

41. Strip Photoresist • Strip off remaining photoresist • Use mixture of acids called piranah etch • Necessary so resist doesn’t melt in next step Fabrication and Layout

42. n-well • n-well is formed with diffusion or ion implantation • Diffusion • Place wafer in furnace with arsenic gas • Heat until As atoms diffuse into exposed Si • Ion Implanatation • Blast wafer with beam of As ions • Ions blocked by SiO2, only enter exposed Si Fabrication and Layout

43. Strip Oxide • Strip off the remaining oxide using HF • Back to bare wafer with n-well • Subsequent steps involve similar series of steps Fabrication and Layout

44. Thin Oxide Layer • The thin oxide is grown in a furnace Fabrication and Layout

45. Polysilicon • Deposit very thin layer of gate oxide • < 20 Å (6-7 atomic layers) • Chemical Vapor Deposition (CVD) of silicon layer • Place wafer in furnace with Silane gas (SiH4) • Forms many small crystals called polysilicon • Heavily doped to be good conductor Fabrication and Layout

46. Polysilicon Patterning • Use same lithography process to pattern polysilicon Fabrication and Layout

47. Self-Aligned Process • Use oxide and masking to expose where n+ dopants should be diffused or implanted • N-diffusion forms nMOS source, drain, and n-well contact Fabrication and Layout

48. N-diffusion • Pattern oxide and form n+ regions • Self-aligned process where gate blocks diffusion • Polysilicon is better than metal for self-aligned gates because it doesn’t melt during later processing Fabrication and Layout

49. N-diffusion • Historically dopants were diffused • Usually ion implantation today • But regions are still called diffusion Fabrication and Layout

50. N-diffusion • Strip off oxide to complete patterning step Fabrication and Layout