Microelectronics Technology

1. Microelectronics Technology Microelectronics Technology Mustafa Arikan University of Iceland

2. Contact info • Mustafa Arıkan (Musti) • arikan@raunvis.hi.is ; mustafa.arikan@gmail.com • Tel : 525-4751 (Ingvarsson Lab., VR-III) • Office hours ???

3. In this course… • Two parts: • Semiconductor processing (from raw material to microelectronic components) • Semicondcutor characterization methods (physical & electrical-optical) • Lectures & Labs • Two lectures on 07.02.2008 and 07.03.2008 • Two labs in two groups on 14.02, 21.02 and 14.03, 21.03.2008

4. Goal of this lectures… • Overview of the fundamentals of microelectronics technology • Fast & quick • The tools we employ to produce and characterize electronic components • Complexity and beauty of the technology • Desired outcome • Understanding of whole process • Big picture • Different approaches

5. What is microelectronics? What is it about? • Microelectronics is a subfield of electronics • study and manufacture of electronic components which are very small (i.e. transistors, diodes…) • Semiconductors , metals, organic & plastic

6. Real small…and impressive…

7. But very complex sometime…

8. What takes to achieve it?

9. What takes to achieve it?

10. Different approaches

11. The basics of semiconductor device fabrication • Proper material for the purpose • Geometry • Material growth and removal (over and over again) by the help of lithography

12. Simple example : MESFET • Metal-Semiconductor Field Effect Transistor

13. MESFET fabrication & The idea of lithography • A real device from substrate to final form • MESFET is relatively simple but not all the devices can be fabricated this easily • Inverter fabrication

14. CMOS Inverter

15. Fabrication of a cmos inverter : Silicon technology • Includes many steps • Many different tools & technologies • Crystal (substrate) growth • Oxidation • Diffusion & implantation • Material growth (metal evaporation, sputtering, vapor deposition, epitaxy) • Lithography & etching

16. We need a substrate ! • How do we get single crystalline Si? • Czochralski • Majority of the wafers • Floating zone (high purity) • High purity – low oxygen & carbon impurity • More complex w.r.t. Czochralski • Bridgman • Easy (melting & cooling) • Low quality • Drip melting, strain annealing and others

17. Czochralski growth

18. Ingot by Czochralski method

19. Czochralski growth • Typically used for Silicon but also • Single crystal semiconductors (Si, Ge, GaAs) • Metals (Pd, Pt, Ag, Au) • Salts etc… • Requires seed crystal • Fast (1-2 mm/min) • Oxygen contamination from crucible • Uniformity of axial resistivity is poor • Segregation problems for dopants

20. We have Si substrate… Next… Let’s focus on individual steps and technologies from now on

21. Oxidation • CVD – LPCVD (chemical vapor deposition (film growth) • Thermally grown oxide (Oxidation) • Photoresist (Lithography & etching)

22. Oxidation • One of the two main advantages of Si • Ge is superior to Si (mobility, power consumption) • SiGe (MOSFET channel), Gd2O3 • Dry oxidation : Si + O2 SiO2 • Wet oxidation : Si + 2H2O  SiO2 + 2H2 • oxygen must diffuse through the oxide to react at the Si/SiO2 interface, so rate depends on the thickness of the oxide and reduces as the oxidation progresses.

23. Oxidation • thermal oxidation is performed in furnaces at temperatures between 800 and 1200°C • Many wafers on the boat (a quartz rack) at the same time • Variants : RTO

24. Oxidation : dry vs. wet • Dry (molecular oxygen) : better oxide but slow (gate oxide) • Wet (steam – water vapor) : fast but porous (isolation) • Deal-Grove model : thickness vs. time - theory

25. Oxidation • Thickness vs. time – practice : Charts !

26. Oxidation

27. Lithography & Pattern Transfer • Used for pattern transfer into metals, oxides and semiconductors • Thin film deposition and lithography (including photo and e-beam, wet etching and lift-off) are the most frequently used method in our labs • 2 types of resists: • Positive : PR pattern is same as mask. On exposure to light, light degrades the polymers resulting in the photoresist being more soluble in developers. The PR can be removed in inexpensive solvents such as acetone. • Negative : PR pattern is the inverse of the mask. On exposure to light, light polymerizes the rubbers in the photoresist to strengthen it’s resistance to dissolution in the developer

28. Lithography & Pattern Transfer • Black areas (PR) are the openings after development of PR

29. Lithography & Pattern Transfer • How do we perform this “lithography” thing? • Dehydration bake or pre-bake • Adhesion promoter (i.e. HMDS) • Apply resist – spinner • Soft bake • UV-exposure with mask • Post-bake • Post processing such as development & etching & lift-off • Other processes required by specific needs (MEMS)

30. Lithography & Pattern Transfer • Baking • spinner

31. Lithography & Pattern Transfer • Expose • Develop

32. Lithography & Pattern Transfer : Uses of lithography • Etching Processes: open windows in oxides for diffusion, masks for ion implantation, etching, metal contact to the semiconductor, or interconnect.

33. Lithography & Pattern Transfer • Lift off Processes: Metalization

34. Lithography & Pattern Transfer • Issues with photolithography • Resolution : feature size (~0.5 micron usually) • Shorter wavelength = better resolution • Registration : alignment of different layers on the same wafer (~ 1/3 of the resolution or 0.06 micron) • Throughput : effective cost and time • Resist thickness ~ 1/spin speed

35. Lithography & Pattern Transfer • Photolithography systems

36. Lithography & Pattern Transfer • Contact  Resist is in contact with the mask: 1:1 magnification • Inepensive, relatively high resolution (~ 0.5 micron), contact with the mask (scratches, particles and dirt are imaged in the wafer) • Proximity Resist is almost but not in contact with the mask: 1:1 magnification • Inexpensive, low resolution (~ 1-2micron), diffraction effects limit accuracy of pattern transfer. Less repeatable than contact methods, • Projection Mask image is projected a distance from the mask and de-magnified to a smaller image: 1:4 -1:10magnification • Can be very high resolution (~0.07 um or slightly better), No mask contact results in almost no mask wear (high production compatible), mask defects or particles on mask are reduced in size on the wafer. Extremely expensive and complicated equipment, Diffraction effects limit accuracy of pattern transfer

37. Lithography & Pattern Transfer

38. Lithography & Pattern Transfer : Light sources • Typically mercury (Hg)- Xenon (Xe) vapor bulbs are used as a light source in visible (>420 nm) and ultraviolet (>250-300 nm and <420 nm) lithography equipment • Lasers are used to increase resolution, and decrease the optical complexity for deep ultraviolet (DUV) lithography systems. Excited dimer (Excimer or Exiplex) pulsed lasers are typically used. These are powerful, extremely expensive to purchase and maintain, optically noisy lasers. • Alternative approaches such as: Nano-imprint, soft, dip-pen, e-beam, FIB, x-ray lithography : Very active research field!

39. Lithography & Pattern Transfer : some examples • Pictures for good and bad lithography

40. Oxidation • Chemical vapor deposition CVD – LPCVD (film growth) • Thermally grown oxide (Oxidation) • Photoresist (Lithography & etching)

41. Diffusion & Implantation • Dopants for N+ and P+ regions (implantation & diffusion)

42. Diffusion & Implantation

43. Diffusion & Implantation

44. Diffusion & Implantation

45. Diffusion & Implantation • What is diffusion? • Diffusion is the spontaneous net movement of particles from an area of high concentration to an area of low concentration (particle penetration from surface into the wafer) • Commonly used for • Bipolar technology (base, emitters) • FET (source, drain) • Use when • Ion implantation damage is not acceptable • Deep junctions are needed • Cheap & easy solutions are seeked • Don’t use for • Ultra-shallow junctions • Forming channel in MOSFET

46. Diffusion & Implantation : Types of diffusion • Instertital • Vacancy • Interstitialcy • Kick-out • Dissociative

47. Diffusion & Implantation • Diffusion equation (derived from Fick’s Law): • Different solution for different approximations • Best solution for an experimentalist: Charts (again!)

48. Diffusion & Implantation • Diffusion depends on: • Diffusion time • Diffusion constant (diffusivity) • Material density • Temperature

49. Diffusion & Implantation • Ion implantation : • Ions (charged atoms or molecules) are created via an enormous electric field stripping away an electron. • These ions are filtered and accelerated toward a target wafer, where they are buried in the wafer. • The depth of the implantation depends on the acceleration energy (voltage). • The dose is very carefully controlled by integrating the measured ion current.

50. Diffusion & Implantation