1 Ion Implant Part 1 Chapter 17: Semiconductor Manufacturing Technology by M. Quirk & J. Serda Spring Semester 2014 Saroj Kumar Patra,, Norwegian University of Science and Technology ( NTNU )
2 Objectives 1. Explain the purpose and applications for doping in wafer fabrication. 2. Discuss the principles and process of dopant diffusion. 3. Provide an overview of ion implantation, including its advantages and disadvantages. 4. Discuss the importance of dose and range in ion implant. 5. List and describe the five major subsystems for an ion implanter. 6. Explain annealing and channeling in ion implantation. 7. Describe different application of ion implantation.
3 Doping Intrinsic silicon - resistivity: 250kOhm-cm Adding small amounts of impurities highly increases conductivity! Impurities contamination Form majority carriers (electrons or holes) in silicon devices Conductive layers in the wafer Alter material properties
4 Common dopants Acceptor Dopant Group IIIA (P-Type) Element Atomic Number Semiconductor Group IVA Element Atomic Number Table 17.1, p.476 Donor Dopant Group VA (N-Type) Element Atomic Number Boron (B) 5 Carbon 6 Nitrogen 7 Aluminum 13 Silicon (Si) 14 Phosphorus (P) 15 Gallium 31 Germanium 32 Arsenic (As) 33 Indium 49 Tin 50 Antimony 51
5 n-type and p-type doping n-type (figure 2.23, p.36) p-type (figure 2.25, p.37)
6 Thermal Diffusion vs Ion Implantation Two methods for introducing dopants to the wafer. Thermal diffusion uses high temperature to move the dopant through the lattice structure. Ion implantation: high-voltage ion bombardment. Today: Ion implantation is primary method Ion impl. lower temperature => less thermal stress Easier to control Diffusion is temperature and time dependent Diffusion used during crystal growing and other steps where we want to dope large surfaces.
7 CMOS Structure CMOS Structure with doped regions Figure 17.1 Green: Ion Implantation Blue: Diffusion Yellow: Ion Implantation or Diffusion
8 Doped regions Dopant gas Oxide Diffused region Oxide N p+ Silicon substrate
9 Process Flow
10 Diffusion Diffusion Principles Three steps Diffusion Process Wafer cleaning Predeposition Dopant sources Drive-in Activation Dopant movement Solid solubility Lateral diffusion
11 Dopant Diffusion in Silicon Si Si Si Si Si Si Dopant Si Si Si Si Si Vacancy Si Si Si Si Si Si a) Silicon lattice structure b) Substitutional diffusion Si Si Si Si Si Si Si Si Si Displaced silicon atom in interstitial site Dopant in interstitial site Si Si Si Si Si Si Si Si Si c) Mechanical interstitial displacement d) Interstitial diffusion
12 Solid Solubility Limits in Silicon at 1100 C Dopant Solubility Limit (atoms/cm 3 ) Arsenic (As) 1.7 x 10 21 Phosphorus (P) 1.1 x 10 21 Boron (B) 2.2 x 10 20 Antimony (Sb) 5.0 x 10 19 Aluminum (Al) 1.8 x 10 19 Table 17.3, p.480
13 Diffusion Process Eight Steps for Successful Diffusion: 1. Run qualification test to ensure the tool meets production quality criteria. 2. Verify wafer properties with a lot control system. 3. Download the process recipe with the desired diffusion parameters. 4. Set up the furnace, including a temperature profile. 5. Clean the wafers and dip in HF to remove native oxide. 6. Perform predeposition: load wafers into the deposition furnace and diffuse the dopant. 7. Perform drive-in: increase furnace temperature to drive-in and activate the dopant bonds, then unload the wafers. 8. Measure, evaluate and record junction depth and sheet resistivity.
14 Typical Dopant Sources for Diffusion Dopant Formula of Source Chemical Name Arsenic (As) AsH 3 Arsine (gas) Phosphorus (P) PH 3 Phosphine (gas) Phosphorus (P) POCl 3 Phosphorus oxychloride (liquid) Boron (B) B 2 H 6 Diborane (gas) Boron (B) BF 3 Boron tri-fluoride (gas) Boron (B) BBr 3 Boron tri-bromide (liquid) Antimony (Sb) SbCl 5 Antimony pentachloride (solid) Table 17.4, p.482
15 Dopant Sources 1 st reaction: B 2 H 6 (g) + 3O 2 (g) B 2 O 3 (s) + 3H 2 O (l) 2 nd reaction: 2B 2 O 3 (s) + 3Si (s) 4B (s) + 3SiO 2 (s)
16 Ion Implantation Overview Controlling Dopant Concentration Ion Implanter Advantages of Ion Implantation Disadvantages of Ion Implantation Dose Range (depth) Classes of Implanters Projected range Energy Loss of an Implanted Dopant Atom
17 Controlling Dopant Concentration and Depth Ion implanter Low energy Low dose Fast scan speed Ion implanter High energy High dose Slow scan speed Dopant ions Beam scan Beam scan Mask x j Mask Mask Mask x j Silicon substrate Silicon substrate a) Low dopant concentration (n, p ) and shallow junction (x j ) Figure 17.5 b) High dopant concentration (n +, p + ) and deep junction (x j )
18 General Schematic of an Ion Implanter Ion source Plasma Extraction assembly Analyzing magnet Ion beam Acceleratio n column Process chamber Scaning disk
19 Ion Implanter Photograph courtesy of Varian Semiconductor, VIISion 80 Source/Terminal side
20 Advantages: 1. Precise Control of Dopant Concentration 2. Good Dopant Uniformity 3. Good Control of Dopant Penetration Depth 4. Produces a Pure Beam of Ions 5. Low Temperature Processing 6. Ability to Implant Dopants Through Films 7. No Solid Solubility Limit Disadvantages: 1. Radiation damage 2. Complex equipment
21 Ion Implant Parameters Dose (Q): Number of implanted ions per unit area Where, Q = dose in atom/cm 2 I = beam current in coulombs/sec (amperes) t = implant time in seconds e= electronic charge n= charge per ion (for instance, B + is equal to 1) A= area implanted in cm 2 Range: Total distance ion travels in the silicon during ion implantation. Higher imlanter energy increased range
22 Classes of Implanters Class of Implanter System Description and Applications Highly pure beam currents <10 ma. Beam energy is usually < 180 kev. Medium Current Most often the ion beam is stationary and the wafer is scanned. Specialized applications of punchthrough stops. Generate beam currents > 10 ma and up to 25 ma for high dose implants. Beam energy is usually <120 kev. High Current Most often the wafer is stationary and the ion beam does the scanning. Ultralow-energy beams (<4keV down to 200 ev) for implanting ultrashallow source/drain junctions. Beam energy exceeds 200 kev up to several MeV. High Energy Place dopants beneath a trench or thick oxide layer. Able to form retrograde wells and buried layers. Oxygen Ion Implanters Class of high current systems used to implant oxygen in siliconon-insulator (SOI) applications. Table 17.6
23 Projected Range of Dopant Ion Incident ion beam Silicon substrate Stopping point for a single ion R p R p dopant distributio n
24 Projected Range Chart 1.0 Implanting into Silicon Projected Range, R p ( m) 0.1 B P As Sb 0.01 10 100 1,000 Implantation Energy (kev) Figure 17.8 Redrawn from B.El-Kareh, Fundamentals of Semiconductor Processing Technologies, (Boston: Kluwer, 1995), p. 388
25 Energy Loss of an Implanted Dopant Atom Electronic collision Energetic dopant ion Silicon crystal lattice Si Si Si Si Si X-rays S i Si Si Si Si Si Si Atomic collision Si Si Si Si Si Displaced Si atom Si Si Si Si Si Si Si
26 Crystal Damage Due to Light and Heavy Ions Light ion impact Heavy ion impact Figure 17.10
27 Thank You