Chapter 3 CMOS processing technology (II) Twin-tub CMOS process 1. Provide separate optimization of the n-type and p-type transistors 2. Make it possible to optimize "Vt", "Body effect", and the "Gain" of n, p devices, independently. 3. Steps: A. Starting material: an n+ or p+ substrate with lightly doped -> "epitaxial" or "epi" layer -> to protect "latch up" B. Epitaxy" a. Grow high-purity silicon layers of controlled thickness b. With accurately determined dopant concentrations c. Electrical properties are determined by the dopant and its concentration in Si C. Process sequence a. Tub formation b. Thin-Oxide construction c. Source & drain implantations d. Contact cut definition e. Metallization Balanced performance of n and p devices can be constructed. (Substrate contacts are included in Fig.3.10) 2003/3/12 CMOS Process (II) 1
(7~8um) 青玉 or SiO2 ( 二氧化矽 ) Anisotropic Etch Form p-island (for n-device) Form n-island (for p-device) 2003/3/12 CMOS Process (II) 2
- Grow gate oxide through thermal oxidation - Deposit Doped Polysilicon Etch Polysilicon Step (h): n-implantation for source & drain Step (i) p-implantation Step (j) - Grow phosphorus glass - Etch glass to form contact cut - Evaporating alumni 2003/3/12 CMOS Process (II) 3
3.3 CMOS Process Enhancement (Interconnection) 3.3.1 Metal Interconnect * CMOS circuit = CMOS logic process + Signal/Power/Clock-routing layers - Second-layer of metal (VIA1=M1 to M2) - Note: M1 must be involved in any contact to underlying areas Contact Etch Isolation layer Form a VIA (polysilicon, diffusion) - Process steps for two-metal process (Omitted) 3.3.1.2 Poly Interconnect - Polysilicon layer is commonly used as interconnection of signals. - Reduce resistance of polysilicon to make long-distance interconnection - Combine polysilicon with a refractory metal (Silicon + Tantalum) 2003/3/12 CMOS Process (II) 4
Ω=20-40Ω/square Ω=1-5Ω/square Make long-distance 3.3.1.3 Local Interconnection (for interconnect) - Local Interconnection allow a direct connection between ploysilicon and diffusion, alleviating the need for area-intensive contacts and metal - Example: Use of Local Interconnect in SRAM (save 25%) 2003/3/12 CMOS Process (II) 5
3.4 Layout Design Rules - Function: obtain a circuit with optimum yield in an area as well as possible - Performance yield * Conservative design rules Functional circuit Good yield * Aggressive design rules Bad yield (A) Line width/spacing Small open circuit Close short circuit (B) Spacing between two independent layers - In process: Compact circuit/layout for low cost and high speed (a) Geometric features for mask-making and lithographical (b) Interactions between different layers (e.g., poly + diffussion) - Rules: a. Micro(μ)-based rules Industry (submicron) b. Lambda-based rules: e.g.,, 1λ=0.6um for 1.2 um CMOS process) for 4-1.2um Scalable CMOS process. 2λ is the minimum channel length (L). - See Table 3.2 and figures (next four pages) 2003/3/12 CMOS Process (II) 6
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Layout Design Rules: 2003/3/12 CMOS Process (II) 9
Contact Rules: There are several generally available contacts: - Metal to p-active (p-diffusion) - Metal to n-active (n-diffusion) - Metal to Polysilicon - VDD and VSS substrate contacts - Split (Substrate contacts) 3.4.5 Layer assignment (Table3.4) - CIF: Caltech Intermediate Form - GDSII Format 2003/3/12 CMOS Process (II) 10
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3.5 Latchup - Latchup : Shorting of VDD and Vss lines Chip breakdown - Latchup Equivalent Circuit: Vertical : pnp - p = source/drain of p device (Emitter) - n = n-well (Base) - p = p-substrate (Collector) Lateral : npn - n = source/drain of n device (Emitter) - p= p-substrate (Base) - n= n-well (Collector) Rsubstrate, Rwell - Parasitic devices and resistors 2003/3/12 CMOS Process (II) 12
Latchup triggering: Transient/Impulse current in start-up A. Lateral triggering: current flows in the emitter of the lateral npn-transistor Trigger point : In,trigger = - Vpnp,on = 0.7V Vpnp-on αnpn Rwell - αnpn = common base gain of the lateral npn device - Rwell = well resistance B. Vertical triggering: Sufficient current is injected into the emitter of the vertical pnp transistor 3.5.3 Latchup prevention - Latchup occur βnpn βpnp > 1 + Where IR,sub = IR,well = (βnpn+1)(ir,sub+ir,wellβpnp) VBE,,npn Rsub VBE,,npn Rsub (IDD-IR,sub) IDD = total supply current 2003/3/12 CMOS Process (II) 13
Observation to prevent latchup: 1. Reduce the resistor values 2. Reduce the gain of the parasitic devices - Approach: 1. Latchup-resistant CMOS process 2. Layout techniques (see section 3.5.4,3.5.5) 3.6 Technology-related CAD tools - Design Rule Check (DRC): On-line DRC and Off-line (Dracula) (3.6.1) - Circuit extraction (Layout Parameter Extraction, LPE) (3.6.2) - CMOS process simulator (Process Input Description Language (PIDL))(sec.3.9) and Supreme by Stanford University. 2003/3/12 CMOS Process (II) 14