EE 434 Lecture 12. Process Flow (wrap up) Device Modeling in Semiconductor Processes

Transcription

1 EE 434 Lecture 12 Process Flow (wrap up) Device Modeling in Semiconductor Processes

2 Quiz 6 How have process engineers configured a process to assure that the thickness of the gate oxide for the p-channel devices is nominally the same as that for the n-channel devices?

3 And the number is

4 1 6 And the number is

5 Quiz 6 How have process engineers configured a process to assure that the thickness of the gate oxide for the p-channel devices is nominally the same as that for the n-channel devices? Solution: The same polysilicon layer is used to form the gates of both the n-channel and the p-channel transistors SiO 2 is stripped and then regrown to assure uniformity of thin oxide across the whole wafer.

6 Process Flow Processing Steps Have Been Discussed Wafer Prep, Photolithography, Deposition, Etching, Diffusion, Combining these Processing Steps to Make Useful Integrated Circuits constitutes a Process Flow Each Process has a unique process flow Process flow constitutes very valuable IP

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14 Basic Devices Devices in Standard Processes MOS Transistors n-channel p-channel Capacitors Resistors Diodes BJT (in some processes) npn pnp Niche Devices Photodetectors MESFET Schottky Diode (not Shockley) MEM Devices. Primary Consideration in This Course Limited Consideration in This Course

15 Basic Devices and Device Models Resistor Capacitor MOSFET Diode BJT

16 Basic Devices and Device Models Resistor Capacitor MOSFET Diode BJT - Resistors and Capacitors were discussed previously in the context of interconnects - Were generally considered parasitics in earlier discussions - Will now be considered as desired components - Models obviously will be very similar or identical

17 Basic Devices and Device Models Resistor Capacitor MOSFET Diode BJT

18 Resistors Generally thin-film devices Almost any thin-film layer can be used as a resistor Diffused resistors Poly Resistors Metal Resistors Thin-film adders (SiCr or NiCr) Subject to process variations, gradient effects and local random variations Often temperature and voltage dependent Ambient temperature Local Heating Nonlinearities often a cause of distortion when used in circuits Trimming possible resistors Laser,links,switches

19 Resistor Model W d L V I Model: R = V I

20 Resistivity Volumetric measure of conduction capability of a material Area is A units : ohm cm L R ρ = AR L for homogeneous material, ρ A, R, L

21 Sheet Resistance W d L R RW R = ( for d << w, d << L ) units : ohms / L for homogeneous materials, R is independent of W, L, R

22 Relationship between ρ and R R = ρ = RW L AR L ρ = A W R = W d W A ρ = R W A = W d R = d x R Number of squares, N S, often used instead of L / W in determining resistance of film resistors R=R N S

23 Example 1 W L R =?

24 Example 1 L L = W N S W

25 Example R =?

26 Example R =? N S =8.4 R = R (8.4)

27 Corners in Film Resistors Corner Rule of Thumb:.55 squares for each corner

28 Example 2 Determine R if R = 100 Ω /

29 Example N S =17.1 R = (17.1) R R = 1710 Ω 1 2 3

30 Resistivity of Materials used in Semiconductor Processing Cu: 1.7E-6 Ωcm Al: 2.7E-4 Ωcm Gold: 2.4E-6 Ωcm Platinum: 3.0E-6 Ωcm n-si:.25 to 5 Ωcm intrinsic Si: 2.5E5 Ωcm SiO 2 : E14 Ωcm

31 Temperature Coefficients Used for indicating temperature sensitivity of resistors & capacitors For a resistor: TCR = 1 R dr dt 6 10 op. temp ppm C This diff eqn can easily be solved if TCR is a constant R R ( T ) R( T ) 2 = e 1 T 2 T TCR ( ) ( ) ( ) T R T 1+ T T TCR 10 Identical Expressions for Capacitors

32 Voltage Coefficients Used for indicating voltage sensitivity of resistors & capacitors For a resistor: VCR = 1 R dr dv 6 10 ref voltage ppm This diff eqn can easily be solved if VCR is a constant R ( V ) R( V ) 2 = e 1 V 2 V VCR ( ) ( ) ( ) V R V + V V R V VCR 10 Identical Expressions for Capacitors

33 Temperature and Voltage Coefficients Temperature and voltage coefficients often quite large for diffused resistors Temperature and voltage coefficients often quite small for poly and metal resistors

34 Capacitance and Resistance in Interconnects See MOSIS WEB site for process parameters that characterize parasitic resistances and capacitances

35 Basic Devices and Device Models Resistor Capacitor MOSFET Diode BJT

36 Capacitance Parallel Plate C A 1 cond 1 insulator cond 2 A 2 C = A A = area of intersection of A 1 & A 2 d d

37 Capacitance Parallel Plate If C d = Cap unit area C = C d A where C d = d

38 Capacitance Junction Capacitor C depletion region C = A d d p n Note: d is voltage dependent -capacitance is voltage dependent -usually parasitic caps -varicapsor varactor diodes exploit voltage dep. of C