Announcements Homework 1 due today September 6 at 5pm drop box in Cory EE105 Fall 2005 Microelectronic Devices and Circuits Homework 2 due next Tuesday September 13 at 5pm All Lab sessions are still on we will cut one for Lab 2 Lecture 3 Integrated Passives 2 Lecture Material IC Fabrication Photo Lithographic Process Last lecture optical mask Carrier concentration and doping Carrier velocity and mobility oxidation IC resistors Drift currents This lecture photoresist removal ashing photoresist coating stepper exposure Diffusion currents Typical operations in a single photolithographic cycle from Fullman Review of electrostatics IC capacitors photoresist development acid etch process step spin rinse dry 3 CMOS Process at a Glance 4 IC Fabrication Si Substrate Pure Si crystal is starting material wafer The Si wafer is extremely pure 1 part in a billion impurities Why so pure Define active areas Etch and fill trenches Implant well regions Si density is about 5 10 22 atoms cm 3 Desire intentional doping from 10 14 10 18 Deposit and pattern polysilicon layer Want unintentional dopants to be about 1 2 orders of magnitude less dense 10 12 Implant source and drain regions and substrate contacts Si wafers are polished to about 700 m thick mirror finish Create contact and via windows Deposit and pattern metal layers 5 The Si forms the substrate for the IC 6 1 IC Fabrication Oxide IC Fabrication Patterning of SiO2 Si has a native oxide SiO2 Chemical or plasma etch SiO2 Quartz is extremely stable and very convenient for fabrication It s an insulator Si substrate Hardened resist SiO 2 a Silicon base material Si substrate Photoresist SiO 2 SiO2 windows are etched using photolithography These openings allow ion implantation into selected regions SiO2 can block ion implantation in other areas Si substrate d After development and etching of resist chemical or plasma etch of SiO 2 Hardened resist SiO 2 b After oxidation and deposition of negative photoresist Si substrate UV light Patterned optical mask e After etching Exposed resist SiO 2 Si substrate Si substrate 7 IC Fabrication Ion Implantation f Final result after removal of resist c Stepper exposure 8 Diffusion Resistor oxide N type Diffusion Region Oxide P type Si Substrate P type Si Substrate Si substrate p type Grow oxide thermally Using ion implantation diffusion the thickness and dopant concentration of resistor is set by process E g 100 unsilicided 10 silicided Shape of the resistor is set by design layout Metal contacts are connected to ends of the resistor N type diffusion region Add photoresist Expose visible or UV source Etch chemical such as HF Ion implantation inject dopants Diffuse increase temperature and allow dopants to diffuse Resistor is capacitively isolation from substrate 9 10 Using Sheet Resistance Rs Poly Film Resistor Polysilicon Film N or P type Reverse biased PN Junction Oxide Ion implanted or diffused IC resistor P type Si Substrate To lower the capacitive parasitics we should build the resistor further away from substrate We can deposit a thin film of poly Si heavily doped material on top of the oxide E g 10 100 unsilicided 1 silicided Bad absolute tollerance very good relative tolerance 11 12 2 Diffusion Diffusion cont Diffusion occurs when there exists a concentration gradient The net motion of gas molecules to the right chamber was due to the concentration gradient In the figure below imagine that we fill the left chamber with a gas at temperate T If each particle moves on average left or right then eventually half will be in the right chamber If we suddenly remove the divider what happens The gas will fill the entire volume of the new chamber How does this occur If the molecules were charged or electrons then there would be a net current flow The diffusion current flows from high concentration to low concentration 13 Diffusion Equations Einstein Relation Assume that the mean free path is Find flux of carriers crossing x 0 plane n 0 n n The thermal velocity is given by kT 1 2 0 J qF qvth mn vth2 12 kT Mean Free Time vth c 1 F vth n n 2 1 dn dn F vth n 0 n 0 2 dx dx 1 dn n vth 2 F vth dx 1 n vth 2 14 vth vth2 c kT J qvth c mn kT q c q mn kT dn dn q n dx q dx kT Dn n q dn dx 15 Electrostatics Review 1 Total Current and Boundary Conditions Electric field go from positive charge to negative charge by convention When both drift and diffusion are present the total current is given by the sum J J drift J diff q n nE qDn dn dx In resistors the carrier concentration is approximately uniform and the second term is nearly zero E For currents flowing uniformly through an interface no charge accumulation the field is discontinous J1 1 J 2 2 In words if the electric field changes magnitude there has to be charge involved Result In a charge free region the electric field must be constant J1 J 2 1 E1 2 E2 E1 2 E2 1 16 17 18 3 Electrostatics Review 2 Electrostatics in 1D Everything simplifies in 1 D Gauss Law equivalently says that if there is a net electric field leaving a region there has to be positive charge in that region E dE dx dE x E x E x0 x0 Recall Q Zero field boundary condition Q E dV dV Q V E dV E dS V V S 0 x1 20 Integrating this basic relation we have that the potential x is the integral of the field d dx r x x0 E dl r dl C In 1D this is a simple integral x x x0 E x dx e dx e x0 E x0 Note An electron should float to a high potential d 1 point d F e Going the other way we have Poisson s equation in 1D x d 2 x dx 2 dx 2 21 Boundary Conditions 22 IC MIM Capacitor Bottom Plate Potential must be a continuous function If not the fields forces would be infinite Electric fields need not be continuous We have already seen that the electric fields diverge on charges In fact across an interface we have E dS 1E1S 2 E2 S Qinside x E1 1 Qinside 0 x 0 Bottom Plate Contacts Q CV By forming a thin oxide and metal or polysilicon plates a capacitor is formed Contacts are made to top and bottom plate Parasitic capacitance exists between bottom plate and substrate E1 2 E2 1 Field discontiuity implies charge density at surface Top Plate Thin Oxide 1 E1S 2 E2 S 0 S x x x E x dx 0 x 0 x1 More Potential Negative sign says that field lines go from high potential points to lower potential points negative slope E2 2 E x 0 19 The electric field force is related to the potential energy Fe qE e x x1 Electrostatic Potential E x dx Consider a uniform charge distribution Electric Fields are Leaving This Box E dS dx …
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