1. EE/MAtE1671 Front-End-Of-Line Variability Considerations EE/MatE 167 David Wahlgren Parent

2. EE/MAtE1672 Introduction FEOL variability affects the response of discrete electrical components. –transistors, capacitors, resistors –MOSFET Poly gate length Spacer widths Gate oxide thickness Device width and edge effects

3. EE/MAtE1673 Threshold Voltage

4. EE/MAtE1674 Inter fab variation Spacer thickness –Simple method Large spacers Leads to larger overlap variations –LDD Narrow spacers –spacer then LDD implant –20% spacer then S/D implant Channel length can vary by 10% on die for even a mature process

5. EE/MAtE1675 Across the Chip Line-width Variation (ACLV) How the line with varies on the die. To combat ACLV have device gates: –have the same physical dimensions –oriented in the same direction –close together –have same nearest neighbor distances –are placed in area with similar pattern density.

6. EE/MAtE1676 NMOS to PMOS Length Tracking The ratios of NMOS and PMOS driving capability vary. –Some styles are very dependant on this. –Even though the gates are etched at the same time, differences in spacers for pmos and nmos can affect LEFF –Some times they do not track leff of nmos gets larger while pmos gets smaller across die. –Keep device length the same. –Keep everything close together.

7. EE/MAtE1677 Channel Width Variations For narrow channel widths the threshold voltage can change. Unlike SCE the effect can be directly proportional to width or inversely proportional with width fro fabrication house to fabrication house.

8. EE/MAtE1678 VT Variations Everything varies but Planck’s constant and Boltzman’s constant –Threshold voltage Gate oxide thickness Doping in the channel Channel Length Fixed oxide charge Mobile charge

9. EE/MAtE1679 Hot Carrier Effects Degrades devices over the lifetime depending on how much they are used. Since all gates are not used the same amount except for clock trees there will be a ID/VT variation that will creep in over time. –NMOS VT goes up ID goes down. –PMOS VT goes down ID goes up.

10. EE/MAtE16710 Type of HCE Conducting –In “pinch off” Inversion region does not reach depletion region and carriers are inject with a high electric field. This gives them a high energy or temperature and it causes crystal damage. –Worse when VDD/2 –Slow slew rates or fast switching degrades FET even faster. Energetic carriers get trapped in the gate oxide.

11. EE/MAtE16711 Drain Resistance Modulation If spacer is too short dopant can get trapped in spacer thus lowering drain conductance. If too large there is too much overlap capacitance and the speed goes down.

12. EE/MAtE16712 Other Variations Negative Bias Temperature Instability –Inter fab variation –not well understood –aging Body effect –Threshold voltage can vary if source is not tied to ground.