Low-capacitance CCD

1. Low-capacitance CCD Chris Damerell (R.A.L.), Brian Hawes (Oxford Univ) • An idea to reduce the inter-gate capacitance in CPCCDs, hoping to achieve busline distribution of clocks. Present designs load as much as 40nF on the Clock Input lines, and require 20-Amps at 50Mhz. Not easy to provide. • Any reduction of capacitance will reduce demands on driver current and Power Supply. • Valuable help and advice from Konstantin, Rainer Richter (MPI) and David Burt – and all who came to the e2V meeting on 18th May. • Started as a cross-check whether the DALSA inter-gate figure of 0.33 pF/cm (per edge) was at all credible, contrary to simulations by Konstantin and Brian and opinions at e2V … LCFI Collaboration Meeting, Bristol – Brian Hawes

2. Source of Inter-Gate Capacitance • 10um pitch – 9.9um Gates; 0.1um Space. Rough approximation to E2V gates • Plot is of Charge Density between gates. The Capacitance is mostly below the Gates in the Substrate and, especially, concentrated in gap between Gates • Is it possible to reduce the inter-Gate Capacitance by increasing the Inter-Gate gap? LCFI Collaboration Meeting, Bristol – Brian Hawes

3. Effect of Increased Inter-Gate Gap • This was tried in 1996: the pnCCD architecture from MPI. The original design failed because the floating surface in the gaps caused major Charge Transfer problems. • The final version had the Inter-Gate strips metallised and biased. These are still working well. • Proposal for CPCCD’s (concept from Chris Damerell): Fabricate with raised Oxide Pedestals between reduced width Gates. These will will lift the necessary conducting surface above the level of the Gates and should further reduce the gate gap Capacitance. LCFI Collaboration Meeting, Bristol – Brian Hawes

4. Simulation of Pedestal Gate Architecture [1] • A series of simulations were run with active gate widths from 0.5 to 9.9 um, on 10um Gate pitch • For each gate width, the Pedestal height was varied from 0.15 to 0.5um • A ‘no pedestal’ case was also simulated, with free space between the gates. • There was no Oxide passivation over the structure in the simulations. This could have been added, but would not make a large difference. LCFI Collaboration Meeting, Bristol – Brian Hawes

5. Simulation of Pedestal Gate Architecture [2] • Example field plot. Parameters: • active gate length = pedestal gate length = 5 mm • gate thickness (active and pedestal) = 0.1 mm • Pedestal height = 0.5 mm oxide • Dielectric isolation above Si: 85 nm oxide plus 85 nm nitride (e2V standard) LCFI Collaboration Meeting, Bristol – Brian Hawes

6. Simulation of Pedestal Gate Architecture [3] LCFI Collaboration Meeting, Bristol – Brian Hawes

7. Simulation of Pedestal Gate Architecture [4] • The curve for no Pedestal agrees closely with results calculated for Microstrip Detectors by D Husson (IEEE NS 41 (1994) 811 LCFI Collaboration Meeting, Bristol – Brian Hawes

8. Charge Distribution with profiled Gate surfaces [1] LCFI Collaboration Meeting, Bristol – Brian Hawes

9. Charge Distribution with profiled Gate Surfaces [2] LCFI Collaboration Meeting, Bristol – Brian Hawes

10. Summary of Results • 2um Active Gates, and 0.5um high Pedestal gives 1.98 pF/cm, including capacitance to substrate. E2V process gives 8.82pF/cm. • This suggests Cg-g can be reduced by a factor 4 • The Capacitance will be modified by true profile of gate edges, but this effect is not large – perhaps a further 20% reduction. • The Capacitance simulations were performed using the FEMM4 program, by David Meeker (http://femm.foster-miller.net). LCFI Collaboration Meeting, Bristol – Brian Hawes

11. What next? [1] (Chris Damerell’s notes) • If this works, one may get close to the DALSA datasheet (Ceff = 0.33x4 = 1.32 pF/cm), so the dream of narrow buslines (my presentation of 17th Jan) could be realised • Even if we don’t quite get there, what about a 1-sided busline architecture? • Several advantages: • Relatively free choice of busline width • metal contacts to the two sets of active gates (f-1 and f-2) will give adequate clock distribution • Eliminates competition for space and risk of crosstalk on ends of ladder between drive and readout • Good width available for bump-bond connections to CPD • Leaves the other edge of the CCD available for a narrow busline connecting to the pedestal gates • Creates a modest increase in material budget, but could still be the winner LCFI Collaboration Meeting, Bristol – Brian Hawes

12. What Next? [2] • There are several possible showstoppers, even if such a structure can be made: • Potential barriers or pockets at the transitions between active gates and pedestal gates • Jitter in potential under the pedestal gate, due to fluctuations in phi-1 and phi-2 waveforms • Radiation-induced shifts in this potential. However, if this is uniform across the device, it can be eliminated by adjusting the pedestal gate voltage • The possibility of greatly reduced CCD capacitance is of general interest. It has always been frustrating that transferring such small signal charges needed such high driver power • If it can be made to work, there could be wider applications • We have started to discuss possible test structures with e2V • We (Brian?) could extend the model outwards from the CCD, back through the buslines (how wide should they really be?) and through bump bonds back to the CPD output LCFI Collaboration Meeting, Bristol – Brian Hawes