Illumination and Charge Transfer : Illumination and Charge Transfer

Requires: SSuprem 4/S-Pisces/Luminous
Minimum Versions: Athena 5.22.3.R, Atlas 5.28.1.R

In this example a CCD structure is constructed using the Athena process simulator. This structure is passed to Atlas for electrical testing. The input file tests consist of four portions:

  • Construction of a CCD device
  • Emptying the storage well of carriers
  • Illuminating the CCD with a light source and measuring the generated carriers
  • Transfer of the carriers to the drain

The first stage of the input constructs the CCD geometry and doping profiles in Athena. The CCD device consists of a storage node on the left of the structure controlled by a polysilicon gate. A transfer node to the right of this is also controlled by both a polysilicon gate and a drain region of heavy n+ doping with a metal contact.

The CCD structure has an n- active region at the surface above a p-type substrate. Under the transfer node an extra p-type implant is given. This implant is of sufficient magnitude to create a potential difference between the storage and transfer nodes, but is not so large as to cause a junction.

The process simulation is designed to run extremely quickly so there is a minimum of diffusion steps included. The final stages of the Athena input performs the electrode definition needed by Atlas. Also some EXTRACT statements are used to measure the junction depth of the active region under the storage and transfer nodes.

The Atlas simulation begins with definition of the models and material parameters of the device. The contact statement is used to define the workfunction of the two polysilicon electrodes. The material statement is used to define minority carrier lifetimes in the semiconductor. The interface charge between the gate oxide and the semiconductor is set by the 'qf' parameter of the interface statement.

The physical models used in this simulation reflect different physical effects that are important in CCD devices. The mobility model CVT includes a surface mobility degradation which is important in the transfer phase of the simulation as the charge moves from the storage node to the drain. The recombination models consrh and auger are needed as the recombination of the carriers is important in determining the capacity and charge transfer characteristics. Band gap narrowing is also needed to model the properties of areas with high carrier concentration.

The initial part of the Atlas input file biases the drain to +20V, whilst the two gates are held at -6V. This is done to deplete the electrons from the n- active layer. The final structure with the active layer empty of carriers is saved. At this point extract statements are used to extract the integrated electron concentration in the storage well. This will be a small number.

Next, Luminous is used to illuminate the storage node. The light beam is defined near the start of the input file using the beam statement. This beam is defined with a 500nm wavelength, an origin at (0,-1) and width of 2um. The angle of 90 degrees specifies normal incidence onto the top of the CCD. Luminous calculates all internal reflections and absorption by the layers within the simulation structure.

To illuminate the structure, a transient simulation is used. The power of the light beam is controlled by the parameter B1 on the SOLVE statement. This is defined to rise from zero to 1 W/cm^2 in 5ns, to remain at this power for a further 15ns, and then to fall to zero in another 5ns. The tstep parameter is used to define the initial timestep of the simulation. All other timesteps are automatically calculated by Atlas based on the local truncation error. The tstop parameter defines the end of the simulation time. The illumination part is set to last 50ns.

At the end of the illumination section a structure is stored. This structure can be plotted to see the charge collected under the storage node. The potential difference between the storage and transfer nodes created by the extra boron implant stops charge from leaking away to the large positive potential on the drain. extract statements are used once again to measure the integrated electron concentration. The value obtained is subtracted from the value obtained from the empty well.

The final stage of the Atlas simulation is the charge transfer. This is also performed as a transient simulation. It is very important to note that during a single Atlas run the time is not reset to zero. Since the illumination section was 50ns, that is the starting time for this section. In this section the transfer gate is ramped from -6V to +2V. This causes the potential barrier between the storage node and the drain to be removed, Charge can now flow form the storage node, under the transfer gate and into the drain.

At each timestep in the transfer simulation a structure is saved. By loading several of these structure files into TonyPlot it is possible to observe the movement of charge across the device.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.