Buried Channel CCDs Research Articles

Increase in silicon charge coupled devices speed with focused ion beam implanted channels

The clocking frequency of long channel CCDs is limited by the time taken to transfer charge from one well to the next. The bulk of the charge transfers rapidly by Coulomb repulsion but residual charge has to transfer by thermal diffusion, which is a slow process. If the n-type dopant density has a gradient in the direction of current flow, a built-in electric field is created, speeding up the charge transfer process. The focused ion beam system is uniquely suited to implant such a gradient of doping and has been used to implant buried-channel CCDs with 26-μm-long storage gates. The devices built to demonstrate this concept were shallow-buried-channel CCDs driven by 2-phase 5-V clocks. Arsenic ions were implanted at an energy of 220 keV and a gradient of doping increasing from 0 to 1.5×1011 ions/cm2 was superimposed on a uniform phosphorus implant. The challenge here was to get a low, monotonically increasing implant dose. Defocusing of the 25-pA As+ + beam to a diameter of ∼1 μm was found to be necessary. The maximum operating (clocking) frequency of the graded-implant devices was 41 MHz compared to a maximum frequency of 2.5 MHz for the same devices with uniformly doped channels. Such long-channel CCDs are of interest in 2-dimensional imaging applications and signal processing applications where high well charge capacity is needed without compromising speed or charge transfer efficiency.

Nonideal behaviour of buried channel CCDs caused by oxide and bulk silicon traps

Analytical expressions are given for noise in FETs due to charge trapping at defects in the silicon and in the oxide. Conditions which lead to 1 ƒ and 1 ƒ 2 spectra are described. A new physical model of the noise mechanisms in ideal depletion mode MOSFETS is fully described. Measurements on various MOSFETs in CCDs and other systems are compared with the predictions of the model. The results show that MOSFETS in CCDs exhibit much higher 1 ƒ noise than predicted. The possible reasons for this excess noise as well as the significance of these findings on charge transfer efficiency are discussed.

Lateral motion of the inversion layer of MOS structures in regions of variable doping density and oxide thickness

We describe computational methods and a new experimental technique for directly measuring the lateral spread of gate-induced surface inversion into the birdsbeak region of LOCOS (local oxidation of silicon) isolation. This method is of general use for characterizing all MOS structures with lateral variations in substrate doping, fixed charge, and/or oxide thickness. The procedure is applied to buried-channel CCDs to determine the location of generation sites along the channel-stop sidewalls.

Low-Temperature Characteristics of Buried-Channel Charge-Coupled Devices

The characteristics of buried-channel CCDs in the temperature range between 40 K and 300 K were studied. It was found that the transfer loss had a peak at about 130 K and increased rapidly below 90 K. In these specific temperature regions, the transfer loss depended on the clock period, the data cycle time and the clock fall time. The characteristics were analyzed based on free-charge transfer mechanisms and the Shockley-Read-Hall theory. The peak at 130 K was caused by a trap at the energy level 0.22 eV below the conduction band, and the increase below 90 K by the carrier freeze-out. Furthermore, the reduction of impurity concentration in the buried channel turned out to be an effective method of improving the transfer efficiency below 90 K.

An electronically programmable transversal input filter

Describes the design and performance of a 64-stage, electronically programmable, binary-analog transversal input filter (PTIF). The monolithic filter consists of an array of two-phase buried-channel CCDs with surface channel inputs, operated in a single-ended configuration and driven with a single clock. Digital NMOS support circuitry permits programming of bipolar weighting coefficients while maintaining code independent offset. The device has been operated as a pseudorandom noise sequence matched filter with a dynamic range of 55 dB at 8 MHz and a near theoretical processing gain of 18 dB. Correlation has been demonstrated at 20 MHz.

Integrated CCD transversal filter matched to a pair of 16-bit complementary series

An integrated filter matched to a pair of 16-bit complementary series was fabricated employing four buried channel CCDs with parallel-in/serial-out approach for high speed operation. This letter presents the structure and operation of the filter and demonstrates various advantages, such as negligible deviation (0.016%) from expected signal-to-noise power gain, high peak-to-sidelobe ratio

Uniphase buried-channel charge-coupled devices

The Uniphase operation of buried-channel CCDs with a single electrode system is described and practical structures are proposed. A stepped-oxide approach using strongly doped/thin-lightly doped/thick regions for the buried layer appears to be best suited for building a uniphase bulk-channel CCD with available technology.

On the design of ion implanted buried channel charge coupled devices (BCCDs)

The allowable range for the channel dose of a buried channel CCD is obtained. The maximum charge handling capacity without surface trapping effect is found to depend on the channel dose and the substrate concentration. In the case of two phase buried channel CCDs, the barrier implant dose can be estimated depending on the charge storage capacity desired. The result is applicable to ion implanted shallow junction devices and can be extended to deeper junction epitaxial buried channel devices.

C-V characteristics of ion implanted depletion IGFETs and buried channel CCDs

Capacitance voltage measurements of ion implanted devices for several circuit connections are presented and interpreted in terms of a simple constant profile approximation. Based on this model the device capacitance is described quantitatively in terms of the series combination of a p− n junction capacitance and a conventional MOS capacitance. It is shown that shallow and deep implants reveal characteristically distinctive C-V curves which provide an immediate rough estimate of the implant depth. Analysis of the model yields directly, important parameters for first order design purposes. It is also shown that the measurements provide a simple diagnostic technique to examine the physics of the implanted structure. The use of the simplified model is justified by the agreement between experimental and calculated values.

Formula omitted] characteristics of ion implanted depletion IGFETs and buried channel CCDs : G. W. Taylor. Solid-State Electronics. 19, 495 (1976)

Des dipôles de dislocations sont produits par bombardement ionique (ions argon d'énergie 4000 eV) de monocristaux d'or d'orientation (001) obtenus par evaporation. Les auteurs étudient l'interaction de deux dipôles situtés à une même profondeur dans l'échantillon et dans des plans parallèles assez proches plans (110). L'expérience montre que durant un recuit ces dipôles peuvent se condenser pour ne donner qu'un seul dipôle. La théorie du phénomène est faite. Elle permet de comprendre cette condensation et de préciser sa cinétique.Dislocations dipoles were produced during ionic bombardement (argon ions, energy—4000 eV of gold crystals, obtained by evaporation, having a (001) orientation. The authors have studied the interaction of two dipoles at the same depth in the specimen on parallel planes near the 110 planes. Experimental evidence shows that during annealing the dipoles can condense to form only one dipole. A theory is proposed which explains this phenomenon as well as its kinetics. A second mode of condensation in addition to that observed is predicted.Durch Ionenbestrahlung (Argonionen, 4 KeV) von (001)-orientierten Goldeinkristallen, die aus der Dampfphase gewonnen wurden, sind Versetzungsdipole erzeugt worden. Die Autoren untersuchen die Wechselwirkung zwischen zwei Dipolen, die in der gleichen Tiefe der Probe und in Ebenen, die nahezu parallel zu den (110)-Ebenen verlaufen, liegen. Das Experiment zeigt, daβ bei Erholung diese Dipole sich zu einem einzigen Dipol zusammenlagern. Eine Theorie dieser Erscheinung wird entwickelt. Sie erlaubt es, diese Kondensation zu verstehen und ihre Kinetik genau anzugeben. Ein Kondensationsmodus, der von dem experimentell beobachteten verschieden ist, wird untersucht.

Effects of radiation on buried-channel CCDs with doped polysilicon gates and undoped polysilicon interelectrode isolation

The effects of gamma radiation on the operation of a buried-channel charge-coupled device having doped polysilicon electrodes and undoped polysilicon interelectrode isolation were studied. Doses up to 1×104 rad caused no increase in the transfer inefficiency. At a dose of 3×104 rad, the flat-band voltage shift was sufficient to drive the buried channel out of depletion and to cause field-induced channeling in the undoped polysilicon isolation regions. Increasing the channel bias returned the channel to depletion. However, channeling of the isolation regions could not be eliminated by changing the clock and bias voltages.