Photon correlator is kernel component of Photon Correlation Spectroscopy particle sizing instrumentation, which must
calculate autocorrelation function in real time. The correlation processing can be carried out by hardware or software.
Hardware implemented with photon correlator has good real-time processing ability, but the cost for hardware resource is
pretty high. Software correlator is comparably cheap but not competent for real-time processing. Hardware and software
photon correlator architecture is analyzed, and a novel least hardware resource real-time correlator architecture taking
advantage of fast processing speed of FPGA devices and vast memory space of microprocessor is presented in this paper.
In this new architecture, high speed and middle speed correlator channels are implemented in FPGA, within which
counter, shift register and multiplier are built with logic resources. At sample times longer than 100μs, photon pulse
count, the output of 16-bit counter in FPGA, is sent to DSP to finish correlation calculation. The 300Mhz float-point DSP
chip TMS320C6713 is chosen, which can finish 256-channel calculation task in 100μs. The combination of FPGA and
DSP significantly decreases the hardware cost of correlator on the premise of real-time processing, and brings great
flexibility to the designing of correlator.
To overcome shortcoming of traditional optical device, a photo system based on scanning electron microscope has been
developed. The photo system is composed of built-in compact piezoelectric stage and electron-beam controller. The stage
is made up of piezoelectric driver and grating encoder system. It can operate in vacuum and non-magnetic environments.
The electron-beam controller includes controlling model, deflection model, image acquisition model and position error
feedback model. Scanning field can be calibrated before scanning. The photo system can scan the IC chip which is wider
than one scanning field quickly by using this nano-positioning stage. The position error can be compensated by the
position error feedback model, so the stitching precision between neighborhood images is improved. Legible IC image
is obtained by using this system.
In photon correlation spectroscopy (PCS) particle sizing techniques, the choice of regularization parameter in the inverse
algorithm of Tikhonov Regularization can be classified into prior and posterior strategies. For prior strategies, the choice
of regularization parameter is made before calculating the regularized solution, but the posterior strategies, based on
some principles, make the choice of regularization parameter matching the error level of original data during the
calculation for the regularized solution. The critical issue of regularization method lies in the proper balance and tradeoff
between the accuracy and stability of the solutions, in other words, the regularization parameter chosen should match the
error lever of autocorrelation function. Using prior and posterior strategies, scattered light signal of 50nm, 100nm and
300 nm particles were simulated and their autocorrelation functions inversed respectively. If noise existed, the stability
was weakened while the deviation of peak value increased. For prior strategy, the noise influence to the inverse is
obvious when noise factor is 0, 0.01, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1 and 1, but inverse results can be obtained on large
noise level even the noise factor is at 1. However, when using posterior strategy, the effect on inverse result was less
when error was small, but convergent inverse results cannot be obtained when the noise level is high.
Correlation techniques are widely used to extract spectral information from light scattering and other stochastic
processes. Within the photon correlation system, the correlating operation must work at a high speed. In this paper, a
photon correlator based on microcontroller C8051F was developed. In the photon correlator, the work of counting and
scratch is completed by the two 4-bits binary adder 74F161, which is connected to form an 8-bits adder., and the
correlation operation of every channel is carried out by the software of C8051F. By probably choosing high speed
devices counting of 10ns in width pulses can be counted. The correlation operations including multiplying and addition
operation of 56 channels with the circulation program within 3μs were made in interrupt service routine of the C8051F.
The work in this paper can be applied in the ultra-fine particle sizing with photon correlation spectroscopy.