We report ellipsometric measurements made on semiconductor samples using photon-correlated beams produced by the process of spontaneous parametric down-conversion. Such a source yields higher accuracy than its quantum-limited conventional counterpart. We also show that our approach has the added advantage of not requiring an external reference sample for calibration. I. BACKGROUNDSince all optical measurements are limited by quantum noise, which dominates at low light levels, there has been a strong interest in developing nonclassical optical sources with sub-Poisson photon statistics that offer sub-shot-noise accuracy. One implementation that has been considered for metrology applications is based on the use of two optical beams, each with Poisson-distributed photons, but also with a fully correlated joint photon counting distribution. Such correlated-photon beams have been generated, for example, by spontaneous parametric down-conversion (SPDC) in a nonlinear optical crystal, and used for applications including quantum cryptography [1], teleportation [2], and metrology [3,4]. If one of the beams is reflected from, or transmitted through, a sample, then measurement of the photon coincidence rate, together with the mean photon counts in each beam, yield estimates of the sample reflectance/transmittance with accuracy greater than the conventional measurement using a single beam [5][6][7][8][9]. In this paper, we consider the use of photon-correlated beams in ellipsometry.Ellipsometry [10-15] is a technique in which the polarization of light is used to determine the optical properties of a material (sample) and infer information such as the thickness of a thin film. The sample is characterized by two parameters = arctan͉ r 2 / r 1 ͉ and ⌬ = arg͑r 2 / r 1 ͒ where r 1 and r 2 are the sample's eigenpolarization complex reflection coefficients [11]. In a conventional ellipsometer, these parameters are extracted by manipulation of the polarization state of the incident or the reflected/transmitted light and measurement of the optical intensities or the photon counting rates. Clearly, such measurements are limited by shot noise, particularly at low light intensities or when using ellipsometers employing a nulling technique. The use of photon-correlated beams in ellipsometry has been previously reported and referred to as "quantum ellipsometry" [16,17]. It was shown that this technique alleviates the need for calibration using an external reference sample.In this paper, we report experimental quantum ellipsometric measurements made on standard optical samples. We also estimate the accuracy advantage attained by the use of quantum relative to conventional ellipsometry. Section II of the paper reviews the theory of correlated-photon ellipsometry, a form of quantum ellipsometry. Although correlated-photon pairs may be generated by a variety of means, correlatedphoton ellipsometry in this paper refers to the use of photon pairs generated by SPDC. In Sec. III we present experimental results obtained from two semiconductor sa...
Register files are becoming one of the critical components of current out-of-order
Abstract-Scaling supply voltage to values near the threshold voltage allows a dramatic decrease in the power consumption of processors; however, the lower the voltage, the higher the sensitivity to process variation, and, hence, the lower the reliability. Large SRAM structures, like the last-level cache (LLC), are extremely vulnerable to process variation because they are aggressively sized to satisfy high density requirements. In this paper, we propose Concertina, an LLC designed to enable reliable operation at low voltages with conventional SRAM cells. Based on the observation that for many applications the LLC contains large amounts of null data, Concertina compresses cache blocks in order that they can be allocated to cache entries with faulty cells, enabling use of 100% of the LLC capacity. To distribute blocks among cache entries, Concertina implements a compression-and fault-aware insertion/replacement policy that reduces the LLC miss rate. Concertina reaches the performance of an ideal system implementing an LLC that does not suffer from parameter variation with a modest storage overhead. Specifically, performance degrades by less than 2%, even when using small SRAM cells, which implies over 90% of cache entries having defective cells, and this represents a notable improvement on previously proposed techniques.
Abstract-The Register File is one of the critical components of current processors in terms of access time and power consumption. Among other things, the potential to exploit instruction-level parallelism is closely related to the size and number of ports of the register file. In conventional register renaming schemes, both register allocation and releasing are conservatively done, the former at the rename stage, before registers are loaded with values, and the latter at the commit stage of the instruction redefining the same register, once registers are not used anymore. In this paper, we introduce VP-LAER, a renaming scheme that allocates registers later and releases them earlier than conventional schemes. Specifically, physical registers are allocated at the end of the execution stage and released as soon as the processor realizes that there will be no further use of them. VP-LAER enhances register utilization, that is, the fraction of allocated registers having a value to be read in the future. Detailed cycle-level simulations show either a significant speedup for a given register file size or a reduction in the register file size for a given performance level, especially for floating-point codes, where the register file pressure is usually high.Index Terms-Register renaming, out-of-order processors, register file optimization, physical register allocation and releasing, precise exceptions.
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