Effect of parameter variations at chip and wafer level on clockskews

In this paper, a methodology is proposed to determine clock skews and the performance of clock architectures considering parameter variations in an early stage of technology development. With this methodology, it is possible to separate process-induced clock skew from other effects like imperfect loading. Parameter variations are seen as one of the most important effects influencing chip performance in future. By comparing a 0.45- and a 0.25-μm technology, it is shown that in the future, process variations will increase clock skew. The clock skews are determined by measuring the relevant device and metal line parameters as a function of position over chip and wafer. In the past, parameters like IDS, Vth, and resistances could be measured very precisely, although it was difficult to measure low capacitances of single metal lines in the range of femto farad. Thus a new measurement method is used to determine interconnect capacitances extremely precisely. Based on these measurement data, a netlist of a defined clock tree is created by a C-program, and the clock signal delay is simulated. From the delay simulation, we calculate the clock skew for each chip dependent on the parameter variations. Experimental results are separated into a basic random fluctuation part and processing-related contributions on the chip and wafer levels. In addition, the effect of temperature gradients on each chip to the clock skew is simulated. The methodology presented is not restricted to just one clocktree but allows investigation of all kinds of clock distribution circuits. The method has clear advantages with respect to chip area against clocktree realizations on a testchip. No direct and costly measurement of signal delays by voltage contrast methods is required, since all parameters are determined by measurement on the device level     

Design methodology for synthesizing clock distribution networksexploiting nonzero localized clock skew

An integrated top-down design methodology is presented in this brief for synthesizing high performance clock distribution networks based on application dependent localized clock skew. The methodology is divided into four phases: (1) determining an optimal clock skew schedule composed of a set of nonzero clock skew values and the related minimum clock path delays; (2) designing the topology of the clock distribution network with delays assigned to each branch based on the circuit hierarchy, the aforementioned clock skew schedule, and minimizing process and environmental delay variations; (3) designing circuit structures to emulate the delay values assigned to the individual branches of the clock tree; and (4) designing the physical layout of the clock distribution network. The clock distribution network synthesis methodology is based on CMOS technology. The clock lines are transformed from distributed resistive capacitive interconnect lines into purely capacitive interconnect lines by partitioning the RC interconnect lines with inverting repeaters. Variations in process parameters are considered during the circuit design of the clock distribution network to guarantee a race-free circuit. Nominal errors of less than 2.5% for the delay of the clock paths and 7% for the clock skew between any two registers belonging to the same global data path as compared with SPICE Level-3 are demonstrated     

Grating-assisted coupling of light between semiconductor and glasswaveguides

Floquet-Bloch theory is used to calculate the electromagnetic fields in a leaky-mode grating-assisted directional coupler (LM-GADC) fabricated with semiconductor and glass materials. One waveguide is made from semiconductor materials (refractive index ≈3.2) while the second is made from glass (refractive index ≈1.45). The coupling of light between the two waveguides is assisted by a grating fabricated at the interface of the glass and semiconductor materials. Unlike typical GADC structures where power is exchanged between two waveguides using bound modes, this semiconductor/glass combination couples power between two waveguides using a bound mode (confined to the semiconductor) and a leaky mode (associated with the glass). The characteristics of the LM-GADC are discussed. Such LM-GADC couplers are expected to have numerous applications in areas such as laser-fiber coupling, photonic integrated circuits, and on-chip optical clock distribution. Analyses indicate that simple LM-GADC's can couple over 40% of the optical power from one waveguide to another in distances less than 1.25 mm     

High-Speed Optical Interconnect Coupler Based on Soft Lithography Ribbons

In this paper, high-speed optical ribbon couplers for card-to-backplane interconnect applications are presented. The ribbon couplers are based on evanescent coupling between flexible multimode waveguide arrays. A soft lithographic technique is utilized to fabricate the ribbons. A flexible nonterminating optical data bus has been developed. Using BeamPROP software, we simulated the evanescent light coupling between two closely spaced ribbon waveguides to study the effects of waveguides separation, interaction length, and misalignment on coupling efficiency. Further experimental analysis and tests have been performed to quantify these effects. To investigate data transmission performance, a 12-channel optical interconnect link has been assembled. Experimental results demonstrated successful evanescent coupling; facilitating auto alignment coupling between card and backplane ribbon waveguides at data speeds as high as 10 Gb/s per channel. The evident high-speed interconnect performance and rapid ribbon prototyping approach can result in overall lower cost coupler fabrication for prospective optical interconnect applications.      

Direct index measurements of diffused channel waveguides

A nondestructive method is described for obtaining information about the refractive index profiles of diffused channel waveguides of the type used in integrated optics. The method assumes that the index profile can be approximated as a product of two functions each of which depends on one of the two transverse coordinates. By observing the focusing effect, which the waveguide has on collimated light passing through it transversely to its axis, the shape of the index profile in a direction parallel to the surface of the substrate can be obtained. From measurements of the reflection coefficients we obtain the refractive index at the upper surface of the waveguide. By combining the two measurements the effective depth of the waveguide can be calculated. Results of measurements on multimode and single-mode waveguides formed in glass and lithium niobate are presented.     

玻璃波导有效折射率的原位测量（特邀）

飞秒激光直写玻璃波导是快速制备三维集成光子芯片的一种重要手段,波导有效折射率的准确测量对于设计光子器件意义重大。设计并制备了一种断臂马赫-曾德尔干涉仪（MZI）结构对玻璃波导有效折射率进行原位精密测量。激光在断线区域和波导内的有效折射率不同,在传输相同长度下产生一定的相位差,最终导致不同的干涉结果。对断臂MZI结构的相位干涉结果进行处理,得到激光直写玻璃波导的有效折射率为1.504+7.7×10?4。利用RSOFT软件光束传播算法对器件进行模拟仿真,仿真结果与实验吻合良好。该精确测量玻璃波导有效折射率的方法对于提升光子芯片设计与制造能力具有重要意义。     

基于狭缝波导的聚合物基微环折射率传感器研究

基于狭缝波导结构,设计了工作波长在890 nm的聚合物基微环。从折射率传感的角度详细分析了狭缝波导的模场特性。分析了波导高度、宽度及狭缝宽度对灵敏度的影响。传统的狭缝波导具有较高的弯曲损耗,这会影响微环谐振器的品质因子Q以及消光比。设计了非对称的狭缝结构,保证波导模式位于波导中央传输,降低弯曲损耗。为了条形波导与狭缝波导更好的耦合,设计了基于多模干涉结构的条形-狭缝波导模式转换器。仿真表明设计的微环谐振器的传感灵敏度达到109 nm/RIU。     

Polymer Optical Interconnects—A Scalable Large-Area Panel Processing Approach

A flexible approach to producing optical interconnects on 609.6$ast ,$609.6 mm large-area panels is demonstrated. Stepwise projection patterning from 101.6$ast ,$101.6 mm masks has generated optical waveguide patterns over the whole panel using large-area projection lithography equipment. The waveguide routing design allows optical waveguides on different 101.6$ast ,$101.6 mm tiles to be interconnected. Four different waveguide connecting geometries in the border region between tiles have been fabricated and tested. Multimode waveguides from inorganic-organic hybrid polymers (ORMOCER) (cross section:$le hbox 50~muhbox mast hbox 10~muhbox m$) with refractive index step between core and cladding$Delta n=hbox 0.01$were produced. The index step was adjusted by mixing two diffrent ORMOCER systems. The materials show good adhesion to numerous substrates, such as glass and silicon. Application concepts such as flexible manufacturing of optoelectrical hybrid backplanes with two-dimensional interconnect, a three-dimensional optical interconnect with optical vias, and a hybrid backplane with the optical interconnect in a strip-format on a separate plane right above the electrical plane are proposed. Promising new technologies are presented along with preliminary demonstrativ viability.     

The design of triple rib waveguide couplers by the discretespectral index method

The operations of three-guide couplers are usually controlled by the width and refractive index of the guides. However, when rib waveguides are used in three-guide couplers and power dividers, we propose to use the dimensions of the ribs as the governing parameters instead. Formulating a new transcendental equation using the discrete spectral index (DSI) method, triple rib waveguide couplers are accurately analyzed for the first time. The new accurate and fast analysis of triple rib waveguides shows that the propagation constants of the various modes are dominantly controlled by the height of the middle rib. A systematic and simple procedure is consequently introduced to design quasi-transverse electric (TE) polarized triple rib waveguide couplers, which ensures maximum power transfer from one outermost guide to the other outermost guide. For triple rib power dividers, the height of the middle rib is found to control the amount of power that can be transferred from the excited middle rib waveguide to the two outer rib waveguides. Maximum power division is achieved when the three ribs of the waveguide are identical     

Integrated optical refractometer based on waveguide bend loss

This paper reports on a refractometer, which is based on measuring the throughput of a bent channel waveguide. A new type of channel waveguide, designed in SiON technology makes the throughput strongly dependent on the refractive index of a measurand fluid. This new waveguide can be designed for any measurand refractive index range between 1.00-5 in an aqueous solution with a refractive index of n=1.37. The sensor characteristics, make this simple sensor very interesting for controlling a chemical concentration during chemical processes, e.g., in the food and beverages industry     

Test structures for characterization of electrooptic waveguidemodulators in lithium niobate

A method of determining the critical parameters of waveguide modulators, using a set of test devices fabricated on a single chip, is presented. The five parameters are the depth and lateral Ti diffusion lengths, the peak index change in the waveguides, the electrooptic coefficient, and the buffer layer dielectric constant. The finite element method is used for calculation of optical modes in waveguides with graded refractive index profiles. The integral equation method is used for calculation of the static electric field due to electrodes in a three-layer structure of air, buffer layer, and LiNbO₃. The test set includes a planar waveguide, Mach-Zehnder modulators, symmetrically perturbed directional couplers, and widened X modulators. Several test chips have been fabricated using different fabrication conditions. The parameter values determined using this method are compared with those reported by other authors     

退火质子交换LiNbO3光波导低损耗耦合分析

利用光束传播法分析了退火质子交换铌酸锂光波导模场参数随波导宽度、深度和表面折射率改变的变化情况,指出波导模场受这些参数的综合影响,并讨论了波导结构参数对光纤．波导耦合效率的影响。结果表明,要实现最佳耦合,不同模场直径的光纤对波导结构参数的要求是不同的,这可供相应波导器件的设计和制备参考。     

Silica-based circular cross-sectioned channel waveguides

A technique for fabricating silica channel waveguides with circular cross sections is presented. The geometrical mismatch between a square or rectangular waveguide and a regular optical fiber is therefore largely reduced. The fiber-waveguide-fiber coupling loss drops from 1.8 dB to 0.5 dB or less for step index multimode waveguides by applying this technique. The heat treatment applied in the waveguide rounding process also smooths the waveguide sidewalls and results in much lower propagation loss in the waveguides. Uniform Y splitters were fabricated to show the feasibility of using this technique to make integrated optical devices.<>     

Polarization-independent thermooptic phase shifters in silicon-oxynitride waveguides

The effective refractive index of dielectric waveguides can be tuned using the thermooptic effect. In general, the tuning efficiency is polarization-dependent owing to temperature-induced stress in the layers, which causes polarization-dependent loss in optical devices. These stress issues are analyzed and tested for a high-index-contrast waveguide structure based on a silicon-oxynitride core. Experimental results are in agreement with simulations. The relative difference in tuning efficiency for transverse electric and transverse magnetic polarized light can be tuned from -3% to +3% by appropriate waveguide technology control. The optimized thermooptic phase shifters show tuning efficiency differences below 0.25%, which are reproducible from wafer to wafer.     

Measurement and modeling of on-chip transmission line effects in a400 MHz microprocessor

On-chip interconnect delays are becoming an increasingly important factor for high-performance microprocessors. Consequently, critical on-chip wiring must be carefully optimized to reduce and control interconnect delays, and accurate interconnect modeling has become more important. This paper shows the importance of including transmission line effects in interconnect modeling of the on-chip clock distribution of a 400 MHz CMOS microprocessor. Measurements of clock waveforms on the microprocessor showing 30 ps skew were made using an electron beam prober. Waveforms from a test chip are also shown to demonstrate the importance of transmission line effects     

Simulation of Tunable Power Multimode Interference Couplers with Deep Rib Structure

The muhimode interference （MMI） couplers, which operate at 1. 55 microns in deep rib InGaAsP/InP waveguide with large lateral confinement and tunable power splitting ratios, are of high interest in integrated optics. The gold contacts are applied on the top of waveguides where tuning is desired and the plasma effect will lead to negative refractive index change. The three-dimensional （3D） finite difference beam propagation method（FD-BPM） is used to model the tunable MMI couplers. The length of a 2 × 2 overlap-MMI is determined by FD-BPM, so the longitudinal position of tuning spots is obtained. The position of gold contacts with two types, the edge-pads or center-pad, are also determined. In our design, the length of MMI is 180 microns. If the width of pads is 50 microns and the refractive index is tuned from 0 to -0. 027, the power ratio is tuned from 50 ： 50 to the maximum 88.5 ： 11.4. For deep rib structure, the effective index（EI） method can not be used to simplify the 3D waveguide to plane waveguide because of its lower precision, and so the direct 3D FD-BPM simulation is necessary for the design of 3D MMI couplers.     

Low-loss, single-mode, organic polymer waveguides utilizingrefractive index tailoring

Low-loss, single-mode optical waveguides have been fabricated from photopolymerizable acrylic monomers. The material system consists of a low-index cladding resin and a high-index core resin. The two resins are miscible so that precise control over the refractive index can be obtained. This allows the fabrication of single-mode waveguides with specific cross-sectional dimensions. One advantage of this is the ability to fabricate waveguides with high coupling efficiencies to other devices such as optical fiber or semiconductor lasers. The materials adhere to a wide variety of substrates and exhibit average waveguide losses of 0.56 dB/cm at 1300 nm for single-mode waveguides. Details of the fabrication procedure, index of refraction tailoring technique, and waveguide loss data are presented     

基于参数化模型的FPGA时钟网络设计和优化

本文在FPGA时钟网络（Clock Distributed Network,CDN）关键结构尺寸的参数化建模基础上,提出一种针对全定制FPGA CDN的设计和优化方法.本文所建立的参数化模型将结构尺寸分为拓扑结构和电路与互连两类,分别给出了这两类尺寸参数的设计原则.在标准CMOS 0.13μm工艺下,对H树型、鱼骨型以及混合型三种类型时钟网络设计了2组结构参数,分别代表优化前和优化后,对比分析延时、偏斜、功耗和面积等性能参数.实验结果显示：混合型结构在绝对延时和时钟偏斜上减小最多,分别达到20.89%和63.20%;鱼骨型结构的面积减小达到50.14%;H树型结构的绝对延时和功耗则均降低了7.37%和8.33%.以上结果充分证明了本文所提设计优化方法的有效性.     

Rotary traveling-wave oscillator arrays: a new clock technology

Rotary traveling-wave oscillators (RTWOs) represent a new transmission-line approach to gigahertz-rate clock generation. Using the inherently stable LC characteristics of on-chip VLSI interconnect, the clock distribution network becomes a low-impedance distributed oscillator. The RTWO operates by creating a rotating traveling wave within a closed-loop differential transmission line. Distributed CMOS inverters serve as both transmission-line amplifiers and latches to power the oscillation and ensure rotational lock. Load capacitance is absorbed into the transmission-line constants whereby energy is recirculated giving an adiabatic quality. Unusually for an LC oscillator, multiphase (360°) square waves are produced directly. RTWO structures are compact and can be wired together to form rotary oscillator arrays (ROAs) to distribute a phase-locked clock over a large chip. The principle is scalable to very high clock frequencies. Issues related to interconnect and field coupling dominate the design process for RTWOs. Taking precautions to avoid unwanted signal couplings, the rise and fall times of 20 ps, suggested by simulation, may be realized at low power consumption. Experimental results of the 0.25-μm CMOS test chip with 950-MHz and 3.4-GHz rings are presented, indicating 5,5-ps jitter and 34-dB power supply rejection ratio (PSRR). Design errors in the test chip precluded meaningful rise and fall time measurements     

Investigation of mode coupling due to ohmic wall losses inovermoded uniform and varying-radius circular waveguides by the methodof cross sections

The effect of ohmic wall losses on mode coupling in overmoded varying-radius circular waveguides is investigated. Mode coupling and multimode propagation in uniform lossy-wall circular waveguides is also discussed. The expressions for the coupling coefficients are given by line integrals of the power-normalized fields of the normal modes along the boundary of the waveguide cross section. Numerical results are presented for the case of propagation of an HE₁₁-like mode excitation in a uniform smooth lossy-wall circular waveguide