Crid Yu

Use of short-loop electrical measurements for yield improvement

Modern submicron processes are more sensitive to both random and systematic wafer-level process variation than ever before. Given the dimensional control limitations of new technologies, the amount of wafer-to-wafer and within wafer nonuniformity of many steps is becoming a significant fraction of the total error budget, which already includes the usual step-to-step allocations. However, a significant portion of the total observed variability is systematic in nature. Accordingly, particle defects may not continue to dominate parametric yield loss without improved understanding of parametric variations. In this paper, we demonstrate the use of short-loop electrical metrology to carefully characterize and decouple wafer-level variability of several critical processing steps. More specifically, we present our method and give results obtained from variability analyses for lithography critical dimension (CD) and inter-level dielectric (ILD) thickness control. Using statistically designed experiments and dedicated test structures, the main factors affecting dielectric thickness variability has been identified. The systematic variability from a wafer stepper has been extracted using a physically based statistical data filter. Once isolated, the deterministic variability can be modeled and controlled to enhance process and circuit design for manufacturability (DFM). We hope that in the future this work will be coupled with novel DFM-oriented CAD tools that encapsulate this information in a fashion that makes it useful to process and circuit designers. >

Trench doping conformality by plasma immersion ion implantation (PIII)

Plasma immersion ion implantation (PIII) is a technique which can be used to conformally dope sidewalls of Si trenches. Using junction staining techniques and subsequently calibrating the observed stained depth to measured dose, dopant distributions inside Si trenches with aspect ratios ranging from 1 to 12 are studied for various bias voltages from 5 to 20 kV. Unlike conventional collimated beam implantation, PIII was able to conformally dope all aspect ratios studied with no evidence of abrupt discontinuities in the dopant distribution along the trench surface as a result of beam shadowing by trench geometry. Furthermore, it is shown that the higher implant biases results in more directional trajectories. Thus, dopant distributions along irregular geometries can be controlled by PDIII process conditions.<<ETX>>

Contributions of stepper lenses to systematic CD errors within exposure fields

We have studied systematic line width (CD) errors as functions of field coordinates for three late-model i-line steppers from different manufacturers by measuring electrical resistance of lines patterned in poly-silicon. The combination of reticle errors with non-linear imaging accounts for a significant fraction of the total line width errors. After removing the effects of reticle errors, CD contour maps are consistent with aberration patterns in which the Strehl intensity is highest at the center of the field.

Patterning tool characterization by causal variability decomposition

A spatial and causal classification of process error provides opportunities for the accurate determination and efficient management of process error budget. Traditional metrology is posed with this dilemma: variability sampling requires cheap, highly repeatable metrology, such as electrical measurements, which also confound error sources of the variability sampled. In response, statistical metrology has been proposed as a novel combination of cost-effective metrology with subsequent statistical or experimental data processing to provide a technique that is capable of error decomposition into equipment causes. The methodology, consisting of 1) reticle and experiment design, 2) data filtering, and 3) error budget formulation, is presented and is general to a short-loop thin-film patterning sequence. A .35-/spl mu/m polygate patterning sequence is chosen to demonstrate this technique. Reticle design and statistical filtering have been presented in a previous publication, and are summarized here. The second causal data filter is presented in this work, Aided by additional experimentation, a physical filter decomposes the separate contributions and interactions of the reticle and stepper. A portion of the error budget is calculated, including the effects of spatial correlation. The results of decomposition yields a numerical metric for equipment and process manufacturability. Results are presented that illustrate the use of the manufacturability metric in equipment selection and process design.

Intrafield linewidth variances in 0.25 μm i‐line lithography

We have studied variations of the widths of polysilicon lines patterned to nominal dimensions of 0.25 μm with an i‐line stepper. The analysis is based on measurements of electrical resistance of the etched polysilicon patterns. Systematic dependence of linewidths on exposure field coordinates accounts for a large fraction of their total variance. We have expressed the systematic intrafield variance as a sum of three terms: one attributed to the stepper, another to the reticle, and a third to both. The systematic intrafield dependence is most significant for small isolated lines; this term alone had a 3σ value of 25 nm at a nominal dimension of 0.25 μm.

Manufacturability evaluation of deep submicron exposure tools using statistical metrology

Statistical Metrology has been proposed as a technique to extract variability components of an IC process sequence through the combined use of conventional metrology and statistical filtering. We have developed a methodology to decompose and categorize CD variability into individual equipment contributions, specifically the steppers and reticles used in 0.35 /spl mu/m polysilicon gate patterning. Spatial variability was sampled using a reticle designed to collect CD measurements over the exposure field and the wafer. Then, a series of statistical and physical filters were implemented to separate the reticle and stepper contributions to CD variability. Results have shown that CD variability has strong spatial and causal components. Decomposition results are applied towards: (1) Identifying bottlenecks in manufacturability by providing an accurate error budget analysis. (2) Using isolated equipment variability components as a manufacturability metric to benchmark exposure tools. (3) Quantifying the correlation between spatial variability components can be manipulated to improve net process manufacturability.

SEM characterization of etch and develop contributions to poly-CD error

CD variability is usually managed using an error budget, which apportions the total allowable error into individual process components. Automated SEM metrology can provide high quantities of samples at intermediate points in a process sequence and can be used to estimate error budget items. However, evidence suggests that SEM CD measurements can be tainted by sequencing artifacts such as trend and autocorrelation. A methodology is developed to characterize the CD variability introduced by automated SEM measurements and remove them through statistical filtering. This technique is applied to estimate variability after resist development.

Effect of Reticle Erros on Systematic Intrafield Line Width Variations

We have studied the contributions of reticle line width ( CD) errors to systematic intrafield CD variations for 0.35 µ m i-line lithography by measuring electrical resistance of lines patterned in polysilicon films. Reticle errors were determined by independent measurements, and their effects on final printed dimensions were analyzed. Both reticles and steppers contribute to the systematic intrafield variation. In some cases, the total variance exceeds the sum of the variances individually associated with steppers and reticles. Because aberrations and reticle errors are not random, analysis of variance must take their correlation into consideration. We have observed cases in which this correlation and nonlinear imaging contribute significantly to total line width variance.

Conformal doping of high aspect ratio trenches by plasma immersion ion implantation (PIII)

Plasma Immersion Ion Implantation (PIII) has been used to conformally dope trenches with aspect ratio up to 12 and trench openings from 1 to 5 μms. The substrate bias affects the directionality of the incoming ions. Optimal processing conditions for conformal doping were found to depend on the aspect ratio of the trench.

Plasma Immersion Ion Implantation: A Perspective

Many kilovolts can be sustained in the sheath between the boundary of a high ion density plasma generated by Electron Cyclotron Resonance (ECR) and a negatively biased substrate in the plasma. Very high dose rate implantation can occur as ions are accelerated from the edge of the sheath towards the substrate. This, combined with simple reactor design, allows plasma immersion ion implantation (PHI) to be used in thin film modification applications that are not viable for conventional implanters. We have used PIII to form sub-100nm p+/n junctions and to getter impurities by backside implantation. By using a separately biased target, ionization of sputtered material can occur and plasma assisted vapor deposition can be achieved. This can also be done with concurrent ion beam mixing of the interface. These capabilities have been demonstrated in Pd seeding for selective electroless copper plating. Through collisions with neutrals in the sheath, the ions can acquire an energy and angular distribution dependent on processing conditions. This has been used to conformally dope high aspect-ratio trenches. As with any high dose implantation, wafer heating is potential problem for PIII as well as substrate sputtering during low implant energy operations. Other potential problems of PIII include pasma/substrate interactions in the forms of etching, deposition, or particulate formation on the substrate.