Michael Gostein

Laser-based surface acoustic wave spectrometer for industrial applications

A compact, robust, and fully automated optical system for laser generation and heterodyne detection of surface acoustic waves (SAWs) in the frequency range 0.1–1.3 GHz is presented. SAWs are generated by two short laser pulses forming an interference pattern at the sample surface, and detected via diffraction of a quasi-cw probe beam off the surface ripples produced by the SAWs. The detection is enhanced by optical heterodyning achieved through the introduction of a fourth (reference) beam. Both excitation and probe/reference beam pairs are produced by a phase grating optimized to provide maximum efficiency for the desirable diffraction orders at the two different optical wavelengths used for excitation and probing. The heterodyne arrangement has an excellent phase stability and provides a number of advantages compared to a conventional transient grating setup. These include enhanced signal level and suppression of “parasitic heterodyning” due to scattered light. The system provides reproducibility of SAW...

Infrared Reflectometry For Metrology Of Trenches In Power Devices

We present examples of the application of Model- Based Infrared Reflectometry (MBIR) to metrology of trench-based power devices at different stages in the fabrication process. These metrology problems are similar to those encountered during the fabrication of deep trench memory devices, but pose some new challenges. We propose analysis methods that overcome the challenges and produce data useful for process control in fabrication facilities. The results indicate that MBIR can become a powerful metrology tool in a variety of applications during the fabrication of silicon power devices.

Measuring Trench Structures For Microelectronics With Model‐Based Infrared Reflectometry

We provide an overview of state‐of‐the‐art metrology of periodic trench structures based on infrared reflectance spectroscopy, focusing on measuring dimensional parameters of trench arrays in silicon. The presentation outlines classes of trench metrology problems for which infrared optical measurement is advantageous versus visible/UV techniques, such as measuring very high aspect ratio trenches used to form dynamic random access memory capacitors. We discuss modeling approaches and compare effective medium models with rigorous coupled wave analysis calculations. The discussion is illustrated by infrared reflectance spectra from both model and real‐life examples of trench structures.

Non-contact metal film metrology using impulsive stimulated thermal scattering

A non-contact, non-destructive method for measuring the thickness of opaque metal films in circuit interconnect layers is presented along with typical metrology applications. The technique, known as Impulsive Stimulated Thermal Scattering or Transient Grating optoacoustics, is based on a special method for the initiation and detection of acoustic waves in the sample film using laser light. The simple optical arrangement and rapid measurement make the technique ideal for in-line on-product process-control applications, particularly for emerging processes for copper-based films. Typical applications include monitoring PVD and ECD Cu deposition and Cu CMP uniformity, on both blanket and patterned wafers.

Thickness measurement for Cu and Ta thin films using optoacoustics

New all-optical, nondestructive metrology instruments for metal film thickness measurement have been developed using the opto-acoustic technique impulsive stimulated thermal scattering (ISTS). The technique uses lasers to initiate and detect acoustic waves in the sample film. In this study, a commercial ISTS-based instrument is evaluated for measuring Cu and Ta thin films, and the results are compared with 4-point probe, SEM, TEM, and GIXR. We also show that ISTS can be used in conjunction with 4-point-probe to determine resistivity for thin PVD Cu films.

Measurement of Electroplated Copper Overburden for Advanced Process Development and Control

We used a non-contact metrology system based on laser-induced surface acoustic waves to evaluate the thickness uniformity of electroplated copper. Several wafers were prepared using different plating conditions that affect the copper overburden thickness and uniformity. Measurements of overburden were made on a variety of pattern features including trench arrays of several pattern densities, solid bond pads, and the surrounding field area. In general, the overburden was found to be thicker above the arrays of narrow linewidth, as compared to the surrounding field. However, the magnitude of this “mounding” effect is dependent on the plating process, pattern density, and position on the wafer. For example, as the accelerator in the plating solution increases, the mounding on a 50% density line array varies from zero to 5000 Angstroms. Under all plating conditions used, the mounding was found to be progressively higher as the pattern density of the array increases (in the range of 20% to 50% copper). The solid pads were found to have conformal filling, in all cases. It was also found that the overburden in the field area is reduced as the adjacent mounding increases, presumably because the reactant in the plating bath becomes depleted in the vicinity of the high overburden regions above the arrays.

Direct Measurement of Planarization Length for Copper Chemical Mechanical Planarization Polishing (CMP) Processes Using a Large Pattern Test Mask

Wehaveusedalargepatterntestmaskandaspecificarrangementofstructuresonawaferfor direct measurement of an average planarization length for copper chemical mechanical polishing (CMP)processes.Weproposenewminimum,maximum,andaverageplanarizationlengthdefini-tions, based on up and down area measurements as a function of trench width. The average pla-narizationlengthisusefulforqualitativelycomparingtheplanarizationcapabilityofcopperCMP processes. We have also performed several experiments that show how the average planarization length depends on polish process settings such as down force and relative speed, as well as on consumables such as pad and slurry.

Surface Wave Metrology for Copper/Low-k Interconnects

We review recent advances in the application of laser‐induced surface acoustic wave metrology to issues in copper/low‐k interconnect development and manufacturing. We illustrate how the metrology technique can be used to measure copper thickness uniformity on a range of features from solid pads to arrays of lines, focusing on specific processing issues in copper electrochemical deposition (ECD) and chemical‐mechanical polishing (CMP). In addition, we review recent developments in surface wave metrology for the characterization of low‐k dielectric elastic modulus, including the ability to measure within‐wafer uniformity of elastic modulus and to characterize porous, anisotropic films.

Modeling of surface acoustic waves in thin film stacks with an arbitrary number of layers (abstract)

An efficient mathematical algorithm for the analysis of surface acoustic waves (SAW) measurements in multilayered thin film stacks has been developed. It allows for a fast and accurate calculation of SAW velocities in stacks composed of an arbitrary number of layers with varying material properties and thicknesses. The inversion algorithm also enables fast determination of multiple stack parameters from SAW dispersion curves. The model is used in a semiconductor metrology tool designed for quality control in the manufacturing of integrated circuits. Surface acoustic waves are described by a special class of solutions of the wave equation satisfying specific boundary conditions at the surface of the media in which they propagate. In the case of a stack composed of thin films, the boundary conditions at the surface of the top layer correspond to a free surface (no stress), at the interface of the individual layers a continuity of both stress and displacement is imposed. In the case of isotropic media, or in...

Characterization of Electroplated Copper Films with Laser-Generated Surface Acoustic Waves

Elastic properties of electroplated copper films fabricated under different plating and annealing conditions have been characterized with laser-generated surface acoustic waves. For acoustic wavelengths longer than the characteristic copper grain size, the effective elastic modulus of copper was found to be unaffected by variations of the plating current as well as by the grain size variation due to different annealing temperatures. Consequently, copper thickness measurements with surface acoustic waves have been found to be more stable with respect to process variations than electrical sheet resistance measurements. For short acoustic wavelengths (∼ 2 νm), grain growth results in a significant increase of acoustic attenuation and a shift in the measured velocity. This finding opens an opportunity for copper structure monitoring by surface wave-based optoacoustic metrology.