Stress minimization on deep sub-micron CMOS processes, measured by a high spatial resolution technique, and its application to 0.15 micron non volatile memories

An innovative experimental method to measure local crystalline structure and deformation with X-rays at high spatial resolution has been developed. A suitable instrument has been designed and built. Experimental procedures, including careful alignment, have been established, and a special software code has been elaborated in order to trace back the strain depth profile from the diffraction profile. The method is based on an X-ray optical element, the X-ray waveguide (WG) that provides, through resonance phenomena similar to the optical interferometer, an X-ray beam highly coherent with spatial dimensions, in the direction perpendicular to the WG surface, of the order of few tens of nanometers and with a divergence of about 1mrad. Furthermore, it causes a strong flux density enhancement (output flux density over input flux density) that in the most recent WG's achieved a value as high as 100. In the course of the project we designed and build a diffractometer suitable to carry out micro-diffraction experiments and set-up experimental and data treatment procedures able to give the strain depth profile with lateral spatial resolution in the 100-300nm range. Local strain measurements with this method on test structures have been compared with CBED and simulations. The method is non-destructive, can be applied to any class of materials from microelectronics to bio-materials but requires a high brilliant source such as the synchrotron radiation. The experimental method and the diffractometer we developed together with the detailed procedures for data acquisition and interpretation are already usable to measure local structural properties of materials. The method is applicable to any kind of crystalline structures, from analysis of microelectronic materials and processes, to study of bio-materials and engineering materials, and in particular to interface problems. The technique can reach sub-micrometer spatial resolution in one direction. It is non destructive and can be applied even to working devices or to materials under stress.

This software can predict the strain and stresses generated by the different process steps of a manufacturing flow. In particular, the extrinsic sources of stresses (water diffusion, phase transformation) have been modelled and calibrated. Furthermore, complex rheological behaviours of thin film materials are available (in particular viscoelastic and elasto-plastic laws). This tool has been validated with 0.15-micron test structures for NVM applications. IMPACT is a 2D process simulator allowing the study of diffusion and stress related problems in silicon technology, a field of major interests in microelectronics. This software has been developed with Fortran 77 and 90 and uses the Finite Element Method (GALERKIN assembly on triangular mesh) to discrete the physical models. It is interfaced to the tools of the companies ISE-AG, SILVACO and AVANT to allow the simulation of some process steps using different tools. In STREAM, the models dedicated to the calculation of the stresses/strains developed during the manufacturing of the devices have been improved. More precisely, the rheological behaviours of thin film materials used in silicon technology have been calibrated using coupled Brillouin Light Scattering and Beam Bending experiments. New rheological laws have been implemented and calibrated (e.g.: elasto-plastic behaviour for (poly)-crystalline materials. In this work, we have also introduced new sources of stress, called extrinsic sources, which typically correspond to absorption/evaporation of water, phase transformation. Finally, the effect of stresses on dopant diffusion has been studied. We have quantified the activation volume of the effect of pressure on boron diffusion and the stress created by extended defects (i.e.: dislocation loops) has been studied. This modelling work has been validated against local measurements of stresses and strains in silicon substrates using the micro-Raman and CBED techniques. This tool has already been used by various major microelectronics companies to study and improve the mechanical reliability of their technologies (either CMOS or Bipolar). IMPACT contains already very advanced models for the prediction of stresses in silicon technologies. However, with the continuous decrease of the dimensions in silicon technology as well as the use of new processes and materials, it is necessary to add continuously new models and to include new physical effects. That is why, on one hand it would be interesting to continue the work with partners who develops or manufacture very advanced CMOS/BICMOS/Bipolar technologies as well as measurement capabilities. On the other hand, IMPACT is still a prototype and in particular, there is no graphical user interface and a low level interactive documentation. As a consequence, in order to commercialise this tool it is necessary to add these functionalities and as a consequence it would be interesting to have a partnership with a software house or a company specialised in these kinds of tools.

Micro-Raman spectroscopy (RS) is a non-destructive optical technique that is used to measure lattice vibration frequencies. The frequency of the Raman spectrum of Si depends on the strain in the Si lattice. In STREAM, the technique was applied to measure mechanical strain induced by shallow trench isolation and subsequent processing steps. The following new results were obtained: Improved spatial resolution: The system was equipped with an autofocus, providing a constant focus of about 0.8 micron (458nm laser light, 100x objective). It was shown that this spatial resolution can be improved to 0.3 micron when combining the autofocus system with an oil-immersion objective. The autofocus system allows to perform RS experiments on larger surfaces such as complete chips. As such, the technique can also be used for studies of IC-packaging induced stress (ex. the stress induced by bonding a chip to a substrate) and stress in MEMS (ex. stress in the Si membrane of a pressure sensor) Raman data were successfully compared with CBED data and FEM results. Raman spectra were measured for different processing conditions of the STI, allowing to determine the effects of these steps on local stress. It was shown that 2-dimensional Raman spectroscopy measurements can be performed on the cross-section of samples thinned for TEM or CBED analysis. This allowed for a direct comparison between CBED and Raman results. It was shown that the relation of the Raman frequency and the stress in the sample is different when measuring on cross-sectional (110) samples and (100) samples. A simple computer program was written that allows to calculate the stress induced by a thermal inclusion (trench) in silicon, and -for some special cases- the related Raman shift. Both two-dimensional and three-dimensional calculations are possible.

Software was developed within the context of a project called STREAM (STREss minimisation and Application to Memories) sponsored by the European Union. It has been jointly developed by SIS and CNR-LAMEL and is presently the only one available with these characteristics. This project involves automatically calculating the strain of TEM/CBED images. This software includes the following tasks: - Automated acquisition of TEM/CBED patterns via a procedure, which can be routinely used by the microelectronics industry. A set of points along a 'cut line' in the cross-sectioned structure will be chosen and the electron probe will be sequentially located in each of them for the CBED pattern acquisition without requiring the operator's control. - Camera automation procedures, e.g. specific functions such as shading correction to suppress dark current offset in cameras and gain correction to compensate for inhomogeneous sensitivity due to CCD sensor deficiencies and fibre optical coupling. - Image and data archiving in a database for later research and higher productivity. A template for the database, which contains selected images and all associated significant data necessary for storing images. - Image Processing and filtering in such a manner that individual lines diffraction patterns may be automatically detected and mathematically represented. This includes computation of HOLZ pattern models to identify the relevant HOLZ lines. - Computation of the effective voltage in a TEM by comparison of simulated HOLZ patterns and experimentally achieved images on perfect silicon. - Computation of the strain tensor and the trace for a given image and a series of images acquired over an area of a semiconductor TEM/CBED images are difficult in contrast depending on the quality of the microscope and the quality of the CCD camera. The HOLZ lines are not always clearly visible and sophisticated image pre-processing must be done in advance to detect the lines automatically. With the CBED module running under analySIS® by Soft Imaging System, convergent beam diffraction TEM images of Si (130), (230) and (150) acquired at 100keV and 200keV can be analysed. The automatic algorithm detects the lines precisely and the results can be obtained within a few minutes. The software calculates the effective voltage and the strain tensor and its trace by simply clicking on a button. Using this software means that a series of images of a semiconductor structure can be analysed within a fraction of the time otherwise needed when using the traditional manual method. The software will guide the user through the various steps of pre-processing and image analysis by a GUI (Graphical User Interface). The GUI has been set up in such a way that user effort is reduced to a minimum of parameters, which makes reproducibility of results much reliable.

Livrables

Partager cette page

Télécharger