Tensile stress positive a

Figure 7.12 Thin-film stresses a film that must be elongated to fit a wafer is under tensile stress (positive) and a film that is compressed to fit a wafer, is under compressive (negative) stress Stresses in thin films cause wafer curvature, as shown in Figure 7.12. Imagine a free film attached to a massive wafer and forcefit to the wafer size. Next, imagine, stress relaxation through the wafer curvature. A film under tensile stress will result in a concave shape, while a compressively stressed...

Etching profiles

The isotropic etching front proceeds as a spherical wave from all points open to the etchant (Figure 11.1). Because the etch profile is rounded, isotropic etching cannot be used to make fine features (Figure 11.2). Undercutting is similar to vertical etched depth. For a thin-film thickness of 500 nm, undercutting is also 500 nm, and etch bias, that is, the difference in etched feature size to mask size, is 1000 nm. The isotropic profile is the most commonly encountered etch profile. Most wet...

Wafer Cleaning and Surface Preparation

Microfabrication takes place under highly controlled conditions all materials for cleanroom construction, processing equipment and wafer-handling tools are carefully selected to minimize particle, molecular or ionic contamination. Water, gases and chemicals are purified of contaminants and filtered of particles. These are, however, passive precautions, and active wafer cleaning must be undertaken before practically every major process step. Wafer-cleaning steps can account for up to 30 of all...

Thermocompression bonding TCB

Thermo-compression bonding (TCB) applies pressure and heat simultaneously on the samples. This is the standard bonding technique for attaching gold leads to ICs. Gold is suitable because it is noble metal there are no gold oxides on the surface prevent TCB, and the low yield point of gold is also advantageous. Typical pressures and temperatures for wafer level TCB with metals are in the range 1 to 10MPa at 300 to 400 C. Bonding times are then minutes or tens of minutes. Nitrogen atmosphere...

Amorphous silicon

PECVD of silicon from silane results in amorphous silicon with a lot of embedded hydrogen. The film is designated a-Si H and its hydrogen content can be up to 30 atomic- (and much less in weight ). The film is amorphous because PECVD temperatures are low, in the range of 150 to 350 C, and the atoms do not have enough energy to find energetically favourable positions but come to rest upon impingement. Amorphous silicon can be deposited on glass, and its biggest industrial application is in the...

Solgel

A sol is a colloidal suspension of small (1-1000nm) particles in a liquid. A gel is 3D solid network that forms in a colloidal liquid. A typical sol-gel process uses metal alkoxides M-(O-CH3)n in organic solvents. Alkoxides hydrolyze according to M(OR) + XH2O > M(OH) + xROH and grow by condensation reaction, (OR)nM-OH + HO-M(OR)n > A great variety of simple methods can be used for sol-gel processing for example, dipping, spraying and spinning. Compositional variation (by changing alkoxides...

Epitaxy

Epitaxial deposition is a very special case of thin-film deposition. Epitaxy means the growth of a single crystalline layer on top of a single crystalline substrate. The growing layer registers the crystalline information from the layer below. In order to do so properly, the crystal lattices of the two layers must be closely matching. Because crystal information is 'transmitted' across the substrate-film interface, surface quality of the starting wafers is of paramount importance. Defects, be...

Thinfilm Materials and Processes

Thin-film processes are needed to make metal wires and to insulate those wires, to make capacitors, resistors, inductors, membranes, mirrors, beams and plates, and to protect those structures against mechanical and chemical damage. Thin films have roles as permanent parts of finished devices, but they are also used intermittently during wafer processing as protective films, sacrificial layers and etch and diffusion masks. Metallic, semiconducting and insulating films are employed (Table 5.1) in...

Contrast

Photoresist contrast is important for both resolution and profile. A sigmoid (non-linear) response function is essential for patternability. Optical wavefronts after mask are not ideal square waves but rather attenuated sine waves, and linear response as a function of exposure dose is rather useless because the photoresist patterns are smoothly curving bumps, and not clearly defined rectangular shapes. Contrast is calculated for positive and negative resists as Yp (log (dc do))-1 Yn (log(d0...

Note on drawings

The z-dimension is enlarged relative to xyy-directions to make drawings easier to read. MOS transistor gate oxide is usually 2 of gate thickness, and if it were drawn to scale, it would not be seen. In bulk micromechanics, the diaphragm of a piezoresistive sensor is, for example, 20 m, or 5 of wafer thickness, and the piezoresistor diffusion depth is 5 of diaphragm thickness, that is 1 m. If the drawing is to scale, it will be specifically notified all other figures in this book have z-scale...

Measurement of adhesion layers and barriers

The first adhesion test is tape-pull test adhesive tape (standard office tape is commonly used) is attached to the thin film and pulled off. If the film peels off with the tape, it has failed the adhesion test. More advanced tests use a quantifiable pull force. Adhesion layer and diffusion-barrier stability can be checked by electrical and physical measurements. Sheetresistance increase is a quick and simple measurement. Copper resistivity is very low, 1.7 ohm-cm, and when the barrier fails,...

Wetetched Silicon Structures

Microsystems technology relies on anisotropic wet etching of silicon for many major applications. Bulk micromechanics depends on silicon crystal plane-dependent etching, and many surface micromechanical and SOI devices make use of silicon wet etching for auxiliary structures, even though main device features are defined by plasma etching. Because < 100> silicon is the workhorse of microsystems, the discussion concentrates on it. Both < 110> and < 111 > etching will be reviewed...

Etching with a hard mask

Anisotropic Etching 111 Wet

In wet etching the resist is usually not consumed by the etchant, and the gravest danger is adhesion loss. This is dependent on priming, feature size, resist thickness and the chemical character of the resist. Generally, thicker resists are mechanically more stable. Interface stability is important for the etched profile because the etchant can easily propagate along the film resist interface. Photoresists are materials that combine photoac-tivity and mechanical thermal chemical stability, and,...

Chemical shrink of dark field structures

Contact Hole Etch

The resist thinning method does not work for dark field patterns any loss of linewidth will result in wider structures. A poor man's method of small DF structures is based on resist flow resist will flow when heated above glass-transition temperature. This flow will, under favourable conditions, make holes and trenches smaller in a controlled fashion. This method has been successfully used in contact hole scaling studies. A more advanced version for making narrow dark field patterns consists of...

Pinhole hole in a chrome

pin spot (extra chrome on a light field area). From the yield and reliability point of view not all defects are equal. Defect must be understood as a Figure 8.4 Mask defects defects smaller than the feature size will affect final dimensions and, therefore, current density, electric field and other device parameters. Redrawn after Skinner, J.G. et al., by permission of SPIE very broad term anything that prints on the wafer or changes critical dimension by more than 10 is counted as a defect....

Measurement of epitaxial deposition

Three measurements must be carried out on epitaxial wafers thickness, resistivity and surface quality. Surface quality is assessed first and foremost by optical inspection pyramids, mounds and hillocks scatter light, which can be detected by optical methods. Nomarski interference contrast microscope detects surface height differences and infrared depolarization reveals stresses. Laser scattering measures particles and microrough-ness. Optical methods are fast, and 100 of wafers are inspected....

Optical Lithography

Lithography work flow consists of the following major steps when viewed from the point of view of the wafer 1. Photosensitive film (photoresist) application The alternative view is that of information flow this will be discussed in Chapter 10 in conjunction with lithography simulation. Optical lithography is basically photography. The original image to be transferred, the photomask, which corresponds to the negative in photography, is set in a mask-aligner exposure tool. It is aligned to the...

Float zone FZ crystal growth

Silicon 110 Wafer Secondary Flat

If high purity or oxygen-free silicon is needed, float zone (FZ) crystal growth is used. In the FZ-method, a polysilicon ingot is placed on top of a single-crystal seed. The polycrystalline ingot is heated externally by an RF coil, which locally melts the ingot. The coil and the melted zone move upwards, and a single crystal solidifies on top of the seed crystal. The highest FZ-silicon resistivities are of the order of 20 000 ohm-cm, compared to 100 to 1000 ohm-cm for CZ. Because there is no...

Characterization of PVD films

PVD films, especially sputter-deposited films, can be modified by a number of parameters. System configuration and geometry come to play via target-substrate distance, base pressure gas phase impurities and power coupling scheme bias voltage and process parameters such as pressure and power affect the momentum of the impinging atoms and ions, and substrate temperature is important for desorption, diffusion and reactions. Collimated sputtering is a technique in which a mechanical grid is placed...

Introduction to Microfabrication

Director of Microelectronics Centre, Helsinki University of Technology, Finland John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Email (for orders and customer service enquiries) cs-books wiley.co.uk Visit our Home Page on www.wileyeurope.com or www.wiley.com AH Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording,...

A

SiO2 18 32 02 12-FEB 3 Oxthi 0.4236 SiO2 18 32 02 12-FEB 3 Oxthi 0.4236 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Depth m 0.00 0.20 0.40 0.60 0.80 1.00 1.20 Depth m Figure 3.3 a 1D simulation ICECREM of arsenic 150keV energy and boron 50keV implantation into silicon, dose 1015 ions cm2 and b dry oxidation of BF2 implanted silicon 20keV, 1015 ions cm2 modified by the user, but default parameters are good for initial simulations and novice users. Simulation examples in Chapters 6, 13, 14 and 15 are...

Lithography metrology

Lithography produces test structures of itself. Test structures must include resolution structures with the same dimensions as the devices themselves, but also smaller and larger structures so that process robustness and linearity can be checked. Optical microscopy and scanning electron microscopy SEM are standard methods. Even when linewidths are below optical microscopy resolution, it is useful as an initial check for instance, resist adhesion loss, delamination and other gross errors can be...

Simulation of Microfabrication Processes

Microfabrication processes consist of tens or hundreds of steps that take weeks or months to complete, and therefore the learning cycles can easily become too long. Simulation is one way of shortening the learning cycles. Simulation accuracy is strongly dependent on the details of the process to be simulated, and even a simple simulator can be extremely valuable if it saves enough experimentation time and effort. Simulators can provide meaningful trend data and comparisons between different...