The search to find a technology to replace surface mounted discrete passive components is being fueled by the recent trends in the microelectronics industry of increasing component density, improved electrical performance, lower costs, and improved reliability.  In order for these trends to continue, surface mount components will need to be replaced by integral passive devices which can be incorporated directly into the printed wiring board (PWB) substrate. One particularly interesting challenge is to identify materials and processes that are compatible with organic PWB substrates that can be used to fabricate such integral passives. Developing new materials for integral capacitors is one of the principle areas of integral passive research.  While the size of the capacitors is decreasing, the charge stored (capacitance) per capacitor necessary to have a working device is relatively constant. Capacitance is inversely proportional to the area of the capacitor, so materials with higher capacitance densities (i.e., higher dielectric constants) will have to be used as device dimensions continue to shrink.  Currently, the two major classes of candidate materials for integral capacitors are polymer-ceramic composites and metal oxides. 

The work proposed for this doctoral thesis is centered around a novel method for direct patterning of metal oxides from organometallic precursors, Photochemical Metal Oxide Deposition (PMOD).  The proposed method uses ultraviolet light to create patterned metal oxide films at ambient temperatures, with no need of separate deposition and patterning steps.  As a result, this method eliminates the need for costly patterning techniques that require the use of vacuum equipment.   Preliminary results have indicated that photochemically deposited films of barium strontium titanate (Ba_1-xSr_xTiO₃) have suitable electrical properties for integral passive uses in the near future.  Fourier Transform Infrared (FTIR) spectroscopy has been used to monitor both thermal and photochemical decomposition of the organometallic precursor materials.  Results indicate that these materials possess sensitivities (i.e., process times) that could be compatible with high volume production.  Qualitative studies on stress induced cracking in the converted oxide films have been carried out, and chemical additives have been evaluated as a means to help prevent cracking.  However, further investigation of the mechanisms behind stress induced cracking will need to be carried out. 

The proposed research for this Ph.D thesis will concentrate on (1) characterizing the thermal and photochemical reaction kinetics for a series of organometallic precursors. (2) Investigating the influence of precursor structure and reaction kinetics on film composition in mixed metal oxide systems. (3) Investigating the influence of processing conditions and methods on film optical, mechanical and electrical properties and (4) optimization of film dielectric constant in the BST mixed metal oxide.   The ultimate goal of the proposed research is to contribute to gaining a better fundamental understanding of the processing and properties of the materials produced using this novel technique, which could lead to further advancements in integral passives technology.

In recent years there has been a significant amount of research focused on the processing of high dielectric constant materials such as barium strontium titanate.  While a large portion of this work has centered around the development of ferroelectric memory devices, other applications such as infrared sensors, microactuators, pyroelectric detectors and both decoupling and bypass capacitors have also been studied.  A majority of the work performed has been centered around silicon technology, and involves high temperature processing of ferroelectric materials. To date, there has been very little work done on low temperature processing (<200°C) of BST for integral capacitor applications.

Regardless of the processing temperature, the creation of integrated passive devices requires the ability to carry out fine patterning of the deposited thin film.  Conventional metal and metal oxide deposition techniques (e.g. CVD, PVD, sol-gel, etc.) require numerous process steps. (see figure 3)  Such methods also often require vacuum

 Figure 1.  (Traditional subtractive etch process for metals and metal-oxides)

and high temperature processing, resulting in material compatibility issues and increased cost. The plasma based subtractive etch, that is often used to pattern blanket-film deposited materials, can be problematic due to substrate damage and difficulties associated with etching certain metals such as platinum. In response to these issues, the proposed research is directed at studying a novel method for direct patterning of metals and mixed-metal oxides using photosensitive organometallic precursors.  This research will concentrate on the evaluation and processing of a new set of materials that can be deposited at room temperature using UV radiation and/or low temperature thermal treatment.  This is a low temperature process that satisfies the temperature requirements for packaging applications , and eliminates the need for costly vacuum processing.  In this process, a metallorganic precursor is spin coated onto the substrate. After a soft bake to remove residual solvent, the precursor film can be converted to the metal or metal oxide directly by exposing it to UV light or a low temperature bake (See figure 2). This process allows one to create both patterned or blanket metal or metal-oxide films.  

 Figure 2: direct patterning of metal/metal-oxide films using organometallic precursors