1. Field of the Invention
The present invention generally relates to microfabrication techniques and, in particular, to a method for forming multiple layers of a wiring substrate and for interconnecting electrical connections on the multiple layers.
2. Related Art
Wiring substrates are typically used to electrically connect various components or devices of an electronic system. For example, a computer or other electrical system often utilizes various components including, but not limited to, many different types of passive elements and integrated circuits. These components are usually coupled to a wiring substrate that provides electrical connections between the components. For example, one type of wiring substrate used by many electrical systems is a printed wiring board, sometimes referred to as a "printed circuit board." Printed wiring boards typically use an epoxy-glass laminate as a dielectric layer and include copper traces that electrically connect the components that are coupled to the printed wiring board. Most copper traces are narrow lines of copper that extend from an electrical contact of one component to an electrical contact of another component. The copper traces do not necessarily follow a straight line path from one component to another component but instead usually bend or curve to prevent the copper traces from shorting with each other.
Other major types of wiring substrates include ceramic and thin film substrates that enable a higher wiring density (i.e., length of wire per unit area) than conventional printed circuit boards. However, the cost of manufacturing the ceramic and thin film substrates is much greater than the cost of printed wiring boards. Therefore, the use of ceramic and/or thin film substrates has been limited.
To increase the capacity of wiring substrates (i.e., printed circuit boards, ceramic substrates, or thin film substrates), the number of layers of the wiring substrate is usually increased. Therefore, many conventional wiring substrates are multilayer structures. Furthermore, to provide vertical interlayer connectivity in multilayer wiring substrates, vias (in multilayer ceramic and multilayer thin film) and plated through holes (in multilayer printed wiring board) are formed.
A via is a vertical connection that electrically connects a conductive connection of one layer to a conductive connection of another layer. Unfortunately, existing techniques for via formation (i.e., filling and stacking) involve expensive and complex planarization processes, thereby increasing the cost and decreasing the yield of manufacturing wiring substrates. In fact, the high costs associated with conventional via filling and stacking prevent their use in many applications.
On the other hand, typical plated through holes are inexpensive and simple to fabricate but are relatively large compared to vias and pass through all of the layers of the wiring substrate. Therefore, a significant amount of space within the wiring substrate is taken up by plated through holes, resulting in a low wiring efficiency.
Consequently, conventional techniques for interconnecting layers of a multilayer substrate are expensive or inefficient. Thus, a heretofore unaddressed need exists in the industry for providing an inexpensive and efficient method of manufacturing a multilayer wiring substrate.