A substrate is an underlying material or layer of material upon which other materials or processes are carried out. It can involve anything from subsoil surfaces in agriculture upon which plants are grown to layers of silicon semiconductor used as a foundation for building electrical circuits, and cells or other biological mediums upon which enzymes act. The term substrate is often used in medicine and microchip manufacturing, but it is also common to geology and other natural processes.
The silicon substrate is, without a doubt one, of the most complex synthetic structures manufactured at a micrometer and nanometer scale. Microchip and solar cell designs utilize multiple horizontal layers, with a typical microchip as of 2011 having several functional substrates, or foundations for layers above them. The semiconductor material starts with a device layer of transistors, which is topped by an interconnect layer to a device layer above it. Between these sections, both insulation and metallization substrate layers are added, as well as bonding layers to keep the entire structure together and functional. Research into interlinked horizontal and vertical layers, making a three-dimensional microchip in cube form, is the next step forward in increasing memory and processing speed.
In contrast to circuit manufacturing, an enzyme substrate is already a natural three-dimensional form. Enzymes are protein molecules that act as catalysts in biochemistry. Nicotinamide adenine dinucleotide (NADH) is an example of a coenzyme of the vitamin B3, that attaches itself to a cell substrate in the human body. The active substrate site where it binds is then altered by the enzyme and this section of the substrate is released as a product of the reaction. Since the enzyme itself is not changed in the process, it then moves on to other locations to continue enabling essential reactions, such as cellular respiration and energy production in the body.
Geological processes often refer to substrates as a method of determining the age and composition of the earth's crust as well. Lower layers of rock deposits, often referred to as strata, are considered older and often contain early fossilized examples of life on Earth, if they were deposited horizontally and not turned over by climate effects. Similar research conducted by examining deep layers in ice sheets in Antarctica is used to determine the makeup of the atmosphere in the distant past through trapped gasses in ice substrates, as well as ice age periods in the history of the Earth's climate. Related processes are used in analyzing the top 6 inches (15.24 centimeters) of soil substrates for nutrients, water retention, trapped air, and so on to determine soil fertility for optimal crop growth.