Chemically sensitive sensor with lightly doped drains
A chemically sensitive sensor with a lightly doped region that affects an overlap capacitance between a gate and an electrode of the chemical sensitive sensor. The lightly doped region extends beneath and adjacent to a gate region of the chemical sensitive sensor. Modifying the gain of the chemically sensitive sensor is achieved by manipulating the lightly doped region under the electrodes.
1. A method of making a chemically sensitive sensor, comprising:
forming a substrate with a first conductivity type of dopant;
building an epitaxial layer using the same conductivity type dopant used to form the substrate, but made less dense than the dopant on the substrate;
forming an electrode layer on the epitaxial layer formed from a different, second conductivity type of dopant than the first conductivity type of dopant used to form the substrate, wherein the density of dopant on both the electrode layer and the substrate are similar;
masking and etching the electrode layer to produce gates and corresponding electrode pairs;
creating a first lightly doped region adjacent to one electrode of each of the electrode pairs using a multidirectional implant technique, wherein the first lightly doped region is formed from a dopant of a conductivity type opposite the epitaxial layer dopant;
producing diffusion nodes that are self-aligned with the electrode pairs, a first of said diffusion nodes contiguous with the first lightly doped region, from a dopant of a conductivity type similar to the gates, electrode pairs, and lightly doped region; and
forming a floating gate electrode, and contacts for the electrode pairs by alternating layers of insulation, dielectric, conductive and metal layers.
2. The method of claim 1 , further comprising:
creating a second lightly doped region adjacent to the other electrode of each of the electrode pairs using a multidirectional implant technique, wherein the second lightly doped region is formed from a dopant of a conductivity type opposite the epitaxial layer dopant.
3. The method of claim 1 , wherein a second of said diffusion nodes is contiguous with a second lightly doped region.
4. The method of claim 1 , further comprising:
forming an additional electrode on the diffusion nodes.
5. The method of claim 1 , further comprising:
forming a microwell to hold a sample above the floating gate.