Method of micromachining an integrated sensor on the surface of a silicon wafer

A method for micromachining a surface of a silicon substrate so as to form a sensing device in the same substrate as at least one semiconductor device, the method comprising the steps of:

• forming an N+ region in the surface of the substrate;
• forming a silicon layer over the surface of the substrate so as to form an N+ buried layer beneath the silicon layer;
• forming an etch mask on the silicon layer;
• etching a plurality of trenches into the substrate and into the N+ buried layer;
• laterally etching the N+ buried layer so as to form a cavity beneath the silicon layer, the lateral etching step being conducted with chlorine gas at a pressure of about 100 to about 1000 mTorr and with the substrate at a temperature of at least about 35.degree. C. so as to preferentially etch the N+ buried layer;
• forming an oxide layer on the substrate and on the surfaces defined by the trenches and the cavity;
• and
• depositing a polysilicon layer over the oxide layer so as to seal the trenches, such that the cavity is a sealed cavity beneath the surface of the substrate;
• whereby a micromachined element is formed between the sealed cavity and the surface of the substrate, the micromachined element being a component of the sensing device.

A method for micromachining a surface of a silicon substrate so as to form a sensing device therein, the method comprising the steps of:

• forming a first N+ region in the surface of the substrate by doping the substrate with a first n-type species;
• growing a first epitaxial silicon layer over the surface of the substrate so as to form a first N+ buried layer beneath the first epitaxial silicon layer;
• forming a second N+ region in the first epitaxial silicon layer by doping a region of the first epitaxial silicon layer with a second n-type species which diffuses relatively slowly in silicon as compared to the first n-type species;
• forming a third N+ region within a portion of the second N+ region and above a portion of the first N+ buried layer, the third N+ region being doped with an n-type species which diffuses relatively quickly in silicon as compared to the second n-type species;
• growing a second epitaxial silicon layer over the first epitaxial silicon layer so as to form second and third N+ buried layers beneath the second epitaxial silicon layer and above the first N+ buried layer;
• heat treating the substrate sufficiently to diffuse a portion of the n-type species of the third N+ buried layer into a portion of the first N+ buried layer, the heat treatment being insufficient to diffuse the first n-type species of the first N+ buried layer into any portion of the second N+ buried layer and being insufficient to diffuse the second n-type species of the second N+ buried layer into any portion of the first N+ buried layer, such that a portion of the first epitaxial silicon layer remains between a portion of the first N+ buried layer and a portion of the second N+ buried layer;
• etching at least one trench through the second epitaxial silicon layer and into at least one of the first, second and third N+ buried layers;
• and
• laterally etching a cavity beneath the second epitaxial silicon layer, the lateral etching step being conducted with a chlorine-containing gas so as to preferentially etch the first, second and third N+ buried layers;
• whereby a micromachined element is formed between an upper portion of the cavity and a lower portion of the cavity, the micromachined element being a component of the sensing device.

A method for encapsulating a structure formed on a surface of a semiconductor material, the method comprising the steps of:

• selectively patterning polyimide on the surface of the semiconductor material so as to cover the structure;
• depositing over the polyimide a first layer of material selected from the group consisting of silicon nitride and silicon dioxide;
• annealing the first layer of material at a temperature and for a duration which are sufficient to relieve stress in the first layer of material;
• forming at least one opening in the first layer of material;
• etching the polyimide through the opening in the first layer of material such that at least a portion of the first layer of material is suspended over the structure on the surface of the semiconductor material;
• and
• depositing over the first layer of material a second layer of material, the second layer of material sealing the opening in the first layer of material so as to form an encapsulating layer over the structure.

A method for micromachining a surface of a silicon substrate so as to form a capacitor therein, the method comprising the steps of:

• forming an N+ region in the surface of the substrate;
• growing an epitaxial silicon layer over the surface of the substrate so as to form an N+ buried layer beneath the epitaxial silicon layer;
• masking the epitaxial silicon layer;
• etching at least one trench through the epitaxial silicon layer and into the N+ buried layer, the trench defining opposing plates of the capacitor;
• laterally etching a cavity beneath the epitaxial silicon layer, the lateral etching step being conducted with chlorine-containing gas such that the N+ buried layer is preferentially etched;
• depositing a conductive film on the opposing plates of the capacitor so as to reduce the distance between the opposing plates;
• and
• etching back the conductive film so as to isolate the opposing plates from each other.