Preventing stray currents in sensors in conductive media

A sensor is provided. The sensor includes a conductive substrate having side-walls; a dielectric layer overlaying a first surface of the conductive substrate, the dielectric layer including a gate dielectric having a first thickness and a field dielectric having a second thickness; a sensing layer overlaying a first surface of the gate dielectric; a non-conductive carrier wherein a second surface of the conductive substrate overlays a portion of the non-conductive carrier; and an insulating layer conformally coating at least the side-walls of the conductive substrate, wherein a first surface of the sensing layer is uncoated by the insulating layer.

BACKGROUND

When sensors and integrated circuits are immersed in a conductive fluid, unwanted electrical current can flow, disrupting the operation of the device. For example, a pH sensor and circuit are immersed in a conductive fluid (seawater). The device's sidewalls of the sensor are exposed and un-passivated due to the die separation step (sawing for example). Electrical current can then flow from the exposed sidewalls into the sensor's circuit.

Currently available pH sensors use O-rings or epoxy to seal the conductive fluid from the sidewalls. However, pH sensors for use in the ocean at depths much greater than a kilometer have failures. Specifically, the high pressure of the ocean at depth causes the O-rings or epoxy to fail. This type of failure is exacerbated by repeated pressure cycling. The failure of the O-rings or epoxy allows conductive fluid leaks and current flow from the unpassivated exposed sidewall of the device.

SUMMARY

Sensors that are immune to stray currents and method of making sensors that are immune to stray currents are described herein and will be understood by reading and studying the following specification. The present application relates to a sensor. The sensor includes a conductive substrate having side-walls; a dielectric layer overlaying a first surface of the conductive substrate, the dielectric layer including a gate dielectric having a first thickness and a field dielectric having a second thickness; a sensing layer overlaying a first surface of the gate dielectric; a non-conductive carrier wherein a second surface of the conductive substrate overlays a portion of the non-conductive carrier; and an insulating layer conformally coating at least the side-walls of the conductive substrate, wherein a first surface of the sensing layer is uncoated by the insulating layer.

In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present invention. Reference characters denote like elements throughout figures and text.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the sensors may be implemented. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.

Embodiments of the sensors described herein are protected from stray currents and do not fail under high pressure or under repeated pressure cycling (e.g., from high pressure to low pressure to high pressure, and so on).

FIGS. 1-3show embodiments of sensors that are immune to stray currents in accordance with the present application.FIG. 1shows a cross-sectional view of the structure of the layers of a sensor10. Sensor10includes a sensor chip90and a non-conductive carrier140. The sensor chip90includes a conductive substrate130, a dielectric layer120, a sensing layer110, and an insulating layer100. The non-conductive carrier140is a chip carrier140. The circuitry used to obtain measurements from the sensor10is not shown inFIG. 1.

The non-conductive carrier140has a first surface141and an opposing second surface143. The conductive substrate130has a first surface131, an opposing second surface132, and side-walls135. The sidewalls135are also referred to herein as “sidewall surfaces135”. The conductive substrate130overlays a portion144of the surface141of the non-conductive carrier140. The second surface132of the conductive substrate130contacts the portion144of the first surface141of the non-conductive carrier140. The dielectric layer120overlays the conductive substrate130. The second surface222of the dielectric layer120contacts the first surface131of the conductive substrate130.

The dielectric layer120includes a gate dielectric121having a first thickness t1and a field dielectric122having a second thickness t2. The field dielectric122has a first surface221and an opposing second surface222. The gate dielectric121has a first surface223and an opposing second surface222. As shown inFIG. 1, the gate dielectric121and the field dielectric122are formed contiguously in the dielectric layer120and they share the second surface222. Since the second thickness t2is greater than the first thickness t1, the gate dielectric121is surrounded by the field dielectric122.

The sensing layer110has first surface111and an opposing second surface112. The sensing layer110overlays the first surface223of the gate dielectric121. The second surface112of the sensing layer110contacts the first surface223of the gate dielectric121.

The insulating layer100conformally coats at least the side-walls135of the conductive substrate130. The first surface111of the sensing layer110is uncoated by the insulating layer100. As shown inFIG. 1, the insulating layer100conformally coats: 1) the side-walls135of the conductive substrate130; 2) the first surface221of the field dielectric122; and 3) an exposed surface142of the non-conductive carrier140. The exposed surface142of the non-conductive carrier140is defined as that portion of the surface141of the non-conductive carrier140that is not overlaid by the conductive substrate130of the sensor chip90.

In one implementation of this embodiment, the conformal coatings are formed by vapor based chemical reactions, which are repeatedly cycled to deposit an atomic layer with each deposition cycle. The insulating layer100deposited in this manner is a high quality dielectric that contiguously and evenly deposits material on all exposed surfaces, even vertical surfaces. The insulating layer100is also referred to herein as “atomic-layer-deposition layer100”, an “ALD layer100”, and “ALD100”.

In one implementation of this embodiment, the insulating layer100is deposited using Atomic Layer Deposition (ALD). In another implementation of this embodiment, the insulating layer100is deposited using one of the types of chemical vapor deposition (CVD). In yet another implementation of this embodiment, the insulating layer100is approximately or less than 1000 Angstroms.

The first surface111of the sensing layer110is the surface that is exposed to the environment to be sensed. As shown inFIG. 1, the first surface111of the sensing layer110is the surface that is exposed to a conductive fluid50.

In one implementation of this embodiment, the sensor10is a pH sensor10used to measure the pH of a liquid (e.g., conductive fluid50). The dielectric layer120and sensing layer110are operable to sense pH when exposed to the conductive fluid50. When pH sensors are used in high pressures situations (e.g., deep in the ocean) or in repeated pressure cycling situations, the pH sensors are known to cause bending of the non-conductive carrier.

In prior art pH sensors that use an O-ring to insulate the sidewalls from the conductive fluid, the bending of the non-conductive carrier under high pressure and/or repeated pressure cycling causes the O-ring to pop off of the device so the sidewalls are exposed. Similarly, in prior art pH sensors that use epoxy to insulate the sidewalls from the conductive fluid, the bending of the non-conductive carrier under high pressure and/or repeated pressure cycling causes the epoxy to delaminate from the sides walls so the sidewalls are exposed to the fluid being sensed. In both cases, a desired current flows from the sidewall to the sensing surface and the prior art pH sensors have erroneous measurements.

Because the ALD layer100of the sensor10has high integrity and good adhesion to the sidewalls135, the ALD layer100is able to withstand high pressure and repeated pressure cycling without cracking or shifting away from the sidewall134. Even when the non-conductive carrier140bends under the pressure, the thin atomic layers of the ALD layer100of the sensor10remain adhered to the sidewalls135. Thus, no undesired stray currents are generated to flow between the sidewalls135and the first surface111of the sensing layer via the conductive fluid50and the sensor10accurately measures the pH of the conductive fluid50.

In one implementation of this embodiment, the insulating layer100conformally coats the side-walls135of the conductive substrate130but does not coat the entire first surface221of the field dielectric122. In another implementation of this embodiment, the conductive substrate130is a silicon substrate. In another implementation of this embodiment, the conductive substrate130is a p doped silicon substrate. In yet another implementation of this embodiment, the non-conductive carrier140is formed from a plastic material as known to one skilled in the art.

FIG. 2shows a layer structure of a sensor11with circuit components195. Sensor11is similar to sensor10in that it includes a non-conductive carrier140and a sensor chip91. The sensor chip91includes a conductive substrate130, a dielectric layer120including the gate dielectric121and the field dielectric122, a sensing layer110, and an insulating layer100(ALD100). The sensor11also includes a first electrode145inlaid in the non-conductive carrier140, a second electrode146inlaid in the non-conductive carrier140, a first via170in the conductive substrate130, and a second via175in the conductive substrate130, and an insulating layer180. The first via170and the second via175are electrically conductive.

A first conductive material181that extends through the insulating layer180is formed in the insulating layer180. In one implementation of this embodiment, the first conductive material181is formed by etching a through hole in the insulating layer180and filling the through hole with a metal or metal alloy. A second conductive material182that extends through the insulating layer180is formed in the insulating layer180. In one implementation of this embodiment, the second conductive material182is formed by etching a second through hole in the insulating layer180and filling the second through hole with a metal or metal alloy. The first and second conductive material181and182are also referred to herein as respective first and second conductive pads181and182.

The first via170electrically connects a source diffusion region represented generally at150to the first electrode145via the first conductive pad181in the insulating layer180. The second via175electrically connects a drain diffusion region represented generally at160to the second electrode146via the second conductive pad182in the insulating layer180. The circuit components195, when electrically connected to the first electrode145and the second electrode146as shown inFIG. 2, are operable to sense a current generated within the sensor11based on the environment of the sensing layer110, e.g., the conductive fluid50. The circuit components195are shown inFIG. 2to be external to the conductive fluid50. However, in embodiments, the circuit components195are packaged with the sensor11and the package is in the conductive fluid50. In one implementation of this embodiment, there is no insulating layer180.

In one implementation of this embodiment, the insulating layer180is formed from silicon oxide. In this case, the insulating layer180is a silicon dioxide layer180. In another implementation of this embodiment, the conductive substrate130is a silicon substrate with appropriate p-n junction layers to form the desired channel. In yet another implementation of this embodiment, the first via170is formed by etching a first cavity171in the conductive substrate130and coating the inner surface of the first cavity171with a conductive material172. In one implementation of this embodiment, the conductive material172is a metal or metal alloy (for example, gold). The conductive material172electrically contacts the first conductive pad181in the insulating layer180. In yet another implementation of this embodiment, the second via175is formed by etching a second cavity176in the conductive substrate130and coating the inner surface of the second cavity176with a conductive material177. In one implementation of this embodiment, the conductive material177is a metal or metal alloy (for example, gold). The conductive material177electrically contacts the second conductive pad182in the insulating layer180.

The sensor11is operable as a pH sensor11. The pH of a conductive fluid50is a function of the number of hydrogen ions in the conductive fluid50as is known to one skilled in the art. The material in the sensing layer110(e.g., gate110) absorbs hydrogen ions (protons) from the conductive fluid50. When hydrogen ions interact with the sensing layer110, the sensor chip91in the sensor11is operable as a field effect transistor (FET)91, in which the sensing layer110functions as a gate110and the conductive substrate130functions as a channel. As the gate110is charged up by the interacting hydrogen ions in the conductive fluid50, the current flow in the channel130increases. The current is measured between the source diffusion region150and the drain diffusion region160.

As is understandable to one skilled in the art upon reading and understanding this document, the sensor10ofFIG. 1can be implemented with the first electrode145, the second electrode146, the first via170in the conductive substrate130, the second via175in the conductive substrate130and the insulating layer180as shown inFIG. 2.

FIG. 3shows a layer structure of a sensor12. The layer structure of sensor12differs from the sensor11ofFIG. 3in that the ALD100does not cover the exposed surface142of the non-conductive carrier140that is not overlaid by the conductive substrate130. In this embodiment, the ALD100is deposited on the sensor chip90formed by the conductive substrate130, the dielectric layer120, and the sensing layer110before the sensor chip90is placed on the non-conductive carrier140. For the embodiment of the sensor10shown inFIG. 1, the sensor chip90is placed on the non-conductive carrier140and then the ALD100is deposited on the sensor chip90and the non-conductive carrier140.

As is understandable to one skilled in the art upon reading and understanding this document, the sensor12can be implemented with the first electrode145, the second electrode146, the first via170in the conductive substrate130, the second via175in the conductive substrate130and the insulating layer180as described above with reference toFIG. 2.

FIG. 4is a flow diagram of a method400of fabricating sensors that are immune to stray currents in accordance with the present application. The method400is described with reference to the sensors10,11, and12of respectiveFIGS. 1,2, and3. However, it is to be understood, that method400is applicable to adaptations or variations of the sensors shown and described herein.

At block402, a first surface131of a conductive substrate130is coated with a dielectric layer120. The dielectric layer120can be deposited using one or more of a variety of techniques for deposition known to one skilled in the art. In one implementation of this embodiment, the dielectric layer120is deposited with a second thickness t2and a portion of the dielectric layer120is etched to form a gate dielectric121with a first thickness t1in the field dielectric122. In this case, the first thickness t1is less than the second thickness t2. In another implementation of this embodiment, a first via170and a second via175are formed in the conductive substrate130(FIG. 2). In yet another implementation of this embodiment, the first via170is formed by etching a first cavity171and coating the inner surface of the first cavity with a conductive material172while ensuring that the conductive material172is isolated from the conductive substrate130as is understandable to one skilled in the art. In yet another implementation of this embodiment, the second via175is formed by etching a second cavity176and coating the inner surface of the second cavity176with a conductive material177while ensuring that the conductive material177is isolated from the conductive substrate130as is understandable to one skilled in the art.

At block404, a portion (e.g., the gate dielectric121) of the dielectric layer120is coated with a sensing layer110. The sensing layer110can be deposited using one or more of a variety of techniques for deposition known to one skilled in the art. In one implementation of this embodiment, the sensing layer110is formed from metal oxides as known to one skilled in the art. The sensing layer110is deposited on the portion of the dielectric layer120that was etched back to form the gate dielectric121with a first thickness t1. In one implementation of this embodiment, the first thickness t1of the gate dielectric121in combination with the thickness of the sensing layer110are less than the second thickness t2of the field dielectric122. In this case, the first surface111of the sensing layer110is closer to the first surface131of the conductive substrate130than the first surface221of the field dielectric122.

At block406, the conductive substrate130is segmented to form sensor chips and sidewalls135of the conductive substrate130are exposed due to the segmenting. In one implementation of this embodiment, the segmenting process is done by sawing the substrate (wafer) into a plurality of sensor chips90. In another implementation of this embodiment, the segmenting process is done by notching the substrate and cleaving the substrate to break along the crystalline planes due to the notches.

At block408, an insulating layer100is conformally formed over at least the sidewalls135of the conductive substrate130that was exposed by the segmenting at block406. The first surface111of the sensing layer110is uncoated by the insulating layer100. In this manner, after die separation exposes the sidewalls135and after the sensor chip90or91is attached to a header (e.g., non-conductive carrier140), the conformal insulating layer100coats the entire sensor10,11, or12to eliminate any conductive path through the conductive fluid50between the first surface111of the sensing layer110and the sidewalls135.

The insulating layer100is conformally formed in vapor based chemical reaction used to deposit a dielectric onto the exposed sidewall surfaces of the conductive substrate130. In particular, Atomic Layer Deposition (ALD) and other forms of chemical vapor deposition (CVD) are able to deposit a high quality dielectric on all exposed surfaces, even vertical surfaces. Because the ALD has high integrity and good adhesion, it withstands high pressure and pressure cycling. A processing step is needed to make clear the first surface111of the sensing layer110of the ALD.

Often the sensing material of the sensing layer110is fragile and can be damaged when the ALD100is removed from the first surface111of the sensing layer110. This damage can be prevented by depositing and patterning a sacrificial layer on the sensing layer110before the ALD100is deposited to protect the material of the sensing layer110. The sacrificial layer is removed later. For example, the first surface111of the sensing layer110is protected from the insulating layer by a deposition of a protective sacrificial material (e.g., aluminum) on the first surface111of the sensing layer110prior to the deposition of the conformal insulating layer100on the sidewalls135of the conductive substrate. Then the protective material (e.g., aluminum) protecting the first surface111of the sensing layer110and the insulating layer100conformally formed over the protective material is lifted off of the first surface111of the sensing layer110so the first surface111of the sensing layer110is exposed to the environment.

In one implementation of this embodiment, the conductive substrate130of the sensor chip90or91is positioned on a non-conductive carrier140prior to step408so that the non-conductive carrier140is also conformally coated with the insulating layer100. This embodiment is shown in the sensor10ofFIG. 1.

In another implementation of this embodiment, the conductive substrate130of the sensor chip90or91is positioned on a non-conductive carrier140after step408so that the non-conductive carrier140is not conformally coated with the insulating layer100. This embodiment is shown in the sensor12ofFIG. 3.

In yet another implementation of this embodiment, a first electrode145and a second electrode146are formed in the non-conductive carrier140along with any required trace lines to communicatively couple the first electrode145and the second electrode146to circuit components195(FIG. 2). In this case, an insulating layer180is formed between the non-conductive carrier140and the second surface132of the conductive substrate130. This ensures the first electrode145and the second electrode146are electrically isolated from the conductive substrate130except for the points of contact between the first via170and the first electrode145and between the second via175and the second electrode146when the sensor chip90or91is positioned on the non-conductive carrier140. In one embodiment, the insulating layer180is a layer of silicon dioxide180formed between the non-conductive carrier140and a second surface132of the conductive substrate130. In another implementation of this embodiment, the insulating layer180is an oxide layer.

Example Embodiments

Example 1 includes a sensor comprising: a conductive substrate having side-walls; a dielectric layer overlaying a first surface of the conductive substrate, the dielectric layer including a gate dielectric having a first thickness and a field dielectric having a second thickness; a sensing layer overlaying a first surface of the gate dielectric; a non-conductive carrier wherein a second surface of the conductive substrate overlays a portion of the non-conductive carrier; and an insulating layer conformally coating at least the side-walls of the conductive substrate, wherein a first surface of the sensing layer is uncoated by the insulating layer.

Example 2 includes the sensor of Example 1, wherein the insulating layer is an atomic layer having a thickness less than 1000 Angstroms.

Example 3 includes the sensor of any of Examples 1-2, wherein the insulating layer conformally coats the field dielectric.

Example 4 includes the sensor of any of Examples 1-3, wherein the insulating layer conformally coats the field dielectric, and an exposed surface of the non-conductive carrier.

Example 5 includes the sensor of any of Examples 1-4, further comprising: circuit components, wherein the dielectric layer, the sensing layer, and the circuit components are operable to sense pH of a conductive fluid when the sensing layer is exposed to the conductive fluid.

Example 6 includes the sensor of any of Examples 1-5, further comprising: a first electrode; a second electrode; a first via formed in the conductive substrate, the first via electrically connecting a source diffusion region to the first electrode; and a second via formed in the conductive substrate, the second via electrically connecting a drain diffusion region to the second electrode, wherein, when the sensor is operable, the sensing layer functions as a gate, and the conductive substrate functions as a channel.

Example 7 includes the sensor of Example 6, further comprising: a layer of silicon dioxide formed between the non-conductive carrier and the second surface of the conductive substrate, the layer of silicon dioxide overlaying the first electrode and the second electrode.

Example 8 includes the sensor of any of Examples 1-7, wherein the conductive substrate is a silicon substrate.

Example 9 includes the sensor of any of Examples 1-8, wherein the conductive substrate is a p doped silicon substrate.

Example 10 includes the sensor of any of Examples 1-9, wherein the dielectric layer and sensing layer sense pH when exposed to a conductive fluid.

Example 11 includes a method of forming a sensor that is immune to stray currents, comprising coating a first surface of a conductive substrate with a dielectric layer; coating a portion of the dielectric layer with a sensing layer; segmenting the conductive substrate, wherein sidewalls are exposed; and conformally forming an insulating layer over at least the sidewalls, wherein a first surface of the sensing layer is uncoated by the insulating layer.

Example 12 includes the method of Example 11, further comprising: positioning the conductive substrate on a non-conductive carrier.

Example 13 includes the method of any of Examples 11-12, further comprising: positioning the conductive substrate on a non-conductive carrier prior to conformally forming the insulating layer over at least the sidewalls, wherein the insulating layer overlays a portion of a surface of the non-conductive carrier.

Example 14 includes the method of any of Examples 11-13, further comprising; etching a portion of the dielectric layer, wherein coating the portion of the dielectric layer with the sensing layer includes: coating the etched portion of the dielectric layer with the sensing layer.

Example 15 includes the method of any of Examples 11-14, further comprising: forming a first electrode in a non-conductive carrier; forming a second electrode in the non-conductive carrier; forming a first via in the conductive substrate; forming a second via in the conductive substrate; and positioning the conductive substrate on a non-conductive carrier.

Example 16 includes the method of Example 15, further comprising: forming an insulating layer between the non-conductive carrier and a second surface of the conductive substrate, the insulating layer overlaying the first electrode and the second electrode; forming a first conductive material in the insulating layer; and forming a second conductive material in the insulating layer.

Example 17 includes the method of any of Examples 15-16, further comprising: forming a layer of silicon dioxide between the non-conductive carrier and a second surface of the conductive substrate, the layer of silicon dioxide overlaying the first electrode and the second electrode; forming a first conductive pad in the layer of silicon dioxide; and forming a second conductive pad in the layer of silicon dioxide.

Example 18 includes the method of any of Examples 15-17, wherein forming the first via in the conductive substrate comprises: etching a first cavity in the conductive substrate; and coating an inner surface of the first cavity with a metal or a metal alloy, and wherein forming the first via in the conductive substrate comprises: etching a second cavity in the conductive substrate; and coating an inner surface of the second cavity with the metal or the metal alloy.

Example 19 includes a pH sensor comprising: a non-conductive carrier; a first electrode inlaid in the non-conductive carrier; a second electrode inlaid in the non-conductive carrier; a conductive substrate having a first surface, a second surface, and side-walls, the conductive substrate overlaying at least a portion of the non-conductive carrier; a dielectric layer overlaying the first surface of the conductive substrate, the dielectric layer including a gate dielectric having a first thickness and a field dielectric having a second thickness; a sensing layer overlaying a first surface of the gate dielectric; an insulating layer conformally coating at least the side-walls of the conductive substrate, wherein a first surface of the sensing layer is uncoated by the insulating layer; a first via formed in the conductive substrate, the first via electrically connecting a source diffusion to the first electrode; and a second via formed in the conductive substrate, the second via electrically connecting a drain diffusion to the second electrode, wherein, when the pH sensor is operable, the sensing layer functions as a gate, and the conductive substrate functions a channel.

Example 20 includes the pH sensor of Example 19, further comprising: a layer of oxide formed between the non-conductive carrier and the second surface of the conductive substrate overlaying the first electrode and the second electrode; a first conductive pad extending through the layer of oxide; and a second conductive pad extending through the layer of oxide.