How to Build a Tiny Pressure Sensor Inside a Silicon Chip

This patent describes a method for building a tiny pressure sensor, called a MEMS sensor, directly inside a silicon chip, where a flexible membrane and a plate form a capacitor that changes with pressure.

What it covers

This patent details a process for manufacturing a MEMS pressure sensor. It starts by creating a 'buried cavity' entirely within a silicon substrate (Claim 1). A 'membrane' is then suspended above this cavity (Claim 1). Next, a 'conductive layer comprising a plate' made of polysilicon is formed above the membrane, separated by an 'empty space' (Claim 1). This empty space is created by growing the polysilicon layer on a temporary 'sacrificial layer,' then etching 'through holes' in the polysilicon and removing the sacrificial material through these holes (Claim 1). An 'access channel' is formed to allow outside air pressure to reach the membrane (Claim 1). Finally, 'electrical-contact elements' are created to connect to the membrane and the plate, which together form a 'sensing capacitor' whose electrical capacitance changes as pressure flexes the membrane (Claim 1). For example, a smartphone might use such a sensor to detect changes in altitude by measuring atmospheric pressure.

What it doesn't cover

• —Does not cover pressure sensors that do not rely on changes in capacitance (e.g., piezoresistive sensors).
• —Does not cover sensors where the cavity is not 'fully enclosed buried within the body' of the semiconductor material (Claim 8).
• —Does not cover manufacturing processes that do not use a sacrificial layer and through-holes to create the empty space between the membrane and the plate (Claim 1).
• —Does not cover sensors where the suspended plate is not made of polysilicon (Claim 1, 8).
• —Does not cover sensors where the fluidic communication to the membrane is not provided by 'a plurality of through holes in the layer of polysilicon' (Claim 8).

The clever bit

The cleverness lies in the specific sequence of microfabrication steps that create a fully integrated, buried capacitive pressure sensor. This includes forming the cavity and membrane *inside* the silicon, then precisely creating the polysilicon plate and its empty space using a sacrificial layer and through-holes, and finally integrating the electrical contacts for the sensing capacitor, all within the chip.

Why it matters

MEMS pressure sensors are essential components in a vast array of modern devices, enabling functions from precise altitude measurement in drones to tire pressure monitoring in cars. This patent's manufacturing process aims to improve the integration and performance of these tiny sensors. STMicroelectronics, the assignee, is a significant global supplier of MEMS devices, making advancements in their manufacturing processes commercially important.

Real-world examples

• 1.Smartphones (for altitude and weather sensing)
• 2.Smartwatches (for activity tracking and elevation)
• 3.Drones (for stable flight and altitude hold)
• 4.Automotive tire pressure monitoring systems (TPMS)
• 5.Medical devices (e.g., blood pressure monitors)
• 6.Industrial process control systems