EP-4742297-A1 - METHOD OF PLASMA ETCHING

EP4742297A1EP 4742297 A1EP4742297 A1EP 4742297A1EP-4742297-A1

Abstract

There is provided a method of inductively coupled plasma etching an additive-containing aluminium nitride film, wherein the additive is selected from scandium, yttrium and erbium. The method comprises placing a workpiece upon a platen assembly within a plasma etch chamber, the workpiece comprising a substrate having the additive-containing aluminium nitride film deposited thereon and a photoresist mask disposed upon the film. The method further comprises powering the plasma etch chamber with an RF power supply, applying a bias power to the platen assembly, and bulk etching the additive-containing aluminium nitride film through the photoresist mask using a switched process by repeatedly alternating between the steps of (i) plasma etching the film by feeding a chlorinated etching gas and an inert diluent gas to the plasma etch chamber at substantially equal flow rates at a target chamber pressure, and (ii) plasma etching the film using the same chlorinated etching gas and inert diluent gas, at the same target chamber pressure, wherein the flow rate of the inert diluent gas is at least four times the flow rate of the chlorinated etching gas.

Inventors

KAZEMI, Samira Binte

Assignees

SPTS Technologies Limited

Dates

Publication Date: 20260513
Application Date: 20250602

Claims (15)

A method of inductively coupled plasma etching an additive-containing aluminium nitride film, wherein the additive is selected from scandium, yttrium and erbium, the method comprising: placing a workpiece upon a platen assembly within a plasma etch chamber, the workpiece comprising a substrate having the additive-containing aluminium nitride film deposited thereon and a photoresist mask disposed upon the film; powering the plasma etch chamber with an RF power supply; applying a bias power to the platen assembly; and bulk etching the additive-containing aluminium nitride film through the photoresist mask using a switched process by repeatedly alternating between the following steps: (i) plasma etching the film by feeding a chlorinated etching gas and an inert diluent gas to the plasma etch chamber at substantially equal flow rates at a target chamber pressure; and (ii) plasma etching the film using the same chlorinated etching gas and inert diluent gas, at the same target chamber pressure, wherein the flow rate of the inert diluent gas is at least four times the flow rate of the chlorinated etching gas.
A method according to claim 1, including increasing the bias power to the platen assembly during step (i).
A method according to claim 1 or 2, wherein alternating between steps (i) and (ii) occurs every 2-6 seconds.
A method according to any preceding claim, wherein the substrate comprises a silicon wafer supporting a molybdenum electrode layer between the wafer and the additive-containing aluminium nitride film.
A method according to any preceding claim, wherein the chlorinated gas comprises chlorine; and/or, wherein the inert diluent gas comprises argon.
A method according to any preceding claim, wherein the plasma etch chamber is powered with an RF power in the range 600-1200 W; and/or wherein a bias power of 800-1400 W is applied to the platen assembly.
A method according to any preceding claim, wherein the bias power is increased by 200-500 W during step (i).
A method according to any preceding claim, wherein during the bulk etching of the additive-containing aluminium nitride film, the plasma etch chamber is maintained at a pressure in the range 2-5 mTorr.
A method according to any preceding claim, including during step (i), introducing each of the chlorinated etching gas and inert diluent gas into the plasma etch chamber at a flow rate of approximately 30-70 sccm.
A method according to any preceding claim, including during step (ii), introducing the chlorinated etching gas into the plasma etch chamber at a flow rate of approximately 60-120 sccm and the inert diluent gas into the plasma etch chamber at a flow rate of approximately 5-25 sccm.
A method according to any preceding claim, further including after the bulk etch, plasma etching the remaining additive-containing aluminium nitride film in a soft landing step by using a chlorinated etching gas comprising boron trichloride and chlorine in a 1:1 ratio and an inert diluent gas comprising argon, wherein the bulk etch produces a microtrench which is removed by the soft landing step.
A method according to any preceding claim, wherein the photoresist mask profile is less than 75 degrees.
A method according to any preceding claim, wherein the additive-containing aluminium nitride film contains scandium, and is defined by the formula AlxScyN, where x + y = 1; and wherein the scandium content y is 0.25 or more, optionally about 0.4.
A method according to any preceding claim, wherein the additive-containing aluminium nitride film has a depth and bulk etching the additive-containing aluminium nitride film comprises bulk etching at least 85% of the depth.
An inductively coupled plasma apparatus for plasma etching an additive-containing aluminium nitride film, the additive being selected from scandium, yttrium and erbium, the apparatus comprising: an ICP plasma etch chamber; a platen assembly disposed within the plasma etch chamber configured to receive a workpiece comprising a substrate having the additive-containing aluminium nitride film deposited thereon and a photoresist mask disposed upon the film; a gas delivery system for feeding a chlorinated etching gas and an inert diluent gas into the plasma etch chamber; a plasma generation device for sustaining a plasma within the plasma etch chamber for etching the additive-containing aluminium nitride film of the workpiece; and a controller configured to control the apparatus to perform a bulk plasma etch of the additive-containing aluminium nitride film through the photoresist mask in a switched process by repeatedly alternating between a first step in which the chlorinated etching gas and the inert diluent gas are fed into the plasma etch chamber in substantially equal amounts and a second step in which at least twice as much of the inert diluent gas is fed into the plasma etch chamber compared to the chlorinated etching gas.

Description

Field This invention relates to a method of plasma etching, with particular reference to a method of plasma etching an additive-containing aluminium nitride film. The invention relates also to an apparatus for plasma etching an additive-containing aluminium nitride film. Background Aluminium Scandium Nitride (AlSCN) is a piezoelectric material used in a range of applications including Bulk Acoustic Wave (BAW) filters for communications (e.g. 5G), microphones, and sensors. Improving the piezoelectric performance for devices (in particular thinner devices) is a major challenge as tolerances become tighter and the integration of the devices on circuit boards becomes more complicated. A key step in the fabrication of devices is etching a layer of aluminium scandium nitride to stop on a molybdenum bottom electrode, whilst minimising electrode loss. Generally, a two-step etch process is used if landing onto a molybdenum electrode. A bulk etch which is optimised for aluminium scandium nitride etch rate and selectivity to the resist mask is followed by a soft-landing etch step with high selectivity to molybdenum but lower aluminium scandium nitride etch rate. Typically, the bulk etch will remove 85-90% of the aluminium scandium nitride film. By switching to the soft-landing process less molybdenum is lost during the over etch. Etching is typically undertaken though a mask, which defines etch regions (i.e. trenches). Ideally the etch will produce steep sidewalls in the aluminium scandium nitride layer without any redeposition, and with minimal footing or micro-trenching. Footing occurs when there is a difference between the etch rates close to the mask and further from the mask and requires additional etch time to eliminate, leading to unwanted molybdenum loss. The causes of foot formation are thought to be ion glancing from the mask sidewalls and/or delay in sputtering of etch byproducts from the sidewalls. Micro-trenching is caused by reflection of ions from the trench sidewalls which results in a local increase in etch rate at the bottom of the trench adjacent the sidewalls. Micro-trenching results in an uneven etch of the aluminium scandium nitride layer and can also result in unwanted molybdenum loss. Whilst the main etch can be controlled to provide steep sidewalls which are redeposition free and avoid footing, the soft landing etch nevertheless leads to a footing. The footing (i.e. caused by an etch depth lag between close to mask and far from mask regions) tends to be more pronounced with increased scandium content and thickness of the aluminium scandium nitride layer. There is a need in the art for an improved method of etching aluminium scandium nitride film which minimises underlying electrode loss and eliminates footing, particularly when using a photoresist mask. To ensure device performance it is necessary to eliminate the footing, but this has proven difficult to achieve without an additional etch, which is undesirable as it tends to result in increased bottom electrode loss. Summary In a first aspect of the invention there is provided a method of inductively coupled plasma etching an additive-containing aluminium nitride film, wherein the additive is selected from scandium, yttrium and erbium. The method comprises placing a workpiece upon a platen assembly within a plasma etch chamber, the workpiece comprising a substrate having the aluminium nitride film deposited thereon and a photoresist mask disposed upon the film, powering the plasma etch chamber with an RF power supply, applying a bias power to the platen assembly, and bulk etching the additive-containing aluminium nitride film through the photoresist mask using a switched process by repeatedly alternating between the following steps: (i) plasma etching the film by feeding a chlorinated etching gas and an inert diluent gas to the plasma etch chamber at substantially equal flow rates at a target chamber pressure; and (ii) plasma etching the film using the same chlorinated etching gas and inert diluent gas, at the same target chamber pressure, wherein the flow rate of the inert diluent gas is at least four times the flow rate of the chlorinated etching gas. Optionally, the method includes increasing the bias power to the platen assembly during step (i). Advantageously, this avoids redeposition build up on the sides of the etched feature. Optionally, alternating between steps (i) and (ii) occurs every 2-6 seconds. Optionally, the substrate comprises a silicon wafer supporting a molybdenum electrode layer between the wafer and the additive-containing aluminium nitride film. Optionally, the chlorinated gas comprises chlorine. Optionally, the inert diluent gas comprises argon. Optionally, the plasma etch chamber is powered with an RF power in the range 600-1200 W. Optionally, a bias power of 800-1400 W is applied to the platen assembly. Optionally, the bias power is increased by 200-500 W during step (i). Optionally, during the bulk etching of the ad