Piezoeletric wet etch process with reduced resist lifting and controlled undercut

A microelectronic device containing a piezoelectric thin film element is formed by oxidizing a top surface of a piezoelectric layer with an oxygen plasma, and subsequently forming an etch mask containing photoresist on the oxidized top surface. The etch mask is conditioned with an oven bake followed by a UV bake. The piezoelectric layer is etched using a three step process: a first step includes a wet etch of an aqueous solution of about 5% NH4F, about 1.2% HF, and about 18% HCl, maintaining a ratio of the HCl to the HF of about 15.0, which removes a majority of the piezoelectric layer. A second step includes an agitated rinse. A third step includes a short etch in the aqueous solution of NH4F, HF, and HCl.

FIELD OF THE INVENTION

This invention relates to the field of microelectronic devices. More particularly, this invention relates to piezoelectric layers in microelectronic devices.

BACKGROUND OF THE INVENTION

Some microelectronic devices containing a component with a piezoelectric element are fabricated by forming an etch mask on the layer of piezoelectric material and etching the piezoelectric material to define the piezoelectric element. Etching the piezoelectric material is problematic. Dry etching the piezoelectric material is difficult for layers over a micron thick due to etch residue buildup and mask erosion. Wet etching the piezoelectric material undesirably undercuts the mask in an uncontrollable manner and lifts the mask from the piezoelectric material, producing undesired profiles in the piezoelectric element.

SUMMARY OF THE INVENTION

A microelectronic device containing a piezoelectric thin film element at least 1 micron thick is formed by oxidizing a top surface of a piezoelectric layer with an oxygen plasma, and subsequently forming an etch mask containing photoresist on the oxidized top surface. The etch mask is conditioned with an oven bake followed by an ultraviolet (UV) bake. The piezoelectric layer is etched using a three step process: a first step includes a wet etch of an aqueous solution of about 5% NH4F, about 1.2% HF, and about 18% HCl, maintaining a ratio of the HCl to the HF of about 15.0, which removes a majority of the piezoelectric layer. A second step includes an agitated rinse. A third step includes a short etch in the aqueous solution of NH4F, HF, and HCl.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1AthroughFIG. 1Pare cross sections of a microelectronic device containing a piezoelectric thin film element, depicted in successive stages of an example fabrication sequence. Referring toFIG. 1A, the microelectronic device100has a structural member102, which may be a monolithic substrate, or may be a movable member such as a membrane or beam. The structural member102may include, for example, silicon, sapphire, ceramic, glass, and/or plastic. The structural member102may include a dielectric layer at a top surface to provide electrical isolation from the subsequently-formed piezoelectric element. A lower contact layer104is formed over the structural member102. The lower contact layer104may include, for example, 75 nanometers to 200 nanometers of platinum with an adhesion layer of titanium. Other materials in the lower contact layer104are within the scope of the instant example. A layer of piezoelectric material106at least 1 micron thick is formed on the lower contact layer104. The layer of piezoelectric material106may include primarily lead zirconium titanate. An upper contact layer108is formed on the layer of piezoelectric material106. The upper contact layer108may include, for example, 75 nanometers to 200 nanometers of platinum.

A top contact mask110is formed over the upper contact layer108so as to cover an area for a subsequently-formed upper contact. The top contact mask110may include photoresist formed by a photolithographic process, and may optionally include an organic anti-reflection layer such as a bottom anti-reflection coat (BARC).

Referring toFIG. 1B, a first dry etch process112removes the upper contact layer108ofFIG. 1Awhere exposed by the top contact mask110to form a top contact114. The first dry etch process112may be, for example, an inductively-coupled reactive ion etch (ICP-RIE) using chlorine radicals, oxygen radicals and argon ions as depicted inFIG. 1B. An example first dry etch process112for a 200 millimeter wafer flows boron trichloride (BCl3) at 30 standard cubic centimeters per minute (sccm), chlorine (Cl2) at 60 sccm to 150 sccm, oxygen (O2) at 5 sccm to 20 sccm, and argon (Ar) at 5 sccm to 30 sccm, maintaining a pressure of 8 millitorr to 12 millitorr. Radio frequency (RF) power is applied to an upper electrode over the microelectronic device100at 400 watts to 600 watts, which is about 1.3 watts/cm2of wafer area to about 1.9 watts/cm2, and RF power is applied to a lower electrode under the microelectronic device100at 250 watts to 350 watts, which is about 0.8 watts/cm2to about 1.1 watts/cm2, for 100 second to 250 seconds, until the upper contact layer108is removed where exposed by the top contact mask110. The first dry etch process112may be endpointed to determine when the upper contact layer108is removed. The first dry etch process112does not remove a significant amount of the piezoelectric layer106.

Referring toFIG. 1C, an overetch process116of the first dry etch process112ofFIG. 1Bconditions a top surface120of the layer of piezoelectric material106where exposed by the top contact114. The overetch process116includes chlorine radicals and oxygen radicals, and is free of argon. Work performed in pursuit of the instant example has shown that argon in the overetch process116deleteriously affects adhesion of a subsequently-formed piezoelectric element mask. In one version of the instant example, the overetch process116may terminate the argon flow and reduce the RF power to the lower electrode by 50 watts, which is about 0.16 watts/cm2, while maintaining other parameters the same as in the first dry etch process112. The overetch process116may be a timed process, for example 90 seconds to 150 seconds long. The overetch process116may erode at least a portion of the top contact mask110.

Referring toFIG. 1D, oxygen radicals from an oxygen plasma process118oxidize the top surface120of the layer of piezoelectric material106where exposed by the top contact114and remove the remaining top contact mask110. The oxygen plasma process118may be performed in a same chamber as the first dry etch process112ofFIG. 1Band the overetch process116ofFIG. 1C, which may advantageously reduce fabrication cost and complexity of the microelectronic device100. An example oxygen plasma process118flows O2at 100 sccm to 250 sccm while maintaining a pressure of 18 millitorr to 30 millitorr. RF power is applied to the upper electrode at 400 watts to 700 watts, which is about 1.3 watts/cm2to about 2.2 watts/cm2, and RF power is applied to the lower electrode at 200 watts to 250 watts, which is about 0.6 watts/cm2to about 0.8 watts/cm2. The oxygen plasma process118may be endpointed so as to terminate when the top contact mask110is removed, or may be a timed etch, for example 75 seconds to 125 seconds. Oxidation of the top surface120of the layer of piezoelectric material106by the oxygen plasma process118may advantageously increase adhesion of the subsequently-formed piezoelectric element mask. In one version of the instant example, the top contact mask110may be removed without using a wet process, such as a wet etch or a wet clean. Removing the top contact mask110without using a wet process may advantageously preserve the oxidation of the top surface120of the layer of piezoelectric material106.

Referring toFIG. 1E, an oxygen over-ash process122further oxidizes the top surface120of the layer of piezoelectric material106where exposed by the top contact114. In one version of the instant example, the oxygen over-ash process122may maintain the parameters of the oxygen plasma process118ofFIG. 1Dfor a fixed time, for example 50 seconds to 100 seconds. Oxidation of the top surface120of the layer of piezoelectric material106by the oxygen over-ash process122may advantageously further increase adhesion of a subsequently-formed piezoelectric element mask.

Referring toFIG. 1F, a piezoelectric element mask124is formed over the layer of piezoelectric material106so as to cover an area for the subsequently-formed piezoelectric element. In the instant example, the piezoelectric element mask124covers and extends past the top contact114by at least a micron. The piezoelectric element mask124may include photoresist, for example a negative photoresist comprising polyisoprene-based polymer, formed by a photolithographic process, and may optionally include a BARC layer. A solvent such as an aqueous amine solution with a pH of 11 to12may optionally be applied to the microelectronic device100prior to forming the piezoelectric element mask124to promote adhesion of the photoresist to the top surface120.

Referring toFIG. 1G, the microelectronic device100is placed on a first heated structure126such as a hot plate or oven plate. The microelectronic device100is baked at 180° C. to 190° C. for 45 minutes to 90 minutes. The microelectronic device100may be baked in an atmospheric ambient, or may be baked in an inert ambient such as a nitrogen ambient. Baking the microelectronic device100may advantageously improve adhesion of the piezoelectric element mask124to the layer of piezoelectric material106and improve resistance of the piezoelectric element mask124to a subsequent wet etch process.

Referring toFIG. 1H, the microelectronic device100is placed on a second heated structure128and exposed to UV radiation130from one or more UV sources132for 90 seconds to 150 seconds. The UV sources132may be, for example, low intensity UV sources with power densities from 80 milliwatts/cm2to 108 milliwatts/cm2or high intensity UV sources with power densities from 225 milliwatts/cm2to 280 milliwatts/cm2. The microelectronic device100may be heated to 200° C. to 250° C. while exposed to the UV sources132. Baking the piezoelectric element mask124under the UV sources132may advantageously further improve adhesion of the piezoelectric element mask124to the layer of piezoelectric material106and further improve resistance of the piezoelectric element mask124to the subsequent wet etch process.

Referring toFIG. 1I, a first wet etch134of a three step process removes the layer of piezoelectric material106ofFIG. 1Hwhere exposed by the piezoelectric element mask124, to form a piezoelectric element136. The first wet etch134is an aqueous solution of 4.5 percent to 5.5 percent ammonium fluoride (NH4F), 1.1 percent to 1.3 percent hydrofluoric acid (HF), and 16.5 percent to 19.5 percent hydrochloric acid (HCl), wherein a ratio of the HCl to the HF is maintained at a value of 14.5 to 15.5. The first wet etch134may expose the lower contact layer104. Maintaining the ratio of the HCl to the HF advantageously controls horizontal etching of the layer of piezoelectric material106so that an undercut distance138of the piezoelectric element136under the piezoelectric element mask124is less than three times a thickness140of the layer of piezoelectric material106. Oxidizing the top surface120of the layer of piezoelectric material106as described in reference toFIG. 1Dand forming the piezoelectric element mask124as described in reference toFIG. 1EthroughFIG. 1Hadvantageously reduce separation of the piezoelectric element mask124from the top surface120during the first wet etch134. Etch residue142which has a low solubility in water may be present on the lower contact layer104after the first wet etch134is completed.

Referring toFIG. 1J, the microelectronic device100is exposed to an agitated rinse process144, which is a second step of the three step process. The agitated rinse process144may include substantially all deionized water (DI H2O), with a source of mechanical agitation. In the instant example, the source of mechanical agitation may be nitrogen bubbling, indicated inFIG. 1Jas bubbles146. Other methods of providing the mechanical agitation are within the scope of the instant example. The mechanical agitation of the agitated rinse process144may advantageously remove a portion of the etch residue142and may advantageously remove a crust on remaining etch residue142to facilitate removal in a third step of the three step process.

Referring toFIG. 1K, a second wet etch148, with an aqueous solution having substantially a same composition as the first wet etch134ofFIG. 1I, removes any remaining etch residue142ofFIG. 1J. The second wet etch148may be performed for a short time, for example 5 seconds to 20 seconds. The undercut distance138of the piezoelectric element136under the piezoelectric element mask124remains less than three times the thickness140of the piezoelectric element136. After the second wet etch148is completed the aqueous solution of the second wet etch148is removed by a rinse process, for example a series of three rinse and dump steps.

Referring toFIG. 1L, oxygen radicals150of an oxygen ash process remove the piezoelectric element mask124. The oxygen radicals150do not remove a significant amount of the piezoelectric element136. A wet clean is not used to remove the piezoelectric element mask124, nor is a wet clean used to remove residue following the oxygen ash process.

Referring toFIG. 1M, a bottom contact mask152is formed over the top contact114and the piezoelectric element136, extending partway onto the lower contact layer104. The bottom contact mask152may include photoresist, and may be thicker than the top contact mask110ofFIG. 1Aso as to cover the piezoelectric element136. The bottom contact mask152may extend past the piezoelectric element136by at least 1 micron.

Referring toFIG. 1N, a second dry etch process154removes the lower contact layer104ofFIG. 1Mwhere exposed by the bottom contact mask152to form a bottom contact156. The second dry etch process154may be similar to the first dry etch process112described in reference toFIG. 1B.

Referring toFIG. 1O, oxygen radicals158from an oxygen plasma process remove the bottom contact mask152. A wet clean is not used to remove the bottom contact mask152, nor is a wet clean used to remove residue following the oxygen ash process.

FIG. 1Pshows the completed piezoelectric element136. Forming the microelectronic device100according to the process described herein may provide the piezoelectric element136with a desired extension past the top contact114and a desired side profile160which is substantially vertical.

FIG. 2depicts the microelectronic device ofFIG. 1Jin an alternate agitated rinse process. In the instant example, the agitated rinse process144is a DI H2O rinse with ultrasonic power applied to the deionized water, possibly a megasonic process, generating ultrasonic waves162in the deionized water which provide the agitation. The ultrasonic waves162may be advantageously effective at eliminating the etch residue142and may be advantageously effective at removing the crust on remaining etch residue142to facilitate removal in the second wet etch process148ofFIG. 1K.

FIG. 3depicts the microelectronic device ofFIG. 1Jin another version of the agitated rinse process. In the instant example, the agitated rinse process144is a DI H2O rinse which is stirred by a stirring mechanism164, generating flow in the deionized water which provides the agitation. The agitation may be advantageously effective at eliminating the etch residue142and may be advantageously effective at removing the crust on remaining etch residue142to facilitate removal in the second wet etch process148ofFIG. 1K.

FIG. 4depicts the microelectronic device ofFIG. 1Jin a further version of the agitated rinse process. In the instant example, the agitated rinse process144is a DI H2O spray rinse166, possibly a high pressure spray rinse, for example above100pounds per square inch (psi), which is provided by one or more spray nozzles168. Mechanical force is generated when the sprayed deionized water impacts the etch residue142, thus providing the agitation. The agitation may be advantageously effective at eliminating the etch residue142and may be advantageously effective at removing the crust on remaining etch residue142to facilitate removal in the second wet etch process148ofFIG. 1K.