Method for formation of 3D structures on a QCM sensor

Abstract

A method for fabricating three-dimensional structures on a QCM sensor (10) is proposed. The method comprises depositing one or more silicon layers (22) to be engraved on the QCM sensor, forming a dot-patterned mask (28) on the Si layer(s), and dry-etching the Si layer(s), leaving pillars (30) where the Si layer(s) are masked by the dots of the mask. Prior to depositing the Si layer(s), the QCM sensor is encapsulated by depositing an oxide or carbide encapsulation coating (20) thereon by a conformal deposition technique, and the Si layer(s) are deposited on the oxide or carbide encapsulation coating.

Description

DP.LIST.0021 / PC 1METHOD FOR FORMATION OF 3D STRUCTURES ON A QCM SENSORBackground of the Disclosure

[0001] The disclosure generally relates to the formation of three-dimensional structures, in particular nanopillar arrays, on a QCM sensor.

[0002] Engineering topographies at the nanoscale (in the range from 1 nm to about 1 pm, preferably from 10 nm to 800 nm) holds significant potential to tailor the material surfaces to biological interactions. Of particular interest are, e.g., protein interaction with nanotopographies. Nanotopographies at the scale of protein dimensions are known to influence biological events on the surface. This fact may be used in various applications, such as, e.g., in-vitro diagnostics, biomedical implants, anti-bacterial surfaces, food packaging and in-vitro cell-expansion.

[0003] The paper “Wafer-Level Self-Organized Copolymer Templates for Nanolithography with Sub-50 nm Feature and Spatial Resolutions”, Advanced Functional Materials, 2011,21(6), 1102, ACS Applied Materials and Interfaces, 2021 , 13, 7, 9113-9121 discloses the fabrication of a highly uniform array of Si nanopillars using diblock copolymer lithography.

[0004] An aspect of critical importance in studying protein adsorption on nanotopographies is the ability to precisely measure the density and kinetics of adsorption. Quartz crystal microbalance (QCM) sensor are preferred tools for generating quantitative, real-time information on protein adsorption further providing the possibility to measure viscoelastic properties of the molecular layers by monitoring the dissipation. However, topographies may exhibit complexity that makes it hard to resolve the contribution from multiple variables including, e.g., multiple length scale roughness, proximity between the features that prevent access to proteins, and large variation in structural attributes of individual features. While well-defined topographies have been achieved in some cases, the fabrication process has not been shown to deliver flexibility in controlling geometric attributes. Well-defined topographies can be modelled more easily, enabling the correlation of the macroscopic outcome of protein adsorption measured using the QCM sensor to what happens at the level of individual nanostructures. This allows for a reductionist approach to surface design, by enabling investigations of one variable at a time and removing interference from others. A method for manufacturing well-defined, periodic nanotopographies, engineered at theDP.LIST.0021 / PC 2length scales of the order of proteins, and spanning large enough areas for investigation by technique like QCM would be a significant contribution to making such investigations more easily accessible and thus more widely usable.

[0005] WO 2021 / 032745 A1 relates to an affinity biosensor, e.g., a QCM sensor, for sensing an analyte (a biomolecule) in a fluid, comprising an interface for contacting the fluid and adsorption of the analyte. The interface comprises a binary pattern of nanoscale regions having affinity for the analyte and a passivated region. The nanoscale regions are isolated from one another by the passivated region in such a way that adsorption of the analyte on the interface is confined to the nanoscale regions.

[0006] JP 2014081297 A discloses a QCM sensor including a crystal plate, an electrode on each of the faces of the crystal plate, and a plurality of patterns on a surface of the electrode. A resist film is used as a mask to dry-etch the electrode with a chlorine-based gas.

[0007] It is an objective of an aspect of the instant disclosure to facilitate the production of three-dimensional Si nanopillar arrays on a QCM sensor.Summary of the Disclosure

[0008] According to a first aspect of the disclosure, a method for fabricating three-dimensional structures on a QCM sensor is proposed. The QCM sensor comprises a piezoelectric crystal substrate and at least two electrodes for inducing deformations (vibrational modes) in the piezoelectric crystal substrate through the piezoelectric effect. The method comprises depositing one or more silicon layers to be engraved on the QCM sensor (e.g. on a front side of the QCM sensor), forming a dot-patterned mask on the one or more Si layers to be engraved, and dry-etching the one or more Si layers to be engraved, leaving pillars where the one or more Si layers to be engraved are masked by the dots of the mask. Prior to depositing the one or more Si layers to be engraved, the QCM sensor is encapsulated by depositing an oxide or carbide encapsulation coating thereon by a conformal deposition technique, and the one or more Si layers to be engraved are deposited on the oxide or carbide encapsulation coating. Preferably, the encapsulation coating is applied on the front and side surfaces of the QCM sensor. Optionally, the encapsulation coating may also be applied on the back side of the QCM sensor. However, the back side does not need to be covered with the encapsulation coating.DP.LIST.0021 / PC 3

[0009] The material of the piezoelectric crystal substrate may be crystalline a-quartz. However, there may be alternatives, such as, e.g., langasite (LasGasSiOu, "LGS") and gallium-orthophosphate (GaPO4). Accordingly, the expression “quartz crystal microbalance” and its abbreviation “QCM” shall not be interpreted as excluding piezoelectric crystal substrate made from other materials than quartz. However, crystalline a-quartz may be the preferred material for the piezoelectric crystal substrate in many applications.

[0010] As used herein, the expression “layer(s) to be engraved” designates layer(s) of the material (Si) that forms the three-dimensional structures, i.e., the pillars, remaining on the QCM sensor after removal of the “superfluous” matter in between.

[0011] It will be appreciated that the various steps carried out after encapsulation of the QCM sensor can be carried out in a semiconductor fabrication plant (a “fab”). Metal contamination being a problem for the fabrication of high-density integrated circuits and computer chips (leading to increased defects, yield losses, degradation, and decreased performance), fabs are extremely vigilant not to allow process steps that could lead to such contaminations. Thanks to the encapsulation, the deposition of the one or more layer(s) to be engraved, e.g., by sputtering, and the subsequent steps are compatible with the requirements of fabs. The ability to use fabs gives access to mass production capacities that will allow significant reduction of the costs of QCM sensors carrying nanostructures. The encapsulation coating has the purpose of preventing the fabrication process involving etching / milling / sputtering type of influence from releasing gold into the reactive ion etching chamber (making the system incompatible for semiconductor fabrication). Therefore, the front and side surfaces of the QCM sensor should be encapsulated. Encapsulation of the back surface is possible. Sometimes, it may help if the back surface is not encapsulated, as it makes a good thermally conducting contact with a support for thermal control of the QCM sensor during the 3D structures fabrication process.

[0012] The one or more Si layers to be engraved may have an overall height from 30 nm to 300 nm, preferably from 60 nm to 250 nm, more preferably from 80 nm to 200 nm.

[0013] The conformal deposition technique for depositing an oxide or carbide encapsulation coating may, preferably, include atomic layer deposition (ALD) and / or chemical vapor deposition (CVD).DP.LIST.0021 / PC 4

[0014] The oxide or carbide encapsulation coating may be applied with a thickness from 3 nm to 25 nm, preferably from 5 nm to 20 nm.

[0015] The oxide or carbide encapsulation coating may comprise or consist of a metal oxide, preferably AI2O3. Additionally, or alternatively, the oxide or carbide encapsulation coating may comprise or consist of a carbide, preferably SiC.

[0016] Preferably, the mask on the pillars may be stripped off, e.g., by dry-etching, after the dry-etching of the one or more Si layers to be engraved.

[0017] According to embodiments, after the stripping off of the mask a surface coating may be deposited, e.g., by sputtering a desired metal or metal oxide, on (the top and side surfaces of) the pillars.

[0018] Preferably, the overall height of the encapsulation coating and the pillars, including any surface coating on the pillars, is between 60 nm and 300 nm, more preferably between 100 nm and 250 nm.

[0019] The pillars may have a base diameter of 15 nm to 100 nm, preferably of 15 nm to 40 nm, more preferably of 20 nm to 30 nm.

[0020] The pillars may have a nearest-neighbour center-to-center distance from 25 nm to 220 nm, preferably from 25 nm to 120 nm, and more preferably from 30 nm to 100 nm.

[0021] The dry-etching of the one or more Si layers to be engraved may, preferably, comprise plasma-etching and / or reactive-ion etching (RIE), e.g., inductively coupled plasma reactive-ion etching.

[0022] According to a preferred embodiment, one or more Si layers to be engraved have an overall height from 30 nm to 300 nm, preferably from 60 nm to 250 nm, more preferably from 80 nm to 200 nm, the conformal deposition technique includes ALD, the oxide or carbide encapsulation coating comprises or consists of AI2O3, the encapsulation coating is applied with a thickness from 3 nm to 25 nm, preferably from 5 nm to 20 nm, and the mask on the pillars is stripped off, e.g., by dry-etching, after the dry-etching of the one or more Si layers to be engraved.

[0023] The dot-patterned mask may be formed using diblock copolymer lithography (e.g., using PS-b-P2VP (polystyrene-block-poly(2-vinylpyridine), PFS (polystyrene-block-polyferrocenyldimethylsilane) or others). This technique may involve depositingDP.LIST.0021 / PC 5a continuous film of diblock copolymer reverse micelles forming a (quasi-) hexagonal array (e.g., by spin-coating) and transferring the array into a dot-pattern comprising inorganic mask particles. It will be understood that the expression “(quasi-) hexagonal array” designates a hexagonal array which has good short-range order but not necessarily a good long-range order. (Quasi-) hexagonal arrays may be obtained in block copolymer lithography, which uses the self-assembling properties of block copolymer reverse micelles. Better control of the long-range order of the (quasi-) hexagonal arrays obtainable with block copolymer lithography may be achieved by structuring the surface (e.g., with shallow linear grooves or dots) before applying the continuous film of the diblock copolymer reverse micelles thereon.

[0024] Before applying the continuous film of diblock copolymer reverse micelles a layer of SiO2 may be deposited on the one or more Si layers to be engraved. The film of diblock copolymer reverse micelles may be formed on the SiO2 layer, and transferring the array into the dot-pattern may comprise plasma-etching coronal block polymers of the reverse micelles to expose the SiO2 layer between the cores of the reverse micelles, followed by plasma-etching the exposed SiO2 layer.

[0025] Additionally, or alternatively, transferring the array into the dot-pattern may comprise exposing the reverse micelles alternately (preferably in several cycles) to TiCk vapor and H2O vapor to incorporate Ti into the cores of the reverse micelles, followed by plasma-etching the reverse micelles with O2 so as to remove the reverse micelles and to form TiO2 nanoparticles in the locations of the cores of the reverse micelles.

[0026] In a further aspect, the disclosure relates to a QCM sensor obtained by the method presented herein. In a preferred aspect, the QCM sensor comprises a piezoelectric crystal substrate and at least two electrodes for inducing deformations (vibrational modes) in the piezoelectric crystal substrate through the piezoelectric effect and an array of pillars made from silicon on the front face of the QCM sensor, the pillars being erect on a continuous oxide or carbide encapsulation coating applied on the front and side surfaces of the QCM sensor.

[0027] In the present document, the verb “to comprise” and the expression “to be comprised of’ are used as open transitional phrases meaning “to include” or “to consist at least of’. Unless otherwise implied by context, the use of singular word form is intended to encompass the plural, except when the cardinal number “one” is used:DP.LIST.0021 / PC 6“one” herein means “exactly one”. Ordinal numbers (“first”, “second”, etc.) are used herein to differentiate between different instances of a generic object; no particular order, importance or hierarchy is intended to be implied by the use of these expressions. Furthermore, when plural instances of an object are referred to by ordinal numbers, this does not necessarily mean that no other instances of that object are present (unless this follows clearly from context).

[0028] When this description refers to “an embodiment”, “one embodiment”, “embodiments”, etc., this means that the features of those embodiments can be used in the combination explicitly presented but also that the features can be combined across embodiments without departing from the disclosure, unless it follows from context that features cannot be combined.Brief Description of the Drawings

[0029] By way of example, preferred, non-limiting embodiments of the disclosure will now be described in detail with reference to the accompanying drawings, in which: Fig. 1 : is a schematic illustration of a method for fabricating nanopillar topographies on a QCM sensor according to a preferred embodiment of the disclosure.Detailed Description of Preferred Embodiments

[0030] Fig. 1 schematically illustrates a workflow for fabricating nanopillar topographies on a QCM sensor in accordance with a preferred embodiment of the disclosure. The process steps enable adapting the fabrication on the QCM sensor to the requirements of semiconductor fabs, which avoid exposure of their devices to metals.

[0031] The QCM sensor 10 comprises a piezoelectric crystal substrate 12 (typically a quartz crystal, as in Fig. 1) and at least two gold electrodes 14a, 14b on the front and back side, respectively, of the QCM piezoelectric crystal substrate 12. An adhesion layer 16 (made, e.g., of Ti, TiO2 or Cr) is arranged between the gold electrodes 14a, 14b and the crystal substrate for compatibility reasons.

[0032] To prevent exposure of the metal surfaces of the electrodes, the QCM sensor 10 is completely encapsulated in an oxide or carbide encapsulation coating 20, e.g., AI2O3, SiO2 or SiC. Before the encapsulation coating 20 is applied, the gold surfacesDP.LIST.0021 / PC 7of the electrodes may be coated with a thin interlayer 18. The interlayer 18 may be made of the same material as the adhesion layer 16, e.g., of Ti, TiCh or Cr.

[0033] The oxide or carbide encapsulation coating 20 encapsulates the gold electrodes, so that no gold surface is exposed in any of the downstream process steps. While AI2O3 may be preferred for encapsulation in certain applications, there are other alternatives to alumina, e.g., TiCh, SiC>2, or other metal oxides. The process used to encapsulate may be ALD (atomic layer deposition), as the process allows conformal coating of the gold on the top and the side walls as well. Nevertheless, other conformal deposition techniques could be used, e.g., sputtering or CVD.

[0034] In a following step, the one or more layers to be engraved 22 are deposited on the encapsulated QCM sensor 10. In the illustrated example, the one or more layers to be engraved 22 consist of silicon that is applied by sputtering. The thickness of the one or more layers to be engraved 22 depends on the desired height of the final pillars. A preferred thickness of the one or more layers to be engraved 22 may, e.g., be between 100 nm and 250 nm, or between 150 nm and 200 nm. The thickness of the one or more layers to be engraved 22, and, thus, the height of the pillars, may be higher than these values, under the condition that the Sauerbrey equation is still valid, i.e. , there is proportionality between the change of the oscillation frequency of the piezoelectric crystal and the change of the mass deposited on its surface. When the pillars are too high, the Sauerbrey equation becomes invalid due to viscoelastic effects, or due to high dissipation. This may be found out in practice by the user checking if the frequency changes at different overtones are no longer the same. As a guideline, the maximum height of the pillars (and thus the maximum thickness of the one or more layers to be engraved 22) preferably does not exceed the sensing depth, 5, of the QCM sensor in the fluid in which the QCM sensor is to be used. The sensing depth 5 is inversely proportional to the square root of the fundamental frequency, fo, of the QCM sensor crystal: 6 oc fQ~1 / 2. The sensing depths in water at room temperature for 5, 10 and 30 MHz QCM sensor crystals amount to, approximately, 250 nm, 180 nm, and 100 nm, respectively. With pillar heights of around 120 nm, the inventors’ experiments showed that the Sauerbrei equation remained valid for a 5 MHz crystal (of the QCM sensor). It may be possible to have pillars with heights of about 0.5 pm but it may be technically more interesting to have pillars with lesser height (e.g., below 200 or below 150 nm) and a high pillar areal number density because the response times areDP.LIST.0021 / PC 8quicker. Pillar areal number density may, e.g., range from about 28 to 780 pm-2(corresponding to a pitch of about 40 to 200 nm - the pitch can be considered the edge of the hexagonal unit cell, with 3 pillars per cell by geometry).

[0035] In the following steps, a dot-patterned mask is formed by diblock copolymer lithography. The steps of a preferred example of this technique are shown in Fig. 1. A SiO2 coating 24 (e.g., having thickness between 20 nm and 30 nm) is made on top of the one or more layers to be engraved 22 so as to prepare a hard mask for reactive ion etching of the one or more layers to be engraved 22.

[0036] A continuous film of diblock copolymer reverse micelles 26 may be applied on the SiO2 coating 24 by spin coating. The reverse micelles organize themselves into a (quasi-) hexagonal array, which may then be transferred into a dot-pattern comprising inorganic mask particles. The reverse micelles may be made of PS-b-P2VP (polystyrene-block-poly(2-vinylpyridine)) by dissolving the copolymer in m-xylene: due to selective dissolution of the PS block, core-shell particles with a with PS shell and a P2VP core are formed. The film of reverse micelles 26 may be treated with an O2 plasma to remove the coronal PS. This isolates the bumps (the P2VP cores) and exposes the underlying SiO2 coating 24. The SiO2 coating 24 may then be etched, e.g., using a C4F8 / CH4 plasma, so that a mask of dot-shaped islands 28 of SiO2 (possibly covered with caps of residual polymer 29) is formed.

[0037] The one or more layers to be engraved 22 are then subjected to reactive ion etching (RIE) to form the pillars 30. Silicon may, e.g., be etched with a CI2 based plasma.

[0038] The mask on the pillars 30 may then be stripped off, e.g., by dry-etching, after the RIE of the one or more layers to be engraved 22. This may be carried out in one or more steps. When the dot-shaped islands 28 still carry a cap of residual polymer, the stripping of the mask preferably comprises a first step of dry-etching the caps 29 followed by a step of dry-etching the inorganic part of the mask.

[0039] In a further step, the pillars 30 formed out of the material(s) of the one or more layers to be engraved 22 may be covered with a surface coating 32, e.g., of a metal or a metal oxide. The surface coating may be applied, e.g., by sputtering. The material of the surface coating may include, e.g., TiCh or ZnO or any other material of interest for bioimplants, antibacterial surfaces, etc.DP.LIST.0021 / PC 9Examples

[0040] QCM chips (nominal frequency of 5M Hz, AT-cut, e.g., from Quartzpro, Jarfalla, Sweden) can be coated with an encapsulation layer of AI2O3 (ALD, 5 nm), then coated with Si (50 to 100nm) and SiCh (20-40nm). Micellar templates may be obtained using PS-6-P2VP with molecular weight of 81.5 kDa (System A) and of 360 kDa (System B) by spin-coating on the QCM chips . Samples with different fill factors may be generated with each system. The surfaces as coated may then be exposed to reactive ion etching for 30-40s in presence of oxygen plasma. The 20-40nm SiO2 layer works as intermediate hard mask able to withstand the subsequent etching process with a selectivity which is higher than what is achievable with the polymeric mask only. The organic template may be first transferred into SiO2 by using a C4F8 / CH4 gas mixture followed by SF6 / C4F8 to etch into the underlying silicon. After the silicon etching and removal of the mask residues, the QCM chips may be coated with sputtered titanium as adhesion layer (e.g., 10 nm) and then with a gold thin film (e.g., 20 nm).

[0041] The resulting three-dimensional structures on the QCM chips may have the following geometric characteristics:

[0042] In the above table, r is the radius of the pillars, h the height, and p is the average pitch of the pillar array. The inter-pillar separation is indicated by s and f is the pillar areal number density. It should be noted that the Ti and Au coatings increase the pillar heights more than the diameters. The diameters also increase, but much less than the heights.

[0043] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting asDP.LIST.0021 / PC 10to the scope of the disclosure, which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims

DP.LIST.0021 / PC 11Claims1. A method for fabricating three-dimensional structures on a quartz crystal microbalance (QCM) sensor, the QCM sensor comprising a piezoelectric crystal substrate and at least two electrodes for inducing deformations in the piezoelectric crystal substrate through the piezoelectric effect, the method comprising:depositing one or more Si layers to be engraved on the QCM sensor; forming a dot-patterned mask on the one or more Si layers to be engraved; and dry-etching the one or more Si layers to be engraved, leaving pillars where the one or more Si layers to be engraved are masked,wherein prior to depositing the one or more Si layers to be engraved, the QCM sensor is encapsulated by depositing an oxide or carbide encapsulation coating thereon by a conformal deposition technique, and wherein the one or more Si layers to be engraved are deposited on the oxide or carbide encapsulation coating.

2. The method as claimed in claim 1, wherein the one or more Si layers to be engraved have an overall height from 30 nm to 300 nm, preferably from 60 nm to 250 nm, more preferably from 80 nm to 200 nm.

3. The method as claimed in claim 1 or 2, wherein the conformal deposition technique includes atomic layer deposition (ALD) and / or chemical vapor deposition (CVD).

4. The method as claimed in any one of claims 1 to 3, wherein the oxide or carbide encapsulation coating is applied with a thickness from 3 nm to 25 nm, preferably from 5 nm to 20 nm.

5. The method as claimed in any one of claims 1 to 4, wherein the oxide or carbide encapsulation coating comprises or consists of a metal oxide, preferably AI2O3.

6. The method as claimed in any one of claims 1 to 4, wherein the oxide or carbide encapsulation coating comprises or consists of a carbide, preferably SiC.

7. The method as claimed in any one of claims 1 to 6, wherein the mask on the pillars is stripped off, e.g., by dry-etching, after the dry-etching of the one or more Si layers to be engraved.DP.LIST.0021 / PC 128. The method as claimed in claim 7, comprising depositing a surface coating on the pillars after the stripping off of the mask.

9. The method as claimed in 8, wherein the surface coating on the pillars is deposited by sputtering of a metal or a metal oxide.

10. The method as claimed in any one of claims 1 to 9, wherein the overall height of the encapsulation coating and the pillars, including any surface coating on the pillars, is between 60 nm and 300 nm, preferably between 100 nm and 250 nm.

11. The method as claimed in any one of claims 1 to 10, wherein the pillars have a base diameter of 15 nm to 100 nm, preferably of 15 nm to 40 nm, more preferably of 20 nm to 30 nm.

12. The method as claimed in any one of claims 1 to 11 , wherein the pillars have a nearest-neighbour center-to-center distance from 25 nm to 220 nm, preferably from 25 nm to 120 nm, more preferably from 30 nm to 100 nm.

13. The method as claimed in any one of claims 1 to 12, wherein the dry-etching comprises plasma-etching and / or reactive-ion etching (RIE), e.g., inductively coupled plasma reactive-ion etching.

14. The method as claimed in claim 2, optionally in combination with any one of claims 3 to 13, wherein the one or more layers to be engraved comprise or consist of one or more silicon layers, wherein the conformal deposition technique includes atomic layer deposition (ALD), wherein the oxide or carbide encapsulation coating comprises or consists of AI2O3, wherein the encapsulation coating is applied with a thickness from 3 nm to 25 nm, preferably from 5 nm to 20 nm, wherein the mask on the pillars is stripped off, e.g., by dry-etching, after the dry-etching of the one or more layers to be engraved.

15. The method as claimed in any one of claims 1 to 14, wherein the dot-patterned mask is formed by diblock copolymer lithography.

16. The method as claimed in claim 15, wherein forming the dot-patterned mask includes depositing a continuous film of PS-b-P2VP reverse micelles forming a (quasi-) hexagonal array and transferring the array into a dot-pattern comprising inorganic mask particles.DP.LIST.0021 / PC 1317. The method as claimed in claim 16, wherein forming the dot-patterned mask includes depositing a layer of SiCh on the one or more layers to be engraved, wherein the continuous film of PS-b-P2VP reverse micelles is formed on the SiCh layer, and wherein transferring the array into the dot-pattern comprises plasma- etching coronal PS of the reverse micelles to expose the SiCh layer between the cores of the reverse micelles, followed by plasma-etching the exposed SiCh layer.

18. The method as claimed in claim 16, wherein transferring the array into the dotpattern comprises exposing the reverse micelles alternately to TiCk vapor and H2O vapor to incorporate Ti into the cores of the reverse micelles, followed by plasma-etching the reverse micelles with O2 so as to remove the reverse micelles and to form TiCh nanoparticles in the locations of the cores of the reverse micelles.

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