TECHNICAL FIELD

[001] This invention relates to micro-electromechanical systems (MEMS) for microfluidic control systems. In particular, it relates to a microvalve structure that may be fabricated with surface micromachining (SMM) technology in which the microvalve may be useful as passive micro-check valve in semiconductor devices for precise and/or automatic dosing in drug delivery and sensing systems.

BACKGROUND ART

[002] Micro-check valve is one of the most critical components of a microfluidic system wherein it acts as the controlling element over fluid flow actuation. Most of the micro-check valves today are active devices wherein power input is required for the actuation of the valve structure. Such active micro-valve structure may be complex and difficult to fabricate and suffers from difficulties in integration into microfluidic systems. This is because the valve structure of these active devices must be fabricated with functionalised parts or surfaces such as providing for a mono- or bi-layer of electro-statically functional or metallic surface of an actuating structure. Bi-layering with 2 layers of materials having different thermal expansion coefficients have also been used in bi-morph and poppet (boss) valves.

[003] A common valve actuating structure employed in microvalves is the cantilever beam structure which may be fabricated with conventional bulk- micromachining. An example of an electrostatic-actuated cantilever beam valve is described in Mueller (1997) ¹ . Such cantilever beams in which the base or anchor to the substrate is provided in form of a step have also been disclosed in U.S. Patent No. 5,819,749 (Lee) wherein an array of such valves is disclosed, and U.S. Patent No. 6,236,491 (Goodwin-Johansson) wherein an air gap between the cantilever and substrate is disclosed to be an important factor in achieving the desired inflection of the cantilever beam.

[004] Large deflection due to heat deformation of cantilever structures in combination with shape memory alloy parallel beam has also been disclosed in U.S. Publication No. 2006/38643 (Xu). For passive microvalves wherein actuation is achieved by mechanical means, a cantilever-type flap made from thin layers of silicon, metal or polymers is described in Kwang W. Oh (2006) ² which is evidently different from beam-type cantilever structures. The non- rigidity and bending characteristics of such prior art cantilever structures made by surface micromachining have been studied and presented as an analytical modelling by the first two of the present co-inventors, Hing Wah Lee (2006) ³ . It is shown in this study that the presence of finite anchor stiffness due to the non-rigid effect yields significant difference on the deflections of the cantilever structure and that the anchor's flexibility should be considered to achieve accuracy.

[005] Apart from the disadvantage that such active microvalve structures consume power and are costly to fabricate due to its complexity, these prior art structures only provide a single direction of fluid flow control, i.e. fluid flow cannot be directed to a specific required path. This in turn creates fluid dead volume in the valve device, making precise fluid dispensing difficult to achieve. Another problem with active valve structures is the need to provide functionalized surfaces or parts on the structure, for example, in order to enable electrostatic actuation. Such functionalized parts or layers are susceptible to undesirable variations in performance due to effects arising from the conducted fluid's pH or ionic properties. [006] It is thus desirable to have a microvalve that could actuate passively, i.e. without the need for power input, e.g. actuated by fluid pressure, so that the design for such microvalve structure and semiconductor body may be simplified and cost-effective, in addition to capability of directing fluid flow in a manner that would avoid dead volume and its performance not affected by the ionic properties of the fluid to be delivered or conducted.

SUMMARY OF DISCLOSURE

[007] Our proposed invention is a passive microvalve which will be actuated by the deformation of the valve structure (such as bending and rotation) due to fluid pressure exerted and does not require any power input for the actuation. The deformation of the valve structure endeavours to direct fluid flow to a desired pathway and reduce occurrence of dead volume. It is also desirable that our proposed microvalve may be fabricated with MEMS processes including surface micromachining (SMM) so that our proposed valve may be fabricated cost-effectively, robust and may be easily integrated into other devices using standard MEMS processes.

[008] A basic embodiment of our microvalve is comprised in a semiconductor- fabricated body regulating fluid flow between at least a first channel and a second channel in a passive manner, said microvalve fabricated as an integral cantilever structure comprising a stem with base rigidly affixed to a substrate; and at least a lever arm extending at about tangentially from distal end of said stem, forming an elbow thereat.

[009] In one aspect of our microvalve structure, the cantilever structure is fabricated to predetermined bending stiffness, anchor stiffness or rotational moment at at least one of said stem base and elbow. Preferably, the cantilever structure is normally in close position against one of said channels and opens by fluid pressure exerting on at least one surface of the stem or lever arm.

[010] In a second aspect, the cantilever structure regulates fluid flow by deformation of at least one of its stem, elbow and/or lever arm. The deformation preferably includes any one or combination of bending and/or rotation of the stem, elbow and/or lever arm. More preferably, the deformation of at least part of the cantilever structure includes deflection to an extent in which the tip of at least one of the lever arm or elbow touches an opposing surface of a channel thereby directing fluid flow in a predetermined direction and avoiding dead volume.

[011] In a third aspect, the cantilever structure may comprise of any one of L-shape or T-shape structure formed via surface micromachining processes. Preferably, the lever arm of the L-shape structure or lever arms of T-shape structure extends to at least 40% of the length of the stem. Preferably still, the base of the cantilever structure's stem is constrained rigidly on a silicon substrate comprising a silicon nitride (SisNα) layer on silicon wafer. [012] In a fourth aspect, the cantilever structure is disposed in the semiconductor- fabricated body as valve control over 2 fluid flow channels, including a first inlet channel to a first outlet channel and a second inlet channel to a second outlet channel. Preferably, the cantilever structure is deformable to simultaneously actuate valve control over both fluid flow channels. Alternatively, the cantilever structure is deformable such that a tip of an extended lever arm is used to crimp or bend a flexible nanotube forming said fluid flow channel.

[013] Our proposed microvalve cantilever structure may be fabricated with surface micromachining techniques, including the steps of (a) depositing a layer of silicon nitride (SiijN-i) (13) on a silicon wafer (11); (b) depositing or growing and forming a pattern of sacrificial oxide layer; (c) depositing a polysilicon layer over the patterned sacrificial formed in step (b) above; (d) patterning said polysilicon layer to form a cantilever valve structure; and (e) removing said sacrificial oxide layer. The sacrificial oxide layer is preferably phosphosilicate glass (PSG) and the removal of the sacrificial oxide layer in step (e) involves buffer oxide etching (BOE).

LIST OF ACCOMPANYING DRAWINGS

[014] The drawings accompanying this disclosure serve to illustrate specific and exemplary embodiments of our microvalve structure and process of fabricating it without limiting or constraining the working principles of our invention.

[015] FIGURE 1 is a schematic drawing of the relative placement of a microvalve in a microfluidic system.

[016] FIGURE 2 schematically illustrates an L-shaped cantilever microvalve structure according to our invention in which the stiffness points are indicated.

[017] FIGURE 3 schematically shows a free body diagram of a hypothetical L-shape microvalve proposed upon application of fluid pressure. [018] FIGURE 4 represents a summary of the displacement for different ratio of beam length (vertical/horizontal) at different applied fluid pressure.

[019] FIGURE 5 displays a numerical simulation showing deformation of an L-microvalve in Z-displacement. [020] FIGURE 6 shows a numerical simulation showing deformation of an L- microvalve in Y-displacement.

[021] FIGURE 7 and FIGURE 8 show an embodiment, of an L-microvalve 5 according to our invention for controlling a single flow path in its normally closed and opened under fluid pressure, respectively.

[022] FIGURE 9 and FIGURE 10 illustrate another embodiment of an L- microvalve of our invention for controlling two flow paths in its normally 10 closed and opened under fluid pressure, respectively.

[023] FIGURE 11 schematically represents a series of steps in the fabrication of an L-shape microvalve structure on a semiconductor device.

15 [024] FIGURE 12 schematically illustrates a T-shaped cantilever microvalve structure according to our invention in which the stiffness points are indicated.

[025] FIGURE 13 schematically shows a free body diagram of a hypothetical T-shape microvalve proposed upon application of fluid pressure.

20

[026] FIGURE 14 and FIGURE 15 illustrate an embodiment of a T- microvalve of our invention for controlling a single flow path in its normally closed and opened under fluid pressure, respectively.

25 [027] FIGURE 16 schematically represents a series of steps in the fabrication of a T-shape microvalve structure on a semiconductor device.

-30- DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[028] In a microfluidic system fabricated within a semiconductor device, such as the one shown schematically in FIGURE 1, fluid flow pathway is typically provided for a fluid to enter the device from an inlet (62) through an incoming microchannel (64). A micropump (66) may be optionally provided to drive the fluid through, or provide the necessary pressure to actuate, the microvalve body (10) which contains a passive actuating element (20) such as a cantilever structure. Upon actuation, the fluid will flow via an outgoing microchannel (74) through an outlet (72).

[029] Such passive actuation of micro check valve may find uses in microfluidic systems for many applications including biochemical analysis, drug delivery, agriculture, environmental monitoring and in microanalyser systems. Our proposed microvalve structure and device incorporating it may be fabricated using semiconductor fabrication processes, particularly those for MEMS such as surface micromachining (SMM).

[030] In the basic configuration, with reference to FIGURE 7, our proposed microvalve is comprised in a semiconductor-fabricated body (10) regulating fluid flow between at least a first channel (12) and a second channel (14) in a passive manner. The proposed microvalve is fabricated as an integral cantilever- structure-(20)-comprising a stem (22) with base (64) rigidly-affixed- to a substrate (15) and at least a lever arm (26) extending at about tangentially from distal end of said stem (22), forming an elbow (28) thereat.

[031] The cantilever structure (60) may take the shape of of an L-shape (20) or T-shape (30) (shown in FIGURES 12 - 16 and further described later) which may be formed via SMM processes. We shall describe the L-shape embodiment first in the following bearing in mind that many of the features described may be common to both L- and T-shape cantilever structures. [032] Our proposed passive cantilever structure (20) may be configured to be in a normally close position against one of the fluid flow channels such that it is only opened by fluid pressure exerting on the cantilever structure (20), e.g. by pressure exerted on a surface of the stem (22), as shown in FIGURE 8 for 5 single valve checking. Fluid pressure may also be arrange to be exerted on a lever arm (26) as shown in FIGURE 10 for bi-directional valving. The actuation of the cantilever structure (20) involves combined deformation of its stem (22), elbow (28) and/or lever arm (26), i.e. may involve bi-directional movement of the microvalve structure (20) in both horizontal and vertical 10 displacement. This can be further achieved with the compounding effect of the rotational stiffness at the anchor (28) of the structure.

[033] Although this embodiment is configured to actuate as a normally closed check valve, it is possible to fabricate one as a normally open valve for a

15 first fluid flow pathway whereby pressure from a second fluid flow pathway may be employed to deform the aforesaid parts of the microvalve structure to close the valve opening of the first fluid flow pathway. In either configuration, no power input is required to actuate our proposed microvalve structure. As the degree of deformation of the microvalve structure is dependent on the fluid

20 pressure exerted, its actuation is more consistent with most types of fluids to be conducted therethrough, regardless of conductive or non-conductive types or regardless of the ionic properties or pH of the fluid.

[034] To achieve the desired amount and direction of deformation of the 25 microvalve structure, it is important that our proposed cantilever structure (20) be fabricated to predetermined bending stiffness, anchor stiffness or rotational moment at the stem base (24) and elbow (28) based on the materials used to form the cantilever structure as well as the relative lengths of the arm (26) and stem (22). Preferably, the cantilever structure is fabricated from

^~ ^Q polysilicon materials with its sterrrbase (24) planted or constrained rigidly on a silicon substrate (15) as illustrated in FIGURE 2 wherein the bending stiffness of the ends of the structure, i.e. base of the stem (24) and end of the lever arm (26) are represented by the constant, K, while the anchor stiffness of the elbow (28) is represented by the constant, Ko. Preferably, the silicon substrate (15) comprises silicon nitride (Si,3N4) (13) layer on a silicon wafer (11) as shown in FIGURES 9 - 10.

[035] The L-shape embodiment of the microvalve structure may have its actuation calculated based on the principle of conventional beam bending theory with the addition of a rotational moment (Mo) at the anchor. The free body diagram is shown in FI(JURE 3 wherein is shown all the resultant forces and moment upon application of external fluid pressure.

[036] Through analytical beam bending analysis, the deflection of the L- microvalve at the vertical direction (y-direction), w can be given as Equation 1 [1] below:

Z = JL ₊ IL [1]

P 8.5/ 2K ₉ and the anchor stiffness due to the rotation can be evaluated in Equation 2 as:

where

I = horizontal beam length (μm)

K— = horizontal-beam stiffness (μm) ^{~~~ ~}

E = modulus of elasticity (MPa)

/ = second moment of inertia (μm ⁴ )

[037] The value of the anchor stiffness will be specific for a given beam length, width and thickness. A comparison of the displacement obtained for conventional cantilever beam with the present invention is shown in Table 1. TABLE 1: Comparison in the analytical model for

deflections of conventional cantilever and L-microvalve

[038] Note that the horizontal displacement of the L-microvalve is similar to the vertical displacement as given by conventional cantilever scheme. A summary of the results obtained from numerical analysis on the displacement of the horizontal beam upon application of fluid pressure is displayed in Table 2 below.

TABLE 2: Results for displacement of

L-cantilever subjected to different fluid pressure

[039] The displacement for different ratio of beam length (vertical/ horizontal) at different applied fluid pressure is shown as a graph in FIGURE 4. Using CoυentorWare™ software for numerical MEMS simulation, the deformation of the L-shape microvalve with results simulated for Z- displacement is obtained and shown in FIGURE 5 while that for Y- displacement is shown in FIGURE 6.

[040] Following the normally closed position of the microvalve structure (20) in FIGURE 7, upon application of the fluid pressure at the top surface of the lever arm (26), the L-shape cantilever microvalve (20) will be displaced both vertically downwards due to the horizontal beam-bending of the lever arm (26) and horizontally forward (shown leftwards in FIGURE 8) due to the vertical beam-bending of the stem (22) by the anchor rotational effect. In contrast to conventional cantilever-type microvalves, the present invention will have a fluid directing effect due to the horizontal forward movement of the stem (22).

This deformation will reduce the occurrence of dead volume due to the fluid- flow-directing-effect - — -- [041] Since the anchor (2H) will also move vertically upwards during actuation, the L-shape microvalve will also be able to be stopped from further displacing when the anchor (28) connects with top substrate of the package (30). In effect, this self-limiting deflection to the extent whereby the tip (27) of the lever arm (26) or elbow (28) touches an opposing surface (29) of a channel, thereby directing tluid flow in a predetermined direction and avoiding dead volume. [042] In a preferred embodiment, the cantilever structure (20) may be constructed within a semiconductor-fabricated body or package (10) in which the valve structure may be used for bi-directional valve -checking, i.e. valve

5 control of 2 separate fluid flows with a single valve structure. This is shown in

FIGURES 9 - 10 wherein the L-shape microvalve structure is used for injection of a first fluid flowing through a first pathway comprising a first inlet channel (12) to a first outlet channel (14) upon actuation of the valve by a second fluid flowing through the stem (22) being bent horizontally forward lfl (shown leftward diagrammatically) due to the second fluid pressure.

[043] Therefore, one (i.e. the second) fluid flowing from a second inlet channel (16) to a second outlet channel (18) may be used to control the release of another (first) fluid from a reservoir due to the bending and rotational effect

15 of the horizontal lever arm (2(5)- In a different aspect, it may also be said that the cantilever structure (20) is deformable to simultaneously actuate valve control over said both fluid flow channels. It should be noted that the first or second inlet channels may be designed to be connected to a reservoir containing fluids to be delivered and controlled by the microvalve. Similarly,

20 the outlet channels may also be provided to be connected to a reservoir for storing fluids to be collected.

[044] _ This .feature is_expectcd to be useful for applications such as pressure monitoring, bio-sensing, flood monitoring etc. For example, in a

25 monolithically-inte grated semiconductor sensor for fluorescence detection on a microfluidic platform, the fluid in the reservoir injected could act as the fluorescent dye (e.g. IR-800 dye in methanol) for optical detection. The bending of the stem (22) also acts as a microchannel directing excess fluid for removal through a fluid passage which opens above the horizontal lever arm (26).

^~ 3ti [045] A general fabrication process for forming an L-shape cantilever microvalve structure is shown in FIGURE 11 which illustrates schematically the process steps (a) to (f). Typically, the fabrication process will involve surface micromachining technology for MEMS, including steps of:

5 (a) depositing a layer of silicon nitride (Si:jN ₄ ) (13) on a silicon wafer (11);

(b) depositing or growing and forming a pattern of sacrificial oxide layer;

(c) depositing a polysilicon layer over the patterned sacrificial formed in step (b) above;

(d) patterning said polysilicon layer to form a cantilever valve structure; 10 and

(e) removing said sacrificial oxide layer

which is readily understood by a person skilled in the art without further elaboration of any special considerations for our present invention.

15 [046] The sacrificial oxide layer is preferably phosphosilicate glass (PSG) while the removal of the sacrificial oxide layer in step (e) ideally involves buffer oxide etching (BOE). A more preferably structure would be a micro- machined cantilever structure (20) with the stem (22) comprising at least 40% of the horizontal lever arm (26) with the stem base (24) rigidly constrained at

20 the silicon substrate (15). Depending on the relative lengths of the stem (22) and lever arm (26), high aspect ratio (HAR) etching techniques may also be useful in our fabrication process.

[047] As explained earlier, apart from the L-shape cantilever (20), it is also

25 possible to fabricate a T-shape cantilever structure (30) that works in about the same manner as the afore-described L-shape structure (20). The corresponding schematic illustration of the stiffness points and free body diagram of the T-shape cantilever (30) are represented in FIGURES 12 - 13 while FIGURES 14 - 15 illustrate its actuation whereby it may be seen that

Tfi its other arm (32) extending in the opposing direction of lever arm 026) may serve as a feature limiting the horizontal forward bending of the stem (22) or the downward bending of the lever arm (26) upon the tip of the other arm (32) contacting the upper substrate. A series of process steps for fabricating a T- shape microvalve structure is suggested in FIGURE 16. [048] It will be obvious to a person skilled in the art that many of the above microvalve configuration, actuation and fabrication process may be adapted or modified in a number of ways which would employ the teachings and working principles disclosed herein. For instance, the bi-directional actuation may be adapted or modified such that one of the 2 fluid flow pathways may be modified to be provided as a flexible nanotube which may be crimped by an adapted tip of the lever arm (26) to stop fluid flow in the nanotube as a second fluid exerts pressure against the cantilever valve to actuate the crimping action. [049] Such configuration may be adapted from Solares (2002) ⁴ which is cited here as an enabling art adaptable from the original electrostatic actuation. Therefore, apart from fabricating our cantilever structure for valve actuation of fluid channel which is integral with a semiconductor package, it is also possible to fabricate it for valve checking purposes of a tubular element forming the fluid channel which is not integral with or within the semiconductor package. These modifications and adaptations are to be considered as falling within the letter and scope of our invention as defined in the following claims.

BIBLIOGRAPHIC LIST OF

NON-PATENT REFERENCES

¹ Juergen Mueller, "A Review and Applicability Assessment of MEMS- based Microvalve Technologies for Microspacecraft Propulsion", 35 ^lh AIAA/ ASME/ SAE /ASEE Joint Propulsion Conference and Exhibit, 20 ^th - 24 ^th June 1999, Jet Propulsion Lab, Caltech, Pasadena, California, U.S.A.

² Kwang W. Oh and Chong H. Ahn, "A Review of Micro valves", Journal of Micromechanics and Microengineering, 16:(2006) R13— R39, Institute of Physics Publishing Ltd, 2006.

³ Hing Wah Lee and Mohd Ismahadi Syono, "Effect of Self- Weight and Non-Rigidity on the Bending Characteristics of Surface Micromachined MEMS Test Structures", ICSE 2006 Proceedings, Kuala Lumpur, MIMOS Bhd, 2006.

⁴ Santiago Solares, Design of a Nano mechanical Fluid Control Valve Based on Functionali∑ed Silicon Cantilevers: Coupling Molecular Mechanics and Classical Engineering Design, Caltech, 2002.