Overview of Microfluidic Analysis Chip Processing Technology

Microfluidic analysis is a micro-full analysis system that uses micropipes as a network to connect micropumps, micro-valves, micro-reservoirs, micro-electrodes, micro-detection elements and other components with optical, electrical and fluid transmission functions, and maximizes sampling, dilution, addition of reagents, reaction, separation, detection and other analysis functions integrated on the chip. At present, the size of microfluidic analysis chips is about a few square centimeters, and the width and depth (height) of microchannels are microns and sub-microns.

Microfluidic analysis chip processing technology originated from semiconductor and integrated circuit chip micro-processing, but it is different from the silicon material two-dimensional and shallow depth processing of integrated circuit chip processing technology. Recently, the research on chip materials and processing technology, which are the basis of microfluidic analysis chips, has been paid attention to by many developed countries.

Materials and Characteristics of Microfluidic Analysis Chip

Materials for Microfluidic Analysis Chips

Rigid materials-monocrystalline silicon, amorphous silicon, glass, quartz, etc.; rigid organic polymer materials such as epoxy, polyurea, polyurethane, polystyrene and polymethyl methacrylate, etc;

Elastomeric material-Dimethylsiloxane (PDMS).

Characteristics of Microfluidic Analysis Chip Materials

Basic requirements for organic polymer chip materials

The material shall be easily processed;

Good optical transparency;

The material shall be inert under the conditions of analysis;

The material shall have good electrical insulation and heat dissipation;

Modifability and sealability of material surfaces.

Lithography (lithography) and etching techniques (etching)

lithography process

Photolithography is the use of photoresist, mask and ultraviolet light for microfabrication, the process is as follows:

(a) Clean the substrate carefully;

(B) Plating a barrier layer on the clean substrate surface, such as chromium, silicon dioxide, silicon nitride, etc;

(c) Then use the glue throwing machine to evenly throw a layer of several hundred A thick photosensitive material-photoresist on the barrier layer. The actual thickness of the photoresist is related to its viscosity and inversely proportional to the square root of the rotational speed of the glue-throwing machine;

(d) A desired channel pattern is prepared on a photomask. The photomask is covered on the substrate, the substrate coated with the photoresist is irradiated with ultraviolet light, and the photoresist is photochemically reacted;

(e) The exposed photoresist is chemically removed by development with a photoresist-associated developer. In this way, the plate-making method can be used to accurately copy the two-dimensional geometric pattern on the negative to the photoresist layer;

(f) After baking, the planar two-dimensional pattern on the negative is precisely etched on the barrier layer by chemical etching using the protective effect of the unexposed photoresist.

Mask preparation

When a microfluidic chip is fabricated using a photolithographic method, a photolithographic mask must first be fabricated. The mask has the following requirements:

a. The contrast between the absorption or transmission of light by the patterned and non-patterned areas of the mask should be as large as possible;

B. mask defects such as pinholes, broken strips, bridges, dirty spots and line bumps should be minimized;

c. The pattern accuracy of the mask should be high.

Photolithographic mask materials commonly used for large-scale integrated circuits are chrome-plated glass plates or quartz plates coated with photoresist. A computer graphics system is used to convert the mask pattern into a data file, and then a special interface circuit is used to control the burst light source, variable diaphragm, workbench and lens in the pattern generator to carve the required pattern on the mask material. However, due to the high cost of equipment, domestic general scientific research units need to be resolved through outsourcing, delaying the research cycle.

Since the resolution of microfluidic chips is much lower than the requirements of large-scale integrated circuits, it has recently been reported that simple methods and equipment are used to prepare masks. After a microcomputer is used to convert the structure diagram of the designed microchannel into an image file through CAD software, the image is printed on a transparent film with a high-resolution printer. This transparent film can be used as a mask for lithography, which can basically meet the requirements of microfluidic analysis chips for masks.

wet etching

The planar two-dimensional pattern on the barrier layer can be processed into a three-dimensional structure with a certain depth by wet etching and dry etching on the lithographed substrate. In recent years, the use of wet etching micro-processing reports more, suitable for silicon, glass and quartz can be chemically etched substrates. It has been widely used for electrophoretic and chromatographic separations.

The wet etching procedure is:

(A) Using the protective effect of the barrier layer, use an appropriate etchant to etch the required channel on the substrate;

(B) After the etching is completed, the optical glue and the blocking layer are removed, and the microchannel of the desired configuration can be obtained on the substrate;

(c) Punch holes in the substrate at appropriate locations (typically at the ends of the microchannels) to serve as reagent, sample and buffer reservoirs. After cleaning, the substrate engraved with the microchannel and the cover sheet of the same material are bonded together under appropriate conditions to obtain the microfluidic analysis chip.

For wet etching of glass and quartz, only etchants containing hydrofluoric acid are available, such as HF/HNO3,HF/NH4. Since the etching occurs on the exposed glass surface, the deeper the channel is carved, the greater the non-parallelism of the two walls of the channel, leading to the channel wide and narrow. This phenomenon limits the use of wet etching of high aspect ratio channels in glass.

Plasma Etching (plasmaetching)

Plasma etching is a dry etching process based on chemical reaction. Under the action of high frequency electric field, plasma is generated by etching gas molecules. The free radical chemistry in the plasma is very active, and the chemical reaction between it and the etched material is used to etch the microfluidic chip.

Plasma etching has been applied to glass, quartz and silicon materials to process microfluidic chips, such as quartz capillary electrophoresis and chromatography microchips. First, a layer of positive glue (light glue that falls off after exposure) is coated on the quartz substrate. After drying at low temperature, the mask is placed and developed after irradiation with ultraviolet light. The image of microstructure will be produced on the light glue. Then the quartz substrate is etched with active CHF3 plasma, and a certain depth channel or microstructure will be produced on the substrate without photoresist. This results in high aspect ratio microstructures. Recently, plasma etching has also been reported for the fabrication of microchannels on polymers.

new technology of micro-machining

Molding method (castmolding)

Photolithography and etching methods are used to first make the positive mold (the desired channel part is raised), and then the liquid polymer material is poured. The cured polymer material is peeled off from the male mold to obtain a chip having a microchannel. This method of preparing a microchip is called a molding method. The key to the molding method is the choice of the mold and the polymer material. The ideal material should have small adhesion to each other and easy demoulding.

The micromold can be made of silicon material, glass, epoxy SU28 negative photoresist, polydimethylsiloxane (PDMS), etc.

Through photolithography, graphics with high aspect ratio (20:1) and resolution as high as several microns can be obtained on SU28 negative optical adhesive, which can be directly used as a mold after development and drying. Polydimethylsiloxane mold can be prepared by pouring polydimethylsiloxane on a master mold made of silicon material, glass and other materials.

The polymer material used for casting should have low viscosity, low curing temperature, and can fill the microchannels and grooves on the mold under the action of gravity. There are two types of materials that can be used: curable polymers and solvent volatile polymers. Curing polymers include polydimethylsiloxane (silicone rubber), epoxy resin and polyurethane, etc., which are mixed with a curing agent and hardened to obtain a microfluidic chip; solvent volatilization polymers include acrylic, rubber and fluoroplastics, etc., and the chip is obtained by slowly volatilizing the solvent.

Although the molding method is limited to some easily curable polymer materials, the method is simple and easy to use, the chip can be copied in large quantities, does not require expensive equipment, and is a method that can produce cheap analytical chips. However, there are few studies on the microfluidic behavior of such chips, and their practical value remains to be discussed.

软刻蚀(softlithography)

Recently, a number of research groups, mainly the research group of Professor Whitesides of Harvard University, have developed a new low-cost micro-processing technology "soft etching" based on self-assembled monolayers (self-assembledmonolayers,SAMs), elastic seal (elastomericstamp) and polymer molding (moldingoforganicpolymers) technology ". The core of soft etching technology is the pattern transfer element-elastic seal. The methods include micro-contact printing, micro-fine molding, transfer micro-molding, micro-replication molding, etc. It can not only manufacture complex three-dimensional microchannels on materials such as polymers, but also change the chemical properties of the material surface. It may become a new method for producing low-cost microfluidic analysis chips.

The best polymer for making elastic stamps is polydimethylsiloxane (PDMS). It has low surface free energy (~ 21.6dyn/cm), stable chemical properties, and does not adhere to other materials. Orthogonal contact with the substrate is tight, easy to take the mold; Soft, easy to deform, good elasticity, can be replicated on the curved surface micro pattern.

Microcontact printing (micro-contactprinting,μCP)

Micro-contact printing method refers to the use of elastic stamps combined with self-assembled monolayer technology to print graphics on flat or curved substrates. Self-assembled monolayers are long-chain molecules containing certain functional groups that spontaneously arrange into a regular structure on a suitable substrate in order to minimize the free energy. The self-assembled monolayer system has been identified with alkyl thiols on the surface of gold and silver and other coinage metals and alkyl siloxanes on the surface of glass, silicon and silicon dioxide. The thickness of the self-assembled monolayer is about 2 to 3nm, and the thickness of the monolayer can be changed within the accuracy range of 0.1nm by changing the number of methylene groups in the alkyl chain.

The elastic seal can be made by first preparing the mold of the relevant pattern by using photolithography and other techniques, and pouring PDMS on the mold. The surface of the seal is coated with alkyl thiol ink, which can print micro graphics on the surface of metals such as gold and silver. In this process, the thiol molecules are automatically arranged into a regular structure in order to minimize the free energy, and have a tendency to automatically heal defects, which can reduce printing defects and ensure printing clarity. The printed surface can be chemically etched or electroless plated to make the graphics visible. If the seal is made very thin and pasted on the surface of the roller, it becomes a micro-printing roller, which can improve the printing efficiency and print large-area graphics.

The microcontact printing method can easily control the chemical and physical properties of the microchannel surface, and has great application prospects in the research of microfabrication, biosensors and surface properties.

Organic Polymer Molding (moldingoforganicpolymers)

The organic polymer molding method includes a capillary micro molding method (micromoldingincapillaries, MIMIC), a micro transfer molding method (microtransfermolding,μTM), a replica molding method (replicamolding), and the like.

In the capillary micro-molding method, a penetrating capillary network is formed between the microchannel on the elastic stamp and the substrate. The polymer prepolymer (such as UV-cured polyurea and heat-cured epoxy) is dropped at the entrance of the network, and the capillary action will suck the prepolymer into the channel network. After curing, a microstructure complementary to the concave and convex of the microchannel on the stamp can be obtained. MIMIC can only process microstructures where the channel network communicates with the inlet.

Micro transfer molding method is to fill the groove on the elastic seal with polymer prepolymer, buckle it on the substrate, after curing, remove the mold, on the substrate is printed on the polymer material composition of the graphics. μTM has been used to fabricate optical waveguides. UV-CURED POLYURETHANE, UV-TM is used to make a micron-level waveguide, and then a layer of covering layer is poured on it. By controlling the ultraviolet irradiation time, the optical index difference between the waveguide and the covering layer is controlled, and the optical coupling effect of the waveguide can be controlled, which is convenient and fast.

The micro-replication molding method is to obtain the microstructure by directly casting the polymer material such as polyurethane on the elastic stamp. This method can efficiently replicate microstructures ranging in size from 30nm to several centimeters. The surface of polymer materials is modified by oxygen plasma treatment, and the capillary function channel can be used for the study of electrophoretic separation.

Molding-based soft etching has the advantages of simplicity, economy, and high fidelity. It can be used to process microstructures on materials such as polymers, inorganic and organic salts, sols and gels, ceramics and carbon, and has been used to prepare microgratings, polymer waveguides, microcapacitance and microresonators. Photolithography can only be processed on this kind of polymer microstructure.

Hot pressing method (imprinting)

The polymethyl methacrylate is heated to 135 ℃ in a hot press, and the silicon male mold is placed under the heat preservation condition and pressurized for 5min, so that the microchannel can be pressed on the polymethyl methacrylate sheet. The substrate with a channel and the cover sheet with a hole are heated and sealed to obtain a microfluidic analysis chip. This method can be copied in large quantities, the equipment is simple and the operation is simple. However, the materials used are limited, and the research on their performance is less, and the application value still needs to be tested.

Laser Cutting (laserablation)

The degradable polymer material is exposed by ultraviolet laser, and the two-dimensional geometric figure on the negative is accurately copied. Adjusting the exposure intensity controls the photolysis depth of the material. The degradation products were removed by pressure purging to obtain a substrate with microchannels. It is thermally bonded to another perforated cover sheet to obtain the desired chip.

This method requires high technical equipment, but the steps are simple, and does not require ultra-clean environment, high precision. It can be used to process microchannels on photolyzable polymer materials such as polymethyl methacrylate and polycarbonate.

LIGA technology

LIGA technology is composed of three links: lithography, electroforming and plastic casting. The first step is synchrotron radiation depth X-ray exposure, which can transfer the pattern on the mask to a photoresist with a thickness of several hundred microns to obtain a three-dimensional structure with the same thickness of several hundred microns and a minimum width of several microns as the mask structure. Electroforming method can be used electroplating. The metal under the photoresist is used for electroplating, the gap on the photoresist pattern is filled with metal, a metal concave-convex layout complementary to the photoresist pattern is formed, the photoresist and the attached base material are removed, and the metal mold for casting is obtained. The plastic is injected into the cavity of the metal mold through the small hole on the metal injection molding plate, and the plastic chip with the same mask structure is obtained after pressure hardening. Polymethyl methacrylate is usually used as a plastic casting material.

Sealing

Thermal bonding (fusionbonding)

Generally, thermal bonding method is used for microstructure etched from glass and quartz materials. The processed substrate and cover sheet of the same material are cleaned, dried, aligned, and placed flat in a high temperature furnace. A polished graphite plate is placed on the upper and lower sides of the substrate and cover sheet respectively, a stainless steel block weighing 0.5Kg is pressed on the upper graphite plate, and heated and bonded in the high temperature furnace. When the glass chip is bonded, the heating rate of the high-temperature furnace is 10 ℃/min, the temperature is kept at 620 ℃ for 3.5 hours, and then the temperature is reduced at a rate of 10 ℃/min. The quartz chip bonding temperature is above 1000 ℃. This method requires high operation technology, and the chip can be thermally bonded many times if there are interference fringes after one seal.

However, thermal bonding cannot be used for chips containing temperature-sensitive reagents, electrodes and waveguides, nor can it be used for sealing materials with different thermal expansion coefficients.

Generally speaking, sealing is more difficult than etching microstructures on glass and silicon wafers, and the yield of thermal bonding is not high.

Anodic bonding (anodicbonding)

Anodic bonding has been widely used in the sealing of glass, quartz and silicon wafers. That is, during the bonding process, an electric field is applied so that the bonding temperature is lower than the softening point temperature.

Glass-to-glass anodic bonding has been of great interest to prevent the phenomena of channel deformation and even collapse that can occur with thermal bonding. A layer of thin film material such as polysilicon, silicon nitride, etc. is deposited on the glass surface as an intermediate layer. The two glass sheets can be bonded when the temperature is raised to 400°C under an electric field of about 700 volts. It is reported in the literature that two glass sheets can be bonded without depositing an interlayer on the glass surface at a temperature of 500°C under an electric field of 500 to 760 volts. When the two glass plates are not bonded, the air gap between the plates bears most of the voltage drop. The glass plates can be regarded as parallel plate capacitors, and the attractive force between the plates is proportional to the square of the electric field strength. Therefore, starting from the closest points in the two glass plates, the movable positive charges (mainly Na) in the lower plate neutralize with the negative charges in the upper plate to generate a layer of oxide (it is this transition layer, the two glass plates are sealed). After the bonding is completed at this point, the surrounding air gap becomes thinner and the electric field force increases, thus the bonding spreads until the whole piece is tightly bonded. The glass surface is polished to reduce the width of the gap between the glass and reduce the bonding temperature.

Other sealing methods

It has been reported that the low-temperature bonding technology of bonding glass with HF and sodium silicate is used. 1% HF is dropped into the gap between two glass sheets, and 40gf/cm2 pressure is added at room temperature. The bonding can be completed in 2h after the temperature rises by 60 ℃ and 1h. Between the two glass sheets, bonding can be carried out by placing overnight at room temperature or 1h at 90 ℃ through the intermediate layer of dilute sodium silicate solution. Sayah et al. also reported two low-temperature bonding methods: a. using 1μm thick epoxy glue between two carefully cleaned glass sheets and hardening at a pressure of 1MPa and 90 ℃; B. adding high pressure for 15h at 100~200 ℃ to make direct bonding, the pressure can be up to 50MPa.

Outlook

Microfluidic analysis chip processing technology originated from semiconductor and integrated circuit chip micro-processing, mainly for silicon material plane two-dimensional or shallow depth processing, has its limitations. In order to meet the requirements of the micro-total analysis system, the microfluidic analysis chip will be developed into a three-dimensional structure that uses micropipes as a network to connect micropumps, microvalves, micro-photoelectric detection and other functional components. Its main development prospects are as follows:

(1) The processing of micro-pumps, micro-valves, micro-photoelectric detection elements and the integration technology with chips will develop rapidly;

(2) Because wet etching can not get high aspect ratio of the channel, also can not accurately process the shape and depth of the channel, it is difficult to reduce the curve effect, improve the integration of the chip, and dry etching is currently used for silicon materials on the processing of high aspect ratio channel reports more, for glass and quartz substrates on the report is still rare. Therefore, the dry etching technique for processing high aspect ratio channels on glass and quartz substrates will be valued;

(3) Polymer chips are expected to enter the market instead of glass and quartz chips due to their ease of mass production and low cost. Reports of high aspect ratio positive die hot pressing or molding processing of polymer chips will continue to emerge;

(4) Low-temperature bonding technology for chips such as glass and quartz will replace thermal bonding to improve the sealing yield and reduce production costs. The sealing technology of polymer chip will be emphasized;

(5) Polymer chip materials and surface modification technologies with better physical and chemical properties will continue to emerge.