ANODIC BONDING OF POLYMERS TO GLASS, SILICON OR

OTHER MATERIALS

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the bonding of polymers, and more particularly to methods for anodically bonding polymers to a substrate.

[0002] The bonding of polymers and the bonding of polymer layers or films to a substrate typically involves either a high temperature melting-based process, or a low temperature process. When bonding a polymer layer with a substrate using a high temperature process, the temperature of the substrate and/or the polymer film is raised to a value that is higher than the glass transition temperature (Tg) of the polymer. Such a melting-based approach involves the use of high temperatures which can damage the substrate and/or the polymer. In addition, such processes are not thermally efficient. Furthermore, a high-temperature method is not suitable when polymers having fabricated micro/nano structures are to be bonded to glass, silicon, or other substrates or another polymer, since the fabricated structures will get deformed or destroyed when the polymer melts. Another disadvantage of high temperature methods is that they tend to create less than optimum bonded surfaces due to their creation of areas with voids which are not bonded at the bond interface.

[0003] In addition, for high temperature bonding of polymers when the temperature is substantially lower than Tg, the pressure must be increased substantially. The use of an increased applied pressure will also adversely affect the desired structures in the polymer.

[0004] Furthermore, in addition to the elevated temperatures and/or pressures that are used to bond polymers to other materials, the bonding process also requires that the polymers wet the other material's surface so the materials can inteipenetrate one-another for bonding to occur. The wetting properties of polymers to materials such as glass and silicon are such that they require an intermediate layer to create a sufficiently bonded interface. Such an additional layer may not always be desirable.

[0005] On the other hand, low temperature bonding processes, while avoiding the disadvantages of a high temperature or high pressure bonding process, conventionally

require the use of an adhesive layer. Similar to the intermediate layer above, an additional or adhesive layer may not always be desirable.

[0006] Anodic bonding is another well-known bonding process. The anodic bonding process is known for silicon-to-glass bonding applications. In micro electro mechanical systems (MEMS), semiconductor and microfluidic applications, the phrase "silicon-to- glass bonding" is almost synonymous with anodic bonding. So, although anodic bonding of silicon to glass may be well-known, anodic bonding is not known for polymer bonding applications. This is because the temperatures employed during a typical anodic bonding process are higher than the Tg of the polymer or because the temperatures employed are sufficiently high that they will adversely affect the desired structures in the polymer. In addition, the required charge separation for an anodic bonding process cannot conventionally be achieved for polymer-to-substrate applications.

[0007] At present, bonding of polymers to glass, silicon or another polymer at a temperature below the polymer's Tg is only possible by using an adhesive layer between the two substrates which are bonded. But the use of adhesives that fill in cavities or structures is not suitable for nano electromechanical (NEMS) and MEMS fluidic devices and sensors.

[0008] There is therefore a need for a low temperature (i.e. T<Tg) bonding process that does not use an adhesive or an intermediate layer to bond a polymer with a substrate.

BRIEF SUMMARY OF THE INVENTION

[0009] In accordance with the embodiments of the present invention, systems and methods are provided for bonding polymers to glass, silicon or another polymer or other substrate at a temperature below the glass transition temperature (Tg) of the polymer, without having an adhesive or an intermediate layer. This allows for the bonding of polymer layers or substrates to glass, silicon or another polymer without destroying the nano/micro features defined on the polymer or on the substrate.

[0010] In one embodiment, the present invention provides a method of anodically bonding a polymer layer with a substrate. The method includes providing a substrate, placing a polymer layer in contact with the substrate, providing a source of cations, configured for supplying cations into the polymer layer, heating the substrate, polymer

layer and cation source, and applying a voltage across the cation source and the substrate layer sufficient to create an electrostatic field at the interface between the substrate and the polymer layer causing the polymer layer and the substrate to come sufficiently close to one another to bond the polymer layer with the substrate.

[0011] For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in.conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Fig. 1 is an exemplary block diagram of one anodic bonding setup in accordance with the embodiments of the present invention.

[0013] Fig. 2 is an exemplary block diagram of an alternative anodic bonding setup in accordance with the embodiments of the present invention.

[0014] Fig. 3 is an exemplary block diagram of an alternative anodic bonding setup in accordance with the embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The embodiments of the present invention provide methods for the anodic bonding of a polymer layer with a substrate.

[0016] The embodiments of the present invention enable the anodic bonding of polymers to glass, silicon, insulators, dielectrics, semiconductors, metals or ceramic substrates or another polymer layer at a temperature below the Tg of the polymer layer, without using an intermediate or an adhesive layer. This enables the bonding of polymers having nanometer, micrometer, or picometer-sized features on the polymer (and/or on the substrate), without destroying the nano-, micro-, or picometer sized features. When anodic bonding of polymers is carried out using the methods in accordance with the present invention, the resulting bonded interface is essentially free of voids even when the two substrates are not flat. As used herein, a flat surface refers to one that has a surface roughness that is less than about 1 to 30 micrometers. The methods of the present invention are useful for fabricating polymer-based nano/microfluidic systems, NEMS/MEMS sensors and systems. For example, the processes in accordance with the embodiments of the present invention may be used for fabrication of polymer-based

micro/nano fluidic systems; the fabrication of disposable sensors; the hermetic sealing of sensors and optical displays, inter-metal dielectric in microelectronics packaging; and the encapsulation of devices with a polymer.

[0017] In accordance with the embodiments of the present invention, one can anodically bond polymers to glass, silicon or another polymer at a much lower temperature (e.g., <Tg) thus enabling the realization of complex polymer-based micro- and nano-systems. The methods in accordance with the embodiments of the present invention enable the formation of hermetic seals in polymer-based NEMS/MEMS devices by using anodic bonding of polymers while avoiding the presence of voids at the bonded interface.

[0018] In accordance with the embodiments of the present invention, anodic bonding is performed on substrates with rough surfaces that have surface features that can be in the range of about 10 to about 300,000 nanometers (or even higher). This is advantageous in that it obviates the need for expensive polishing of the substrates prior to their bonding with the polymer layer.

[0019] The novel anodic bonding methods in accordance with the embodiments of the present invention enable the bonding of polymers to glass, polymer to insulator, polymer to dielectric, polymer to semiconductor, polymer to ceramic, polymer to metal oxide, polymer to metal nitride, polymer to metal oxy nitride, polymer to non metallic oxy nitride, polymer to silicon or polymer to any other suitable material, without using an adhesive or an intermediate layer. Although anodic bonding of silicon to glass is known, it has not been possible to successfully perform an anodic bonding of other combinations listed herein, without using the novel methods of the present invention.

[0020] In accordance with one embodiment of the anodic bonding processes of the present invention, one substrate is doped with positive ions or cations such as sodium ions. In certain aspects, the doping of the polymer substrate with cations (e.g., sodium ions) is done by immersing the substrate in a saturated NaCl solution. One embodiment of the anodic bonding setup of the present invention is described in conjunction with Fig. 1.

[0021] Fig. 1 shows an exemplary block diagram 100 of one anodic bonding setup in accordance with the embodiments of the present invention. As shown in Fig. 1, material 2 (102) is connected with the negative electrode and material 1 (104) is connected with the

positive electrode of the power supply or voltage source (106). Heating block 108 is used to heat the setup.

[0022] The anodic bonding processes in accordance with the embodiments of the present invention can advantageously be carried out successfully even with very thick polymer layers due to the presence of a significant number of cations present in the polymer itself, which is much higher than the level present for pure or undoped polymers. This, in turn, creates the attraction force (which realizes the anodic bond) between polymer layer 104 and material 2 (102) (e.g., glass, silicon, ceramic or other substrate including thin film coated substrates such as indium tin oxide coated glass or metal coated glass or metal oxide coated substrates or metal nitride coated substrates or metal oxy nitride coated substrates). The polymer layer can include a spin coated film with a thickness as small as a few nanometers or as thick as a few centimeters. Polymer sheets can also be used with thickness up to a few millimeters or even a few centimeters.

[0023] In accordance with an alternative embodiment, the bonding is realized by using a third substrate 110 which acts as a source of cations, as described in conjunction with Fig. 2. Fig. 2 shows an exemplary block diagram 400 of an alternative anodic bonding setup in accordance with the embodiments of the present invention. One exemplary substrate that can act as a source of cations is a borosilicate glass (material 3) 110 doped with a high concentration of sodium ions or sodium like ions. The borosilicate glass (material 3) 110 acts as an ion injection layer. In accordance with this alternative embodiment, the polymer layer 104 does not need to be doped with cations (e.g., ions such as sodium) before bonding. As an alternative to the doped borosilicate glass, a material with a high concentration of ions, such as sodium ions or sodium like ions can be used as an ion injection source. Such a material can be, for example, a soda lime glass. In addition to borosilicate glass, other substrates including fused silica, quartz, Pyrex or and soda lime glass may also be used as substrates that can act as a source of cations. The substrates that can act as a source of cations need not have a specific cation (e.g., sodium ion) doping concentration, so long as the substrate is able to deliver a sufficient amount of sodium or sodium like ions into the polymer when the voltage is applied.

[0024] In accordance with the embodiments of the present invention, the substrate to which the polymer layer is bonded may be a glass substrate, a silicon substrate, a polymer substrate, a ceramic substrate, a silicon dioxide substrate, a metal substrate, a metal oxide

substrate, a metal nitride substrate, a dielectric substrate, a semiconductor substrate, a metal oxy nitride substrate, or thin film coated substrates of the material listed above or any other suitable substrate.

[0025] As described above, for one embodiment of the present invention, the polymer layer is doped with cations. The cations may include alkali metal ions, sodium-like ions, or sodium ions. As used herein, a sodium-like ion is a low atomic weight ion that can be mobilized by the action of an electric field at temperatures in the range of 0 - 500 ⁰ C , such as sodium, lithium and potassium ions.

[0026] One way of doping the polymer layer includes immersing the polymer layer in a sodium chloride solution or NaCl like solution. As used herein a NaCl like solution is a solution that contains a salt made up of a sodium-like ion. NaCl-like solutions include other effective salt solutions such as a LiCl solution.

[0027] Another way of doping the polymers to be bonded is by ion implantation by irradiating the substrates with high energy ions. Yet another way of doping the polymers to be bonded is by ion exchange.

[0028] As described above, the cations that enable the anodic bonding process may be provided by placing a cation-doped substrate adjacent to the polymer layer. Using this technique, the cation-doped substrate is configured for providing cations into the polymer layer under the direction of the voltage applied across the substrate, the polymer layer and the source of cations. The cations in the cation-doped substrate may include sodium ions or sodium-like ions. In accordance with this embodiment of the present invention, the cation-doped substrate is formed by immersing a substrate into a sodium chloride solution. The cation-doped substrate can also be replaced by a substrate where cations are naturally present in the substrate, for example borosilicate glass, soda lime glass and other such substrates. An alternative way of doping the ion injection substrate is by ion implantation by irradiating the substrates with high energy ions.

[0029] In accordance with the anodic bonding embodiments of the present invention, the substrate or the polymer layer or both can posses micrometer, nanometer, or picometer scale features, that are substantially unaffected as a result of the bonding.

[0030] In addition, as described above, either the substrate or the polymer layer or both can have smooth surfaces (i.e., surfaces having a surface roughness that is in the range

between 1 nano meter and 1 micrometer) at the interface between the substrate and polymer; or either the substrate or the polymer layer or both have a high surface roughness at the interface between the substrate and the polymer layer. As used herein, a high surface roughness is a surface roughness that is in the range between 1 and 500 micrometers or higher.

[0031] The applied voltage used to anodically bond the polymer layer with the substrate may be a direct current (DC) voltage in the range of about 10 V to about 3000 V across the substrate and the polymer layer; or an alternating current (AC) voltage in the range of about +10 V to about +3000 V across the substrate and the polymer layer or a pulse voltage in the range of about 10V to about 3000V. It is also possible that the minimum voltage can be much lower than +10V and the maximum voltage can be much higher than ±3000V.

[0032] The embodiments of the present invention do not require for there to be a specific relationship between the thermal expansion coefficients of the layers being bonded. There are however advantages to having the thermal expansion coefficients of substrates which are to be bonded be not too different from one-another in order to reduce mechanical stresses at the bonded interface. Depending on the desired bond strength, it may be preferred that the substrate and the polymer layer have thermal expansion coefficients that are within about twenty percent of one another. Alternately, it may be preferred that the substrate and the polymer layer have approximately equal thermal expansion coefficients. However, it should be appreciated that the substrate and the polymer layer can have thermal expansion coefficients that differ by greater than twenty percent relative to each other.

[0033] In accordance with another embodiment, the present invention provides a method of anodically bonding a glass substrate 502 to a Si substrate 504 having a thick layer of SiO ₂ , SiN or any other material 506 disposed thereon. As used herein a thick layer of SiO ₂ , SiN or any other material is a layer having a thickness greater than about 0.4 μm. It is known that when a SiO ₂ layer on a Si substrate is thicker than 0.4 μm, one is unable to achieve a satisfactory bond between the glass and the Si substrate. Fig. 3 shows an exemplary block diagram 500 of an alternative anodic bonding setup in accordance with the embodiments of the present invention. As shown in Fig. 3, the method in accordance with the embodiments of the present invention includes providing a Si substrate 504 with a

thick layer of SiO ₂ , SiN or any other material 506, placing a glass substrate 502 in contact with the substrate 504, 506, providing a sodium like ion source (e.g. glass) of cations 508, configured for supplying cations into the Si substrate 504, heating the substrate 504, 506 and glass 502 and sodium like ion source 508 (e.g., such that the temperature can range from room temperature up to 400°C or above), and applying a voltage across the glass 502 and the sodium like ion source 508, sufficient to create an anodic bonding between the glass substrate 502 and Si substrate 504, 506.

Examples

Anodic side glass preparation (substrate connected to the positive terminal of the power supply)

[0034] This step is not critical, and so the present invention is not limited to include this step. Glass substrates with a fair surface smoothness were cleaned in piranha solution, H ₂ SO ₄ IH ₂ O ₂ , 2:1, for 10 minutes thereafter, rinsed with DI water and dried. Subsequently the substrates were doped with sodium ions by immersing the glass into a saturated solution of sodium chloride for 24 hours. Alternatively substrates are boiled for 1 h in saturated NaCl solution. Substrates were then rinsed and dried before use.

Cathodic side substrate preparation (substrate connected to the negative terminal of the power supply)

[0035] This step is not critical, and so the present invention is not limited to include this step. The substrates to be bonded were cleaned in piranha solution, H ₂ SO ₄ :H ₂ O ₂ (2:1) for 10 minutes subsequently rinsed with DI water and dried.

Preparation of polymer films

[0036] This step is not critical, and so the present invention is not limited to include this step. Polyimide films of about 12 micrometer were prepared by spin coating (3000 rpm) a liquid form of polyimide followed by thermal curing. PI-2525 from HD Microsystems was used.

Bonding process

[0037] This step is not critical, and so the present invention is not limited to include this step. To carry out the process a Carls Suss MA-8 anodic bonding system was employed.

The prepared substrates were assembled such that the positive bias is applied to the doped glass substrate and the negative bias to the substrate to be bonded. The bonding conditions used are as follows: Temperature: 300° C; DC voltage: 1000 KV; voltage was applied for one hour and the current was limited to a maximum of 0.2 mA to avoid overheating.

Experimental details: Bond Strength

[0038] To determine the bond strength, the tensile strength method was employed. Araldite glue was used to hold the bonded substrates to the test probe.

[0039] The results of several bonding tests are summarized below.

Example 1: Soda lime glass - Polyimide bond

[0040] Thickness of anodic substrate: 2 mm; width: 25 mm; bond strength of the polyimide-glass bond was 1379 N.

Example 2: Soda lime glass (ITO coated) - Polyimide

[0041] Thickness of anodic substrate: 2 mm; width: 25 mm; bond strength of the polyimide-glass bond was above 1326 N.

Example 3: Pyrex glass - Polyimide

[0042] Thickness of anodic substrate: 1.5 mm; width: 25 mm; length: 25 mms; bond strength of the polyimide-pyrex bond was 130 N.

Example 4: Polyimide - Silicon bond

[0043] Thickness anodic substrate: 1.5 mm; width: 25 mm; length: 25 mms; bond strength of the polyimide-silicon bond was 200 N.

Low Temperature Bonding Examples

Example 5: Soda lime glass -Polyimide (Kapton film) bond

[0044] Bonding conditions: Temperature: 150 °C; DC voltage: 2000 KV; voltage was applied for one hour. It should be noted that the applied voltage can be much lower or

much higher. Thickness of anodic substrate: 2 mm; width: 25 mm; length:25 mms; bond strength of the polyimide-silicon bond was 480 N.

[0045] The examples summarized above teach the methods of the present invention for the anodic bonding of polymers to glass, silicon, indium tin oxide (ITO) coated glass and/or other polymers, but are not limited to the above examples. Anodic bonding in accordance with the embodiments of the present invention is advantageous as compared with existing polymer bonding or other bonding methods for the reasons set forth below. Conventional anodic bonding processes cannot be implemented for the bonding of polymers, because in order to create a high electric field between substrates it is necessary to have an anodic side substrate with a high concentration of cations (e.g., sodium ions) which with the application of voltage migrate producing charge separation and as a result produce a high electric field across the interface. In addition, in a conventional setup, polymers do not have movable cations (e.g., "sodium like" ions) which migrate under the influence of an electric field, producing charge separation across the material surfaces and as a result, very high electric field forces across the materials interfaces. Additionally, in a conventional setup, anodic bonding of glass-to-silicon is possible only above about 350- 400 ⁰ C, which is above the Tg of most polymers. Due to the above reasons, conventional anodic bonding processes cannot be used with polymers. The process of charge separation occurs at temperatures around 150°C when using the methods in accordance with the embodiments of the present invention. The novel processes of the present invention are advantageous since by working below the Tg of the polymer, the structures on the polymer surface can be preserved.

[0046] As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. For example, the application of the voltage and the heating may be carried out simultaneously or sequentially, or more than one polymer layer or more than one substrate may be anodically bonded using the methods of the present invention. These other embodiments are intended to be included within the scope of the present invention, which is set forth in the following claims.