Patent Application: US-46203406-A

Abstract:
a chemical or corrosive environment sensing system , comprising a giant magnetoresistive effect device having at least one environmentally exposed film , and a device , for sensing changes in the gmr effect device resulting from environmental exposure of the at least one environmentally exposed film . the film may be reversibly or irreversibly altered by the exposure , and is preferably nano - textured to alter a reaction rate and surface area . the sensor may be enzyme linked , that is , respond to an enzyme reaction product rather than the substrate directly . the gmr property altered and / or sensed may be , for example , a lower or upper switching field , an electrical resistance , and the gmr value . the device may be used as a sensor or as part of a control system .

Description:
this invention utilizes combined property of corrosion protection and controlled chemical reaction rates for a cobalt ( co ) top layer in a magnetic spin valve . the degree of chemical reaction is monitored by measuring the changes in magnetic property of the top co layer , which is very sensitive to the occurrence of corrosion and chemical reaction . it has been demonstrated that the magnetic property of a thin co layer to chemical reaction at the atomic scale can be easily quantitatively measured using spin valves as a sensing unit . the most noticeable magnetic property change that is affected by corrosion and chemical reaction is the magnetic switching field in a small applied magnetic field . the change in switching field can be easily measured using a spin valve with gmr effect with co as the top magnetic layer . to achieve ultra - high sensitivity , the corrosive - condition sensitive media in the sensors is preferably in the form of nano - structured magnetic materials such as patterned arrays , nano - scaled granular thin films , or magnetic ion loaded mesoporous media . there are four important reasons for choosing nano - scaled materials for corrosion testing , based on geometrical , structural and magnetic considerations : 1 ) the very small feature sizes of the corrosion media will enhance the sensitivity of the sensors due to a comparable length scale to the corrosion thickness . for example , the thickness of oxide layers formed on ferromagnetic metals is usually several nanometers ( after several days of exposure ), comparable to the characteristic sizes of nano - structured materials . due to very small size of the corroding objects the sensitivity of the sensor will be greatly enhanced because a large part of the material can be corroded in a short time . 2 ) a related property of the nano - sized and porous materials is their large surface to volume ratio , which can be orders of magnitude higher than for conventional polycrystalline alloys . the corrosion rate and the usable exposure time in corrosion testing can be controlled by varying the characteristic size of the corroding objects , tailored to have suitable sensor response for different materials . the corrosion rates for bulk fe , ni and their alloys are typically in the range of 2 to 500 nm / hour , while that for co is lower . the corrosion rates of nano - structured materials can be hundreds times higher than that of the bulk materials . 3 ) the microstructure of these materials is well defined upon reaching nanometer scales . corrosion of very fine objects with uniform sizes ( i . e . grain or pattern size ) and well defined crystallographic orientations is expected to be highly repeatable for a given corrosive environment . because of specific structure of these materials the stress corrosion and pitting corrosion mechanisms can be neglected . 4 ) magnetic grains with dimensions below 150 nm can be single domain . they have simple inner magnetic structure , which makes the interpretation of the magnetic behavior much simplified comparing to larger grains where domain wall movements should be taken into account . the magnetic behavior of single domain magnetic grains has been thoroughly studied and understood . for example , the domain magnetization re - orientation is considered to be a coherent rotation . the dependence of the magnetization on the grain size has been well established . there are many magnetic properties of nano - structured magnetic materials , which are suitable for use in corrosion testing . the most relevant properties are : 1 ) magnetization — the saturation magnetization should decrease , approximately proportional to the mass of corroded magnetic material . some deviations from this relation are expected only for very thin films ( of the order of several atomic layers ) or for very small isolated particles . the decrease in the magnetization for ultra - fine particles , with the particle size , is more pronounced due to superparamagnetism and particle surface effect . the saturation magnetization can be measured using highly sensitive magnetic field sensors to monitor corrosion - related changes . the magnetic field sensor is preferably a gmr spin - valve , although a spin - tunneling device or magnetic hall effect device might also be used . 2 ) initial permeability — the inductive sensing of dynamic permeability can be applied to measure permeability , which is sensitive to magnetic anisotropy . it was demonstrated that increasing the distances between nano - crystalline grains results in a dramatic change in magnetic properties . the advantage of the inductive method is that it is a noncontact method , hence , the testing material can be easily replaced . it is also important that this method works in a wide temperature range , so both low temperature and high temperature corrosion can be monitored . special interests lie in measuring small ferromagnetic particles , which demonstrate superparamagnetic behavior at the ambient temperature . this may be the case for mesoporous media or thin film alloys containing ferromagnetic particles . corrosion of such materials will decrease the grain size of the ferromagnetic particles , lowering their blocking temperature . the consequence is a rapid decrease in the permeability at the onset of corrosion . the spin valve has a simple structure of co / cu / co with a proper buffer layer ( e . g . fe ) on a substrate ( si or glass ). the top and bottom magnetic layers respond to applied magnetic field differently due to different response property , layer thickness , and chemical environment . the spin valve possesses unique property of having different resistance values with different magnetic configuration of the two co layers : the magneto - electronic response curve sensitively depends on the relative magnetic orientation of the two magnetic layers , with the resistance having a higher value when two magnetic vectors are in opposite direction . thus , such spin valves provide a practical means to measure the magnetic orientation of individual layers . since the magnetic responses critically depend on the chemical environment and reaction , this provides a sensitive method of determining the degree of corrosion and chemical reaction , and allows implementation of an integrated all - electronic sensor and circuit . it is also possible to build remotely monitored unit by incorporating wireless data communication . the spin valve sensor according to the present invention is generally compatible with various semiconductor processing techniques , and thus the sensor may be provided on the same substrate or integrated with processing circuitry and other circuitry . the magneto - electronic properly of spin valves can be modified by applying thermal annealing up to 350 ° c . fig1 shows the typical magnetoelectronic response curve of a series of co / cu / co / fe spin valves annealed at different temperatures ranging from 200 ° c . to 350 ° c . according to one embodiment of the invention , the spin valve design has a layering configuration of co ( 2 nm ), cu ( 3 nm ), co ( 5 nm ), and fe ( 5 nm ). the spin valve films were deposited in a high vacuum system with a base pressure better than 1 . 5 × 10 − 7 torr . the film deposition was done using magnetron sputtering with elemental targets and deposition rates of about 0 . 2 nm / s . the deposition ar pressure was kept at about 4 . 0 mtorr . the annealing at various temperatures was done in a following reducing gas of composition h8 . 5 %- ar for 30 minutes or more . proper ramping was allowed in the annealing procedure . these spin valves have a gmr value of about 1 to 10 %, and a lower switching field of about 20 oe , which corresponds to the magnetic switching of the lower co / fe bi - layer . the higher switching field is highly dependent on the annealing temperature . as shown in fig1 , annealing temperature has a strong effect on the gmr values and high switching field . initially , annealing at lower temperatures ( below 250 ° c . ), the gmr values remains unchanged , while the higher switching field is reduced slightly from about 150 oe to 120 oe . subsequently , the gmr value is slightly enhanced and the higher switching field is increased ( to about 220 oe ). upon annealing at 350 ° c ., this particular spin valve lost its gmr response . fig2 and 3 show the influence of chemical environment on the gmr response . this spin valve was annealed at 250 ° c . before subject to chemical sensing tests . after being corroded in air for two days , the gmr value is increased by 20 % ( from 1 . 6 % to 2 . 0 %), and the high switching field was increased dramatically from 80 oe to 4200 e ( fig2 ). upon further annealing , the high switching is somewhat reduced . the same spin valve film , after submerged in dionized water for 10 minutes or less , lost its gmr response altogether ( fig3 ). this demonstrates the dramatic sensitivity of such spin valves to the chemical reaction and its selectivity . the materials for corrosion testing are initially produced in the form of thin magnetic films of fe , co and ni ( or other magnetic alloys or compounds ) with thickness values ranging from 100 nm to 2 μm , although thinner films , e . g ., 10 - 200 nm , may be used . the thin films may be formed using magnetron sputtering deposition . magnetron sputtering is a versatile technique that allows production of a large variety of thin films with repeatable microstructures ( epitaxial or polycrystalline films ) and compositions . nano - sized arrays and micron - sized device structures may be patterned from these films using x - ray lithography and ion milling . granular nanocrystalline films can be produced either by the magnetron sputtering deposition or using a laser ablation deposition . the mcm - 41 mesoporous media can be made by chemical methods in known manner . mcm - 41 has hexagonal arrays of micellar rods , which are used as templates for silicon oxide growth and after removing the organic medium by calcination they form an array of tubes with diameters of 2 to 10 nm . these tubes can be loaded by ferromagnetic materials and used as a corrosion media . the new composite corrosion sensors are based on the vitality of most magnetic materials with reactive chemical species and ultra - sensitivity of the magnetic field sensors made from specially structured and selected layered or composite magnetic systems . the spin - valve and spin - tunneling devices all have maximum sensitivity in current - perpendicular - to - plane configuration . since the thickness of these magnetic structures is generally in the range of several to tens nanometers , to achieve reasonably detectable signals the lateral dimensions need to be in the micrometer range . also , to induce substantial magnetic fields at the field sensors , the corrosion media will be required to reach micrometer sizes . the micrometer and submicrometer dimensions may be achieved by photolithography in combination with ion milling and micromachining using hard x - ray . one consideration is whether the magnetic field sensors are sensitive and stable enough to detect the slight variation in magnetic field produced by the magnetic corrosion media during the course of monitoring corrosive environments . let us assume that fe or co particles are used as corrosion agents that fill a porous medium with 50 % of the total volume . alternatively , one considers patterned arrays of magnetic strips or dots with 50 % magnetic volume . we also assume that all the magnetic particles are magnetized along the same orientation . the magnetic field , b generated by these particles at a distance of r from the surface of the magnetic medium is found to be b = p ( 4πm )( r / l ) ln ( i + t / r ). in this expression , t is the thickness and l the lateral dimension of the overall magnetic medium , m the magnetization density of the magnetic component , and p the magnetic volume percentage . for bulk fe and co , ( 4πm )= 2 . 2 and 1 . 8 tesla , respectively , thus , on the average we have 4πm = 2 tesla . for nano - sized magnetic particles it is known that the curie temperature decreases substantially as compared with the bulk value . there is a large reduction in the saturation magnetization at the ambient temperature . for example , it has been found that for fe particles with their sizes ranging from 2 to 6 nm the m value at the ambient temperature is reduced by 40 % compared to the bulk value . thus , the realistic value for ( 4πm ) should be somewhat smaller . let us further assume that it is 60 % of the bulk value , and that the thickness , t , of the magnetic medium equals the distance , r , from the surface . the practical limit for the lateral dimensions of micro - sized structures by micromachining is about 10 micrometers . if we assume a l / r ratio of 10 ( about 1 micrometer thick magnetic film ), the estimated magnetic field for the magnetized medium is about 690 gauss , for 50 % magnetic volume fraction . this field is strong enough to be easily detected by gmr sensing devices . when one atomic layer on the magnetic particle surfaces is fully corroded the change in the overall magnetization can be as large as 30 % due to a large surface area . this corresponds to a field variation of 210 gauss . for the case of magnetization parallel to the medium plane , the induced magnetic field is reduced further by a factor of about 0 . 2 . the total induced field and its variation for one atomic layer corrosion correspond to , respectively , 140 and 40 gauss . these values can be further enhanced by decreasing the lateral dimensions of the magnetic corrosion media and by positioning the field sensors closer to the corrosion media . the fact that the magnetic media consist of cluster arrays instead of uniformly dispersed magnetic atoms further increases the local fields . the parallel magnetization configuration is required for spin valve and spin - tunneling magnetic field - sensing configuration since the easy axis of the magnetization for the sensor structure is in plane . these structures are the most sensitive field - sensing devices known to date , ideally suited for the parallel magnetization configuration . the best demonstrated spin - valve sensors have a field sensitivity of 3 %/ gauss . such sensitivity limits are well suited for accurate determination of these field variations . exchange biased gmr spin valves are a preferred sensor type . the direction of the magnetization of one magnetic layer is pinned by the exchange biasing to a contiguous antiferromagnetic layer , while that of the other magnetic layer is free to rotate and thus probe the magnetic field . specific systems include permalloy / cu and co / cu spin - valves using femn and nio as the antiferromagnetic pinning layers . magnetron sputtering has been shown to be one of the best deposition methods to grow spin - valve structures . the magnetic structures or thin films for sensing magnetic fields are deposited on si wafers , and then patterned according to predefined geometry and dimensions , by combined lithography , ion milling , and micromachining after being patterned into field sensing devices , the structures are embedded in a non - magnetic and non - conductive protective medium such as sio 2 . the resulting assembly is then polished , reactively ion etched and annealed to produce a smooth surface . annealing also serves to modify the internal microstructures of the field - sensing structures for improved performance . further fabrication of desirable corrosion thin films suitable for specific corrosive environments is then done on the treated surface . the new film is patterned into micrometer sizes , and nano - sized structures of strip or dot arrays are fabricated on the patterned corrosion medium to enhance the exposure area . after patterning , the sample is placed in a high vacuum chamber to be ion etched to remove the oxide layers to expose the magnetic surfaces . then , an intermediate porous corrosion - damping layer , such as porous sio 2 can be deposited to modify the corrosion rate of the medium . another protective capping layer can be deposited to prevent corrosion before testing and / or use . the approach of directly fabricating the corrosion medium on a protective spacer on the field - sensing structure is desirable due to minimized intermediate processes and the need to expose the corrosion medium to testing or corrosive environments to be analyzed . the present invention provides a method for integrating the two separate processes of making the field - sensing device layer and magnetic corrosion layer into a single process . in case that the corrosion medium is fabricated separately , such as by wet chemical methods , slices of the fabricated corrosion medium are bonded onto the magnetic - field sensing structures . according to the formulation provided above , an acceptable geometrical configuration is with a comparable thickness of the corrosion medium to the distance of the field - sensing structure to the medium . the sensitivity will be otherwise affected in two aspects . for too thick a corrosion medium , the effective corrosion will tends to be in the surface region . the effectiveness of inducing significant field change is minimized and difficult to control . for a thin corrosion medium the induced field effect is too weak to be effective . reactive ion etching is employed to remove undesirable materials to modify the corrosion layer thickness once the two components are bonded together , to achieve optimal performance . after desirable thickness is achieved a protective cap layer is deposited . corrosion starts and mostly occurs on the surface within a layer with a typical thickness of 2 to 10 nm . in regular magnetic recording media with thick magnetic films , there is little correlation between the topography and the magnetic domain structure as determined by spm : because the latter is a bulk effect whereas the former is a surface effect . for nano - scaled magnetic corrosion media , there should be a correlation between the changes in the magnetic domain structure and the topography variation by corrosion , since in such systems the magnetic film thickness is comparable to corrosion product layer thickness . for relative corrosive materials such as ni , fe , and their alloys , gas phase exposure to corrosive species ( with different electric negativities ) under controlled temperature , concentration and flow conditions can be sensed . factors affecting corrosion rates include temperature , humidity , gas content and flow rates . chlorine and hydrogen ions are known strong corrosive agents , while oxygen is relatively weaker . for corrosion resistive co and its alloys , the sensor may be suitable for analyzing both gaseous and aqueous environments . magnetron sputtering deposition is utilized to fabricate magnetic thin films for both corrosion media and magnetic field - sensing structures . the process is controlled to achieve controlled material properties , such as crystalline orientation , surface smoothness , interface quality , film adhesion , and their dependence on deposition conditions . the film systems for corrosion media may include thin films of one or more of ni , co , fe , gd , etc ., and their magnetic granular alloys with other non - magnetic elements . for comparison with the resulting corrosion products , the corresponding magnetic - oxide and sulfate films may be fabricated and studied . magnetic thin films and multilayers for field sensing devices may include giant moment magnetic films , such as fe — pt , gmr spin - valve structures ( e . g . feni / cu / feni / mnfe ), and spin - tunneling structures . the individual processes in the production of a preferred sensor , in sequential order , are : 1 ) the field - sensitive thin film structures deposited on si wafers , followed by property measurement and characterization . 2 ) the field - sensitive structures processed into micron - sized devices including proper signal readouts . 3 ) buffer layer deposited on the devices followed by reactive ion etching to gain desirable thickness and a smooth surface . 4 ) magnetic corrosive films deposited on the buffer layer , followed by patterning into micron - sized structure . 5 ) for mesoporous medium containing magnetic particles , thin slices will be bonded to the buffer layer , thinned down by reactive ion etching to desirable thickness , and patterned into micron - 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