GMR biosensor with enhanced sensitivity

Abstract

A method of forming a sensor array comprising a series connection of parallel GMR sensor stripes that provides a sensitive mechanism for detecting the presence of magnetized particles bonded to biological molecules that are affixed to a substrate. The adverse effect of hysteresis on the maintenance of a stable bias point for the magnetic moment of the sensor free layer is eliminated by a combination of biasing the sensor along its longitudinal direction rather than the usual transverse direction and by using the overcoat stress and magnetostriction of magnetic layers to create a compensatory transverse magnetic anisotropy. By making the spaces between the stripes narrower than the dimension of the magnetized particle and by making the width of the stripes equal to the dimension of the particle, the sensitivity of the sensor array is enhanced.

Claims

1. A method of forming an array of GMR sensor stripes whereby said array detects the presence of small magnetized particles and whereby the individual GMR stripes forming said array are not adversely affected by magnetic hysteresis, comprising: providing a substrate; forming on said substrate a plurality of substantially identical, planar, horizontal, rectangular GMR stripes, each stripe having parallel lateral edges in the lengthwise direction and parallel transverse edges in the widthwise direction, wherein the length of said lateral edges is greater than the length of said transverse edges, creating a condition of longitudinal magnetic anisotropy by the formation of said shape; placing said GMR stripes so that proximal lateral edges of adjacent stripes are parallel and separated by a separation distance that is smaller than the size of the small magnetized particles and aligned along said lateral edges so that corresponding transverse edges are co-linear; then connecting transverse edges of adjacent stripes with an electrically conducting element so that said plurality of GMR stripes forms a series circuit having a continuous linearly connected serpentine configuration through which a single continuous electrical current can pass; encapsulating said stripes within a surrounding layer of widthwise-directed stress-inducing insulating material, thereby creating a magnetically stable bias point by combining stress induced magnetostriction with said longitudinal magnetic anisotropy to create a magnetically stable bias point; and wherein each GMR stripe is formed by a method comprising: providing a substrate; forming on the substrate a magnetically pinning layer; forming on said pinning layer a magnetically pinned layer; forming on said pinned layer a conducting, non-magnetic spacer layer; forming on said spacer layer a magnetically free layer; forming on said magnetically free layer a capping layer; wherein all said layers are substantially rectangular and longer in a lengthwise direction than in a transverse direction; and magnetizing said pinned layer and said free layer along a lengthwise direction corresponding to a lengthwise shape anisotropy and inducing a counterbalancing magnetostriction by a widthwise directed stress produced by subjecting said GMR stripe to an encapsulation in a stress-inducing insulating material, whereby an inherently stable bias point is produced by the combined effects of shape anisotropy and stress-induced magnetostriction.

2. The method of claim 1 wherein said magnetically free layer is formed of a CoFe or NiFe ferromagnetic alloy to a thickness between approximately 10 angstroms and 100 angstroms.

3. The method of claim 1 wherein said spacer layer is formed of Cu to a thickness between approximately 10 angstroms and 20 angstroms.

4. The method of claim 1 wherein said encapsulating layer is formed of the insulating material alumina, silicon oxide or silicon nitride, to a thickness between approximately 1000 angstroms and 2 microns.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The objects, features, and advantages of the present invention are understood within the context of the Description of the Preferred Embodiment as set forth below. The Description of the Preferred Embodiment is understood within the context of the accompanying figures, wherein:

(2) FIG. 1 (prior art) is a schematic representation of a magnetic bead bonded to a target molecule and the target molecule bonded to a receptor site.

(3) FIG. 2a (prior art) is a schematic cross-sectional representation of a GMR sensor such as is positioned beneath the substrate of FIG. 1.

(4) FIG. 2b (prior art) is a schematic illustration of an overhead view of the sensor of FIG. 2a, showing also the presence of an external field produced by a magnetized particle.

(5) FIG. 3 (prior art) is a schematic perspective representation of a typical biased GMR sensor stripe over which a magnetized particle is positioned.

(6) FIG. 4a is a schematic overhead view of a sensor array formed of the sensor stripes of the present invention.

(7) FIG. 4b is a cross-sectional schematic view of one sensor stripe of the array.

(8) FIG. 5 is a schematic overhead view of two interconnected sensor stripes of the present invention showing the effects of a magnetized bead.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) The preferred embodiments of the present invention are a GMR sensor stripe and an array of such GMR sensor stripes, capable of detecting the presence of magnetic particles or beads, typically bonded to chemical molecules. The GMR stripe and the array of stripes, by virtue of their formation, are not adversely affected by instability of a free layer bias point due to hysteresis. We use the term stripe to characterize a GMR sensor element and to emphasize the fact that it is deposited in the shape of a long, approximately rectangular strip or stripe. When used to detect magnetic particles bonded to target molecules (eg. in a bio-chemical assay) the array is formed beneath a surface on which are affixed bonding sites for target molecules. To perform the detection process, the target molecules whose presence is to be detected, as well as others that are not targets, are first magnetically tagged, by being bonded to small magnetic particles or beads that are subsequently magnetized by an external magnetic field.

(10) The advantages of the present invention reside in the fact that the bias point of the free and pinned layer magnetizations of each GMR sensor stripe in the array is oriented along the lengthwise direction of the stripe. The fact that the stripes are thin and longer then they are wide, provides a shape anisotropy that maintains a bias point in the lengthwise direction that is stable with respect to hysteresis effects produced by the cyclic motion of the free layer magnetic moment during its use in detection processes. In order to ensure that the shape anisotropy does not adversely affect the sensitivity of the sensor to small external fields that move the magnetization away from the longitudinal bias direction, a compensating anisotropy is produced by combining a stress induced anisotropy due to magnetostriction of the sensor magnetic layers with the stresses in the magnetic layers produced by tension or compression of the various surrounding sensor overlayers that encapsulate the sensor. This combination of magnetostriction and compressional or tensile overlayer stress can be adjusted to reduce the overall magnetic anisotropy. Finally, the sensor free layer is made as thin as possible while not sacrificing the GMR ratio, dR/R, and the interlayer coupling between the free and pinned layers is adjusted to be smaller than the magnetic anisotropies.

(11) The sensor stripes produced by the methods of this invention are then connected in electrical series in a serpentine fashion that places individual stripes side-by-side in a parallel configuration, with a narrow space between adjacent stripes and with the bias directions of their magnetizations (i.e., their magnetic moments) parallel. To achieve this configuration, the individual stripes are placed side-by-side as desired and then electrically connected between the aligned top and bottom edges of adjacent stripes with a conducting element to create a continuous electrical circuit. Because the stripes are very narrowly spaced (less than a bead diameter) and are very narrow themselves (approximately a bead diameter) there is a great likelihood that individual beads located above the stripes will straddle two adjacent stripes, thereby, enhancing the response of the array.

(12) Because the methods of forming the binding surface, the nature and formation method of the binding sites and the means of attaching the magnetic beads to the target molecules are all well known in the art (see the above cited journal article and the prior art patents), the detailed description of the invention that now follows will be restricted to the construction of the sensor stripes and the array configuration.

(13) Referring now to FIG. 4a, there is shown a schematic overhead view of a small array of GMR stripes or, equivalently, what could be a segment of a larger array, in which there are three electrically connected GMR sensor stripes of the present invention, denoted for reference purposes as stripes 1, 2 and 3. These stripes are of generally rectangular shape, having parallel lateral edges (101), (202), (303) of length between approximately 10 microns and 200 microns and parallel transverse edges (111), (222), (333) of width between approximately 1 micron and 5 microns. The stripes are connected in electrical series in an electrically conductive continuous serpentine configuration that aligns successive stripes adjacent to each other with their magnetic moments, when in a quiescent state, oriented in parallel (arrows (11), (22), (33)). The separation (44) between adjacent stripes (filled by the surrounding layers of insulation (45)) is less than the diameter of the magnetic particles to be detected, which are typically between approximately 0.2 microns and 1 micron. As can be seen in the figure, the co-linear upper transverse edges (111), (222) of stripes 1 and 2, are electrically connected with a conducting element (500), as are the lower transverse edges (222), (333) of stripes 2 and 3 (600). The free lower edge of stripe 1 (111) and the free upper edge of stripe 3 (333) are each conductively connected to terminal connectors (550) for the purpose of engaging the array within an external circuit (not shown). If the three stripes are part of a larger array, the terminal connectors would be absent and connections to other GMR stripes would be made. As can be envisioned, if the array consisted of M stripes, the connections would proceed, pairwise, in like fashion, with end stripes 1 and M being connected to terminals. It is understood that the array of FIG. 4a will be encapsulated within surrounding layers of insulation (45).

(14) The dimensional difference between the length and width of each sensor stripe gives the stripe a shape asymmetry that produces a magnetic anisotropy along the lengthwise dimension. This anisotropy assists in maintaining the bias point (the magnetic moment under quiescent conditions) of the free layer when that bias point is also in the lengthwise direction as shown in FIG. 4a. However, the magnetic anisotropy cannot be too great or it will impede the variations in magnetic moment of the free layer under the action of external magnetic fields. Thus, some degree of additional magnetic anisotropy must be incorporated into the sensor stripe in order to produce the required sensor sensitivity. This will now be discussed with reference to FIG. 4b.

(15) Referring to FIG. 4b there is shown a cross-sectional view of a single GMR stripe, such as either of the three stripes in FIG. 4a, illustrating, schematically, the preferred sequence of layers that form the GMR sensor stripe. Looking from the bottom up, there is shown a substrate (55), which can be a layer of oxide, a pinning layer (66), which can be a single layer of antiferromagnetic material, a pinned layer (77) which can be either a single layer of ferromagnetic material, such as CoFe or NiFe, formed to a thickness between approximately 10 and 100 angstroms or a laminated synthetic antiferromagnetic layer formed of two such ferromagnetic layers coupled by a non-magnetic coupling layer, a spacer layer (88) of a non-magnetic, electrically conducting material such as Cu, formed to a thickness between approximately 10 and 20 angstroms, a free layer (99) formed of a ferromagnetic material such as CoFe or NiFe, to a thickness between approximately 10 and 100 angstroms, and an overlayer (100) or capping layer to protect the sensor structure. The overlayer can be a portion of the surrounding insulating layers, which are formed of oxides or nitrides of Si or it can be a portion of the layer that supports the bonding sites for the magnetically tagged particles, the supporting layers being typically formed of similar insulating materials. After the sensor stripe is fabricated, the pinned and free layers are annealed to set their magnetic moment directions (i.e, their magnetizations) along the lengthwise dimension of the stripe as shown here as (777) and (999) also in FIG. 4a (as (11), (22) and (33)) so that the bias point (direction of the magnetic moment when the stripe is quiescent, i.e. is not acted on by external fields) is along the lengthwise direction of the stripe. It is further noted that the stripe is surrounded by layers of insulation (45), such as alumina or oxides or nitrides of silicon formed to thicknesses between approximately 1000 angstroms and 2 microns, to isolate it electrically from neighboring circuit elements (not shown) and that such insulating material will contribute to stresses exerted on the stripe.

(16) By adjusting the spacer layer (88) the interlayer coupling between the free (99) and pinned (77) layers can be reduced so that the variation of the free layer magnetization in response to small external fields produces the required response of the sensor. Further, the free layer itself must be made as thin as possible, without sacrificing the dR/R of the sensor (the measure of its sensitivity), so that the free layer is responsive to small external fields. In addition, as is known in the art, the ferromagnetic layers exhibit the phenomenon of magnetostriction, which is typically defined in terms of a coefficient of magnetostriction. For example, NiFe alloy has a coefficient of magnetostriction that approaches zero at a composition of about 19% Fe. The coefficient becomes negative with less Fe and positive with more Fe. A thin layer (such as is formed herein) of positive coefficient of magnetostriction will exhibit a magnetic anisotropy in a direction of tensile stress on the layer. Likewise, a film having a negative coefficient of magnetostriction will exhibit a magnetic anisotropy in a direction of compressive stress on the layer. As the GMR sensor is a metallic stripe (as shown in FIG. 4b) encapsulated in surrounding insulation layers from above and below (and possibly including the substrate itself), it will generally be under anisotropic compressive or tensile stress that is substantially within the plane of the sensor layers. The magnitude of this stress will depend on the material forming the surrounding insulation layers and specific processes involved in their fabrication. The magnetostriction coefficient of the GMR sensor can be adjusted by its composition to give a magnetostriction coefficient that, when combined with the anisotropic stress of the surroundings, will result in a stress induced magnetic anisotropy that is perpendicular to the lengthwise direction of the stripe. For example, if the anisotropic stress of the GMR sensor is tensile in the lengthwise direction, the magnetically free layer magnetostriction coefficient is adjusted to be slightly negative, so that the stress induced magnetic anisotropy will be perpendicular to the lengthwise direction of the stripe while the magnitude is small enough so that the net anisotropy is still in the lengthwise direction.

(17) Referring now to FIG. 5, there is shown, schematically, just stripes 1 and 2 of the array in FIG. 4a. A magnetized bead (not shown) is located above the separation between the stripes and produces two lobes (1001) and (2002) defining equal-strength field lines of its magnetic field. The field vectors are directed as shown by the enclosed arrows, within the plane of the stripes. It can be seen that the parallel configuration of the adjacent stripes 1 and 2 and the new orientation of their magnetic moments (11) and (22) caused by the field of the magnetized bead, combined with the narrow separation between stripes, the narrowness of each stripe and the series connection of the stripes, produces a significant enhancement of the sensor's response. The maximum response of the sensor array to the presence of a magnetized particle occurs when the particle is over the separation between adjacent stripes, as shown in this figure. In that position, each of the two lobes causes a strong deflection of the magnetic moments of the respective stripes. Because of the series connection of the two stripes, the dR/R of each stripe effectively add to produce a doubling of the voltage drop across the array. If the magnetic bead is not precisely over the separation between stripes, the narrow width of each stripe still ensures that the magnetic field of the bead impinges on more than one stripe and enhances the response of the array.

(18) As is finally understood by a person skilled in the art, the preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. Revisions and modifications may be made to methods, materials, structures and dimensions employed in forming and providing a GMR sensor stripe array with a stable free layer bias point, while still forming and providing such an array and its method of formation in accord with the spirit and scope of the present invention as defined by the appended claims.