FIELD-ASSISTED BONDING

Abstract: The invention relates to a method of bonding together two workpieces, of which at least one is (or has a surface which is) of a material which is electrically substantially non-conductive, by the field-assisted bonding process known as electrostatic bonding which comprises the steps of positioning the workpieces in contact with one another at a common interface, maintaining at least their interface at an elevated temperature, applying a substantial first voltage between them so as to establish at the interface an electrostatic attraction which forces the workpiece surfaces into intimate contact with one another and causes them to bond to one another, and thereafter removing the applied voltage and allowing the workpieces to cool. According to the invention this process is improved by including the additional step, after application of the first voltage and while still maintaining the said interface at elevated temperature, of applying between the workpieces a second voltage of opposite polarity to the first voltage.

Description: 05575 1 - FIELD-ASSISTED BONDING This invention relates to a method of bonding workpieces together by the field-assisted bonding process known as electrostatic bonding, anodic bonding or Mallory bonding. In use of the electrostatic bonding process, two workpieces which are to be bonded together, and of which at least one is (or has a surface which is) of a material which is electrically substantially non-conductive, are positioned 1n contact with one another at a common Interface, the workpieces are maintained at least at their interface at an elevated temperature, and a voltage is applied between them so as to establish at the interface an electrostatic attraction which forces the workplece surfaces into intimate contact with one another and causes them to bond to one another, whereafter the voltage is removed and the workpieces are allowed to cool. The process is commonly employed in bonding a silicon or other semiconductive wafer to a glass substrate, the voltage being so applied as to make the semiconductive wafer electrically positive relative to the glass; and it may also be used for bonding one glass to another if the positive voltage is applied to the glass having the greater alkali content. An important use of the electrostatic bonding process is in the manufacture of semiconductive cavity devices for use as sensors, of pressure or acceleration for example, in which a semiconductive wafer has an operative or diaphragm region, which is parallel to but spaced from a glass substrate surface, and a surrounding region which is bonded to the substrate surface to provide a hermetically sealed cavity between the diaphragm region of the wafer and the corresponding part of the substrate surface. The spacing of the diaphragm region of the wafer from the substrate surface may be achieved either by machining or etching a well or recess in the wafer surface or by depositing an intimately adherent upstanding surrounding ring of Insulating material, such as silicon nitride, on either the wafer or the 75 - 2 - glass substrate, before bringing the wafer and substrate together and effecting the bond between them (or between the ring of insulating material and the surface on which it was not originally deposited). In such cavity devices, the substrate surface within the cavity may have deposited upon it a metal electrode layer forming with the wafer diaphragm, as described, for example, in our international patent application No. WO91/10120, a capacitor whose changes in capacitance indicate changes in the cavity depth and thus accelerations or pressure changes which are to be measured. Another cavity device of the kind here in question is described in our international patent application No. W091/10119. When the electrostatic bonding process is applied in the manufacture of such a device, involving bringing the materials to a temperature of, say, 400°C and applying a voltage of at least several hundred volts or even kilovolts (with the semiconductor wafer positive relative to the substrate), a movement of positive ions, away from the Interface, occurs in the substrate. This movement of ions creates a depleted (high resistivity) region in the substrate immediately adjacent the interface, so that a high proportion of the applied voltage is dropped over that region and a very large electric field is produced there. This results in a large electrostatic attraction at the interface, pulling the materials of the two adjacent workpieces into intimate (atomic) contact and thus promoting the creation of a chemical bond between the workpieces. However, the ionic movement produces a change in the composition of the substrate at and adjacent the interface, where a depletion of mobile ionic species such as lithium occurs, and also at the opposite face and in the body of the substrate, where an increased concentration of the mobile ionic species results. Now, with a view to obtaining a stress-free bond between wafer and substrate, the composition of the substrate is normally chosen to give a thermal expansion characteristic which matches that of the semiconductor wafer almost exactly; but the redistribution of ionic concentration results in this matching being lost and also in a variation of the physical properties of the substrate across its thickness. In fact, the depleted substrate region at and near the interface has a reduced thermal coefficient of expansion, whereas the coefficient at and near the opposite face Increases. It results that, as the bonded assembly cools from the bonding temperature, differential contraction occurs and a bowed assembly is produced. In an assembly of 75mm diameter for example, this bowing may be of the order of hundreds of micrometres, and even in a smaller assembly the amount of bowing, between the periphery and the centre of the cavity formed between the substrate and the wafer, may be equal to a significant fraction of the nominal depth of the cavity. Such bowing of the bonded assembly is undesirable for several reasons. Firstly, it produces a distortion of the nominal shape and dimensions of the cavity between the wafer and the substrate, which may affect, to an unpredictable and variable degree, the characteristics and performance of the device in use. Secondly, the bonding operation may be followed by other operations (such as lapping the semiconductor wafer to reduce Its thickness, thereby to convert its operative region bounding the cavity into a diaphragm of accurately controlled thickness) which require the use of the opposite, exposed surface of the substrate as a reference plane from which to gauge the wafer thickness, and any bowing of the nominally flat reference surface must reduce the accuracy of such operations. Accordingly, it is an object of the present invention to provide a method of bonding workpieces together, by the electrostatic bonding process, in which bowing of the workpieces is avoided or greatly reduced; and this object is achieved, according to the invention, by applying the bond-assisting voltage first with one polarity, to effect bonding, and then reversing the polarity of the applied voltage. According to the invention, therefore, there is provided a method of bonding together two workpieces, of which at least one is (or has a surface which is) of a material which is electrically substantially non-conductive, comprising the steps of positioning the workpieces in contact with one another at a common interface, maintaining at least their interface at an elevated temperature, applying a substantial first voltage between them so as to establish at the interface an electrostatic attraction which forces the workpiece surfaces Into Intimate contact with one another and causes them to bond to one another, and thereafter removing the applied voltage and allowing the workpieces to cool, characterised by the additional step, after application of the first voltage and while still maintaining the said interface at elevated temperature, of applying between the workpieces a second voltage of opposite polarity to the first voltage. Preferably the magnitude of the second applied voltage Is at least approximately equal to that of the first voltage, though of opposite polarity, and the reversed-polarity second voltage is applied for a period at least approximately equal to that during which the bond-assisting first voltage is applied. The effect of applying the second voltage, with reversed polarity, appears to be to reverse the direction of the migration of mobile ions which has occurred, as described above, during application of the bond-assisting first voltage. Accordingly, in the case described above where the workpieces being bonded are respectively a semiconductor wafer and a glass substrate, the mobile ion depletion in the substrate at and near the interface, and the build-up of a mobile ion excess in other parts of the substrate, are reversed and the compositional homogeneity of the substrate is in large measure restored during the period of application of the reversed-polarity second voltage. In consequence, the uniformity of the substrate in terms of thermal coefficient of expansion is also in large measure restored, with the result that, when the second voltage is removed and the bonded assembly is allowed to cool, the substrate contracts uniformly and virtually without any bowing or other distortion. For example, bonded assemblies which, without application of the reversed polarity voltage in accordance with the invention might have shown bowing of hundreds of micrometres as mentioned above, have been obtained with a bowing of as little as 2 micrometres when made in accordance with the invention. Furthermore, if (as will usually be the case) the composition of the substrate has been chosen to match Its thermal coefficient of expansion to that of the semiconductor wafer being bonded to it, this matching is not lost when the bonding is effected by the method of invention, and, as intended, the bond is therefore not subjected to substantial stresses due to differential contraction rates as the bonded assembly cools. It will be understood that the method according to this present invention may be employed as an improvement on the bonding methods described in the specifications of our aforementioned international patent applications, the disclosures of which are hereby incorporated by reference in the present specification.

Claims: 75 - 6 - CLAIMS 1. A method of bonding together two workpieces, of which at least one is (or has a surface which is) of a material which is electrically substantially non-conductive, comprising the steps of positioning the workpieces in contact with one another at a common interface, maintaining at least their interface at an elevated temperature, applying a substantial first voltage between them so as to establish at the interface an electrostatic attraction which forces the workplece surfaces Into intimate contact with one another and causes them to bond to one another, and thereafter removing the applied voltage and allowing the workpieces to cool, characterised by the additional step, after application of the first voltage and while still maintaining the said interface at elevated temperature, of applying between the workpieces a second voltage of opposite polarity to the first voltage. 2. A method as claimed in Claim 1, wherein the first and second voltages are of at least approximately equal magnitude though of opposite polarity. 3. A method as claimed in Claim 1 or Claim 2, wherein the first and second voltages are applied for at least approximately equal periods of time. 4. A method as claimed in any of Claims 1 to 3, wherein the workpieces are respectively a wafer of single-crystal silicon and a substrate of glass, characterised in that with these materials maintained at a temperature of at least approximately 400°C the said first voltage is applied with a magnitude of at least several hundred volts and a polarity such as to maintain the silicon wafer positive relative to the glass substrate.

Inventors: ROGERS TONY WILLIAM JAMES, ROGERS, TONY, WILLIAM, JAMES
Applicants: NAT RES DEV, NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1991
Main classification: H01L21
IPC codes: H01L21, H01L29