Apparatus and Method for Flip-Chip Bonding

This invention relates in general to flip-chip bonding of hybrid solid state systems and more particularly to thermal (infrared) imaging systems and methods of fabrication.

Various techniques are available for electrically and mechanically connecting one substrate with another substrate to form a semiconductor device. One technique includes placing leads or terminals on one face or side of a first substrate and coupling the first substrate with matching leads or terminals on one face or side of a second substrate. Frequently, bead-like projections or bumps are formed on the face of the first substrate which is then registered with matching terminals on the second substrate and the substrates are bonded together. Various types of semiconductor substrates such as diodes, transistors and integrated circuits may be formed with their terminals on one face or one side of the substrates. Such substrates are often referred to as flip-chips and the method of coupling two flip-chip substrates is frequently referred to as flip-chip bonding.

For several years hybrid solid state systems such as infrared detectors have been successfully incorporated into integrated circuits for mass production and miniaturization. Typically, such infrared detectors are fabricated with an N.times.M array of infrared detector elements or thermal sensors (sometimes referred to as a focal plane array). The focal plane array is generally bonded with an integrated circuit substrate to form the thermal imaging system. Bump bonding is a common technique used to couple a focal plane array with its associated integrated circuit substrate.

One procedure used in bump bonding a focal plane array with an integrated circuit substrate includes placing the integrated circuit substrate on a heated pedestal. The focal plane array is next placed in a gimbal and aligned with the integrated circuit substrate. Frequently, the gimbal is formed as part of a pivoting arm assembly. Linear motion may also be used to move the gimbal assembly. After alignment and registration of the focal plane array with the integrated circuit substrate, the assembly may be used to apply a pre-selected amount of force to the focal plane array and integrated circuit substrate. Heat may also be applied if required to aid in the bonding process. Typically such bonding is performed in either a vacuum environment or inert gas environment.

Prior bonding equipment and methods include the use of heat lamps, resistant heaters, heated and cooled lines, and thermoelectric means to obtain the required temperature profile depending upon the type of materials used in the bonding process. Likewise, gravity, load cells, air cylinders and diaphragms have been used to provide the necessary force during the bonding process.

In accordance with the present invention, apparatus and methods are provided which substantially eliminate or reduce disadvantages and problems associated with prior flip-chip bonding equipment and techniques. The present invention allows high volume fabrication of semiconductor devices formed from two or more substrates with flip-chip type interconnections between the substrates. One aspect of the present invention includes apparatus for alignment and registration of two substrates with matching flip-chip interconnections and applying a predetermined amount of force to the substrates. The apparatus may also be used to maintain the desired alignment and force while transporting the substrates and during temperature cycling of the substrates in an oven or other heating system to complete the bonding process.

An important technical advantage of the present invention includes fabricating a thermal imaging system using flip-chip bonding techniques such as bump bonding to couple a focal plane array with its associated integrated circuit substrate. The bonding apparatus may include a permanent magnet to apply a predetermined amount of force to the substrates. The bonding apparatus also allows transportation of the focal plane array and its associated integrated circuit substrate to a furnace or heater assembly for temperature cycling while maintaining the desired alignment and force on the substrates. An air gap associated with the permanent magnet may also be used to adjust the force placed on the substrates.

Another aspect of the present invention includes providing bonding apparatus which allows low cost, high volume, bump bonding of focal plane arrays with their associated integrated circuit substrates. For some applications, more than one thousand semiconductor devices may be fabricated with relatively short cycle times between alignment of the substrates, movement of the substrates to a furnace or heater and completion of the bonding process by temperature cycling.

Another significant technical advantage of the present invention includes providing bonding apparatus which compensates for thermal expansion and contraction of two or more substrates and their associated interconnections during temperature cycling while at the same time, maintaining the desired alignment and force on the substrates.

A further significant technical advantage of the present invention includes the ability to vary the force applied to two or more substrates during a flip-chip bonding process. For those embodiments of the present invention which include the use of one or more permanent magnets, the number of magnets, their size, type and associated air gap may be easily modified to vary the force applied to the substrates. For those embodiments of the present invention which include the use of electromagnets, the physical configuration of the magnets may be modified and/or the electrical power supplied to the magnets may be adjusted to vary the force applied to the substrates.

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an isometric drawing showing two substrates having flip-chip type interconnections which may be bonded or coupled with each other in accordance with the present invention;

FIG. 2 is a schematic drawing in section with portions broken away showing apparatus including a moveable pedestal incorporating the present invention for aligning and registering two substrates such as shown in FIG. 1;

FIG. 3 is a schematic drawing in section with portions broken away showing the apparatus of FIG. 2 with the substrates of FIG. 1 in contact with each other and a predetermined amount of force applied to the substrates; and

FIG. 4 is a schematic drawing in section with portions broken away showing a heater assembly with the movable pedestal disposed therein and the substrates secured to the movable pedestal in contact with each other and a predetermined amount of force applied to the substrates.

The preferred embodiments of the present invention and its advantages are best understood by referring to FIGS. 1 through 4 of the drawings, like numerals being used for like and corresponding parts of the various drawings.

Two or more substrates are frequently coupled with each other during fabrication of semiconductor devices and hybrid solid state systems. The present invention may be used to couple or bond substrates with matching flip-chip type interconnections. FIG. 1 is a schematic representation of such substrates which may be coupled or bonded with each other in accordance with the present invention. First substrate 20 may be an integrated circuit substrate (sometimes referred to as a silicon processor). Second substrate 22 may be a focal plane array associated with an infrared detector or thermal imaging system (not shown). As will be described later in more detail, the present invention allows fabricating a thermal imaging system by bump bonding a focal plane array with an integrated circuit substrate. A thermal imaging system is only one example of the various types of semiconductor devices and hybrid solid state systems which may be fabricated using the present invention. For some applications, the present invention may be used to bond leads associated with tab type or surface mounting techniques, especially as technology allows additional geometry reductions and maintaining precise alignment during fabrication becomes increasingly critical.

Infrared detectors or thermal imaging systems typically function based upon either the generation of a change in voltage due to a change in temperature resulting from incident infrared radiation striking a thermal sensor, or the generation of a change in voltage due to a photoelectron interaction within the material used to form the thermal sensor. This latter effect is sometimes called the internal photoelectric effect.

Thermal sensors having ferroelectric elements typically generate a change in voltage due to a change in temperature of the ferroelectric material resulting from incident infrared radiation. Such thermal sensors may be formed from barium strontium titanate or other suitable ferroelectric materials.

Second substrate or focal plane array 22 may be formed from a plurality of thermal sensors (not shown) which function based upon the generation of a change in voltage due to a change in temperature of ferroelectric material resulting from the incident infrared radiation. Alternatively, second substrate or focal plane array 22 may be formed from a plurality of thermal sensors (not shown) which function based on the generation of electron-hole pairs resulting from the internal photoelectric effect. The present invention may be satisfactorily used to couple focal plane array 22 having either type of thermal sensor to an associated integrated circuit substrate 20.

Various types of thermal sensors may also be satisfactorily coupled or bonded using the present invention. U.S. Pat. No. 4,143,269 entitled Ferroelectric Imaging System, to McCormak, et al., provides information concerning infrared detectors fabricated from ferroelectric materials and a silicon switching matrix or integrated circuit substrate. Second substrate 22 may be formed with a plurality of ferroelectric thermal sensors and first substrate 20 may comprise an integrated circuit substrate as shown in U.S. Pat. No. 4,143,269. U.S. Pat. No. 4,143,269 is incorporated by reference for all purposes in this patent application.

Focal plane array or second substrate 22 may also be manufactured from thermal sensors which are structured as photodiodes and/or photocapacitors. U.S. Pat. No. 4,447,291 entitled Methods for Via Formation in HgCdTe, issued to Eric Schulte, and U.S. Pat. No. 5,144,138 entitled Infrared Detector and Method, issued to Kinch, et al., provide information concerning infrared detectors fabricated from HgCdTe semiconductor materials and alloys which produce electron-hole pairs in response to incident infrared radiation. U.S. Pat. Nos. 4,447,291 and 5,144,138 are incorporated by reference for all purposes in this application.

Various types of silicon processors and/or integrated circuit substrates may also be satisfactorily used to manufacture infrared detectors in accordance with the present invention. U.S. Pat. No. 4,684,812 entitled Switching Circuit for a Detector Array, issued to Tew, et al., provides information concerning one type of silicon processor satisfactory for use as first substrate 20. U.S. Pat. No. 4,684,812 is incorporated by reference for all purposes in this application.

Typical bump bonding procedures for fabricating a thermal imaging system involve bringing the integrated circuit substrate and the focal plane array into initial close proximity and aligning the two substrates with respect to each other. A predetermined amount of force is then applied to the substrates and the entire assembly is cycled to the correct temperature to obtain the desired bond between the substrates. This procedure is often performed in one piece of equipment providing the necessary alignment, coupling forces, and heat source. Depending upon the bonding medium, the bonding procedure may be performed under vacuum, inert or atmospheric conditions.

Examples of bump bonds associated with thermal imaging systems are shown in U.S. Pat. No. 5,047,644 entitled Polyamide Thermal Isolation Mesa for a Thermal Imaging System, issued to Meissner, et al. The fabrication techniques and the materials used in U.S. Pat. No. 5,047,644 may be used in fabricating substrates 20 and/or 22. U.S. Pat. No. 5,047,644 is incorporated by reference for all purposes in this patent application.

FIGS. 1 through 4 are schematic representations of selected steps in the fabrication of a semiconductor device or hybrid solid state system such as an infrared detector or thermal imaging system. The principal components of the thermal imaging system include focal plane array 22 and integrated circuit substrate 20.

As shown in FIG. 1, integrated circuit substrate or silicon processor 20 and focal plane array 22 comprise a plurality of metal interconnections 24 and 26 respectively. Interconnections 24 are formed on surface or side 28 of integrated circuit substrate 20. Each interconnection 24 is preferably located adjacent to a contact pad (not shown) used to provide signals to silicon processor 20. Each interconnection 26 is preferably associated with an individual thermal sensor (not shown) in focal plane array 22. Each interconnection 26 is preferably designed to mate with a corresponding interconnection 24 to allow transmission of signals from the respective thermal sensor to silicon processor 20 through a bond formed by the associated interconnections 24 and 26.

Substrates 20 and 22 are shown with a generally rectangle configuration. However, the present invention may be used to satisfactorily couple a wide variety of substrate and wafer configurations including, but not limited to, circular, oval and square.

For purposes of illustration and description only, reference is made in FIGS. 2, 3 and 4 to bonding integrated circuit substrate 20 with focal plane array 22. However, various types of substrates may be bonded using the apparatus shown in FIGS. 2, 3 and 4. Also, for some applications, more than two substrates may be simultaneously bonded using the apparatus shown in FIGS. 2, 3 and 4 and related fabrication techniques incorporating the present invention.

The principal components of bonding apparatus 40 shown in FIGS. 2 and 3 include supporting structure 42, base unit 44, pedestal assembly 50 and alignment assembly 90. Pedestal assembly 50 is preferably movably disposed on its associated base unit 44. Base unit 44 cooperates with alignment assembly 90 to position pedestal assembly 50 and retainer plate 96 with respect to each other.

Pedestal assembly 50 comprises housing 52 with chamber 54 disposed therein. Housing 52 may have the general configuration of a hollow cylinder. However, for some applications, housing 52 may have a rectangular, square or other geometric configuration as desired for the specific application.

For some application, a plurality of base units 44 and their associated pedestal assemblies 50 may be disposed on supporting structure 42. An important feature of the present invention includes the ability to use base unit 44, pedestal assembly 50, and alignment or gimbal assembly 90 in cooperation with each other for high volume production.

One end of housing 52 is preferably attached to carrier 56 which may be used to position pedestal assembly 50 on base unit 44 and to transport pedestal assembly 50 between bonding apparatus 40 and heater assembly or furnace 110. Base unit 44 preferably includes first passageway 46 which allows communicating a vacuum from a remote source (not shown) with chamber 54.

Bushing 58 with one or more magnets 60 is slidably disposed on the exterior of housing 52. End plate 62 is provided on the end of housing 52 opposite from carrier 56. End plate 62 comprises a portion of the means for releasably holding first substrate 20 on pedestal assembly 50. End plate 62 also forms shoulder 64 on the exterior of housing 52. Bushing 58 is preferably sized to slide longitudinally over the exterior of housing 52 between carrier 56 and shoulder 64. For some applications, bushing 58 may be replaced with a bearing assembly (not shown) or a slide housing (not shown) which allows linear movement of magnet 60 relative to the exterior of housing 52.

As will be explained later in more detail, shoulder 64 cooperates with bushing 58 to provide a portion of the means for determining the amount of force applied to first substrate 20 and second substrate 22 during the bonding process. For some applications, the position of shoulder 64 on housing 52 may be adjustable to allow varying the force applied to first substrate 20 and second substrate 22. Groove 70 is provided on the exterior of bushing 58 to assist with moving magnet 60 relative to end plate 62. One or more openings 68 are provided in end plate 62 to allow vacuum from chamber 54 to releasably hold first substrate 20 on end plate 62.

Alignment assembly 90 preferably includes alignment head 92 and arm 94. For some applications, alignment head 92 and arm 94 may function as a pivoting arm assembly with respect to supporting structure 42. However, various electric, pneumatic, and/or mechanical mechanisms may be used to adjust the position of alignment head 92 with respect to its associated pedestal assembly 50. Retainer plate 96 is releasably secured to the portion of alignment head 92 facing the associated pedestal assembly 50. For some applications of the present invention, a plurality of base units 44 with their respective alignment assembly 90 and alignment head 92 will be disposed on supporting structure 42.

Retainer plate 96 provides a portion of the means for releasably securing second substrate 22 with alignment head 92. Retainer plate 96 and/or alignment head 92 will preferably include a gimbal assembly (not shown) which allows adjusting the position of second substrate 22 with respect to first substrate 20. Second passageway 98 is preferably provided in alignment head 90 to communicate vacuum from a remote source (not shown) to retainer plate 96 to releasably secure retainer plate 96 to alignment head 92. One or more openings 100 are provided in retainer plate 96 to allow the vacuum within second passageway 98 to releasably secure second substrate 22 with retainer plate 96. Retainer plate 96 provides a portion of the means for applying a preselected amount of force to couple first substrate 20 with second substrate 22.

One or more pins 66 are preferably provided on base unit 44 for engagement with carrier 56 to position carrier 56 and its associated pedestal assembly 50 with respect to first passageway 46 of base unit 44 and alignment head 92. For some applications, alignment assembly 90 and pedestal assembly 50 may be adjusted relative to each other by the same alignment and control system (not shown). For other applications carrier 56 may remain fixed relative to base unit 44 while base unit 44 is positioned on supporting structure 42 relative to alignment head 92.

A typical bonding procedure using the present invention includes the following steps. First substrate or silicon processor 20 is placed on end plate 62 of pedestal assembly 50. Vacuum supplied from chamber 54 through opening 68 may be used to releasably hold first substrate 20 on pedestal assembly 50. Second substrate or focal plane array 22 is next releasably attached to retainer plate 96 and mounted on the respective alignment head 92. Vacuum from second passageway 98 and opening 100 may be used to releasably secure second substrate 22 and retainer plate 96 with alignment head 92.

Alignment assembly 90 and pedestal assembly 50 may be adjusted relative to each other using base unit 44 to establish the desired alignment between interconnections 24 and 26 on their respective substrates 20 and 22. Alignment assembly 90 may also be used to position second substrate 22 in close proximity with first substrate 20. Depending upon the type of interconnections 24 and 26, and the positioning capabilities of alignment assembly 90, substrates 20 and 22 may be positioned such that interconnections 24 and 26 physically contact each other. For other applications, interconnections 24 and 26 may be held in very close proximity to each other by alignment head 92.

After first substrate 20 and second substrate 22 have been positioned with the desired alignment relative to each other, bushing 58 may be moved longitudinally over the exterior of housing 52 until bushing 58 contacts shoulder 64. Retainer plate 96 is preferably formed from material which will be attracted to magnet 60. For some applications, retainer plate 96 may also contain one or more permanent magnets. The magnetic attraction between magnet 60 and retainer plate 96 determines the force applied to first substrate 20 and second substrate 22. As best shown in FIGS. 3 and 4, air gap 102 is preferably provided between retainer plate 96 and magnet 60. The dimensions of end plate 62, shoulder 64 and bushing 58 cooperate to define in part the size of air gap 102 and therefore the magnetic force applied to retainer plate 96. For some applications, the size of air gap 104 may be adjustable to allow varying the force applied to first substrate 20 and second substrate 22.

After first substrate 20 and second substrate 22 have been coupled with each other by magnet 60 and retainer plate 96 as shown in FIG. 3, the vacuum may be released from first passageway 46 and second passageway 98. Alignment assembly 90 and alignment head 92 may be moved away from the associated pedestal assembly 50. Carrier 56 may be used to transport the respective pedestal assembly 50 with substrates 20 and 22 coupled with each other to heater assembly 110. Depending upon the type of interconnections 24 and 26, heater assembly 110 may be cycled through the desired temperature profile to complete the bonding process between first substrate 20 and second substrate 22. If interconnections 24 and 26 are formed from indium based materials, heater assembly 110 will preferably have either a vacuum or inert environment.

For many applications, interconnections 24 and 26 may be formed from indium. For other applications, various epoxy compounds may be disposed between substrates 20 and 22 to aid the bonding process in addition to interconnections 24 and 26. The temperature profile provided within heater assembly 110 will be a function of the type of interconnection and desired bonding between substrates 20 and 22.

The use of carrier 56 and associated pin 66 facilitate establishing an automated process to move pedestal assembly 50 from its respective base unit 44 to heater assembly 110. Since retainer plates 96 and second substrate 22 may be released from their respective alignment head 92, alignment assembly 90 and each base unit 44 may be used to couple a large number of substrates 20 and 22 with each other while other pedestal assemblies 50 and their associated substrates 20 and 22 are undergoing temperature cycling in heater assembly 110. Various types of furnaces may be satisfactorily used as heater assembly 110.

The present invention is particularly beneficial for automating the process of bonding two or more substrates having flip-chip type interconnections. Bonding apparatus 40 may be used to couple a large number of first substrates 20 with their associated second substrate 22. The number of substrates coupled is dependent only upon the number of pedestal assemblies 50 and the cycling time required for mounting each substrate 20 on the respective pedestal assembly 50 and each substrate 22 on its respective alignment head 92. At the same time, a large number of pedestal assemblies 50 with their associated substrates 20 and 22 may be disposed within heater assembly 110 for cycling through the desired temperature profile. Thus, bonding apparatus 40 and heater assembly 110 may function independent from each other without hindering their respective operations.

The temperature cycling process used to form the desired bond between interconnections 24 and 26 may be conducted in various types of heaters. One of the advantages of pedestal assembly 50 is that temperature cycling may be performed as part of a continuous process as compared to other methods of bonding interconnections 24 and 26. Also, the magnetic forces between magnet 60 and retainer plate 96 will accommodate thermal expansion and contraction of substrates 20 and 22 and their respective interconnections 24 and 26 during the temperature cycling process. Magnet 60, retainer plate 96, and air gap 102 cooperate to maintain the preselected force on interconnections 24 and 26 during thermal cycling without adversely effecting the alignment of substrates 20 and 22 with respect to each other.

Prior fabrication processes often involved alignment, registration, and temperature cycling within the same piece of equipment to form the desired bonding between first substrate 20 and second substrate 22 in either a vacuum or inert environment. Using one piece of equipment frequently resulted in significant delay in completing the bonding process.

Various metals and metal alloys may be used to form interconnections 24 and 26. Examples include indium (In), indium/lead/indium (In/Pb/In), aluminum (Al), or aluminum/titanium-tungsten (Al/TiW). The present invention may also be used to bond interconnections formed from refractory metals such as titanium, tungsten, titanium-tungsten alloys, tantalum, molybdenum and alloys such as titanium silicon (Ti.sub.4 Si.sub.3) and titanium nitride (TIN). Interconnections 24 and 26 may also have various configurations such as mesas with bump bonding material, via interconnections, metal connectors, or metal strip conductors.

For one embodiment of the present invention, magnet 60 is preferably a permanent magnet disposed on the exterior of bushing 58 and retainer plate 96 is formed from material which will be attracted to magnet 60. Alignment head 92 will preferably be formed from nonmagnetic material to facilitate the release of retainer plate 96 and second substrate 22 after substrates 20 and 22 have been placed in the desired alignment with respect to each other. For some applications, retainer plate 96 may also include one or more permanent magnets which will be attracted to magnet 60. For other applications, magnet 60 may be placed on retainer plate 96 and bushing 58 formed from suitable material which will be attracted to magnet 60 on retainer plate 96.

For another embodiment of the present invention, magnet 60 may be an electromagnet which receives electricity from a remote source (not shown). By placing an electromagnet on bushing 58, the magnetic force applied to retainer plate 96 may be adjusted by modifying the electrical power supplied to the magnet 60. This embodiment of the present invention will allow varying the magnetic force applied to retainer plate 96 based upon the type of substrates and their connections which will be coupled using apparatus 40.

The present invention may be used with bump bonding, soldering, epoxy bonding or any other technique used to couple flip-chip type substrates with each other. The requirements for forming a bond in accordance with the present invention are positioning two or more substrates on pedestal assembly 50 and cycling pedestal assembly 50 with the attached substrates through the desired temperature profile. Bonding apparatus 40 may also be used with ultrasonic bonding techniques and cold welding or bonding procedures.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.