Patent Application
20070235744
This invention relates to semiconductors and semiconductor processing. More particularly, this invention relates to a method of bonding a carrier to the backside of ultra-thin semiconductor wafers and devices.
Semiconductor devices made from gallium arsenide (GaAs), indium phosphide (InP), and gallium nitride (GaN) are considered to be high speed or ultra high-speed devices. The substrates for these devices need to be kept thin and as a result, they are fragile and easily cracked. Notwithstanding their fragility, they still need a carrier by which they can be mounted to a circuit board or other planar surface through which power and information-bearing signals can be coupled into and out of them.
A recent development in semiconductor processing of high and ultra-high speed devices has been to attach a semiconductor substrate directly to a circuit board using conductive epoxy or solder. A problem with such prior art attachment methods includes defects in the connection adhesive, such as voids, oxidation or sloping. A method and a carrier structure by which a delicate, ultra-thin high-speed semiconductor devices can be attached to a substrate would be an improvement over the prior art.
Ultra-high speed semiconductors can be directly applied to a circuit board or other substrate using a backside metallization layer or carrier that is grown or deposited onto the backside of an ultra-high speed electronic device. The backside metallization applied to the wafer allows individual chips or cut from the wafer to be attached to a circuit or package housing by hot-gas injection, hot plate or solder reflow, without using prior art conductive epoxy or solder perform and without sustaining physical damage.
The method of applying backside metallization is to apply an infrared alignment photolithography followed in sequence by a titanium adhesive layer. Platinum and gold protective layers follow the titanium and lift off the photoresist. An interconducting layer of aluminum is deposited over entire back side wafer surface follows photoresist lift-off process.
A conventional photolithography is applied over the aluminum layer and the aluminum layer is etched away so as to remove aluminum from beneath where devices (electronic circuit or die) will be located on the wafer and to leave only the aluminum between the devices to act as interconducting path for next processing step. A relatively thick copper carrier layer is applied over the etched aluminum layer area (on backside devices) followed by a layer of solder SnAg, or SnAu or SnAuCu. After the copper and, solder layers are applied, the wafer can be cut into individual electronic devices, circuits or die.
As used herein, the term “wafer” means a relatively large disk or plate in which several electronic circuits, components or photonics devices are formed at the same time during the various steps of forming a semiconductor device. Inasmuch as a wafer is made of semiconductor material, a wafer is also considered herein to be a “substrate” in which one or more electronic circuits, components or photonic devices are formed using semiconductor device fabrication techniques.
Various processes for forming semiconductors and semiconductor wafers are well known in the art but such processes are not germane to the invention described and claimed herein other than to note that back-side metallization and the process for applying backside metallization described herein is particularly applicable to thin semiconductor wafers. The method and apparatus disclosed and claimed herein actually finds application with virtually all semiconductors, including those formed from germanium (Ge), silicon (Si), cadmium telluride (CdTe), gallium arsenide (GaAs), indium phosphide (InP) and gallium arsenide (GaN).
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Copper is a good electrical conductor. It is also a good heat conductor. By making the carrier layer (7) thick and making it of pure copper, it provides a layer that will support the semiconductor and resist cracking and other damage that can be happen during handling.
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As stated above, the substrate/wafer thickness can be less than or equal to 10 mils.
In the method described above, the layers deposited on the back side of the wafer substrate as depicted in
In the embodiment described above, the titanium adhesive layer has a thickness that is typically less than or equal to 500 Angstroms. Although thicker adhesive layers are certainly possible, the function of the adhesive layer is to provide a layer that itself adheres to semiconductor but which will allow other materials that do not adhere well to a semiconductor to be applied to it.
The intermetal layer combined of titanium, platinum and gold has a thickness that can vary from less than or equal to 500 Angstroms, to less than or equal to 1000 Angstroms. The interconducting aluminum layer is typically less than or equal to one thousand angstroms.
The nickel layer, which is considered herein to be a binding layer, has a thickness that is less than or equal to 10,000 Angstroms. As stated above, the copper carrier metal layer is relatively thick. It's thickness can range up to 20 mils.
Those of ordinary skill in the art will appreciate that the foregoing method and structure for eutectic bonding of a carrier to the backside of an ultra-high speed semiconductor wafer or substrate will enable such an integrated circuit device to be attached to a circuit board using known methods that include hot-gas injection, a hot plate or a solder reflow process. By applying the relatively thick copper carrier as described above, ultra-high speed integrated circuits that must be kept as thin as possible are less susceptible to cracking and other physical damage.
