Wafer backside removal to complete through-holes and provide wafer singulation during the formation of a semiconductor device

[0001] This invention relates to the field of semiconductor manufacture and, more particularly, to a method which provides one or more vias through a silicon substrate, and also provides dicing of a wafer simultaneously with via formation.

[0002] To form individual semiconductor devices from a semiconductor wafer, various features such as transistors, storage capacitors, and other electronic circuitry are formed on the front of the semiconductor wafer, then the wafer is diced or singulated (singularized) to form a plurality of individual semiconductor dice.

[0003] One method to dice a semiconductor wafer is to attach a plasticized adhesive tape or film to a frame, adhere the semiconductor wafer to the adhesive film, and then cut through the wafer from the circuit (front) side with a wafer or dicing saw at a wafer scribe (kerf or “street”) area and partially into the adhesive film to ensure complete singulation of each semiconductor die. Each die which has been determined to be functional from a prior probe test is then removed from the adhesive film for packaging.

[0004] Semiconductor devices having small outlines and thin profiles are desirable to minimize the size of electronic devices such as notebook computers, cell phones, and generally any device into which they are installed. However, processing semiconductor devices on a very thin semiconductor wafer is difficult because an already fragile semiconductor wafer becomes even more susceptible to breakage as it becomes thinner.

[0005] Methods of wafer singulation which aid in the formation of very thin semiconductor devices have been proposed. In one method, electronic circuitry is formed on a relatively thick semiconductor wafer, then grooves are only partially cut into the front of the semiconductor wafer at the scribe areas. Subsequently, the wafer is etched or ground from the back of the wafer to the level of the cuts. Back grinding the wafer first thins it to a desired profile, then dices the wafer once the level of the cuts is reached.

[0006] To minimize the outline of a semiconductor device, attempts have been made to stack a plurality of semiconductor dice. While this increases the thickness of a completed device, it is sometimes preferable to have a thicker package than to increase the overall outline or “footprint” of the device. Stacking semiconductor device requires electrical interconnections from one device to another, which is often performed through an interconnection running along the outside of the dice, or through vias which have been etched from the front through the dice prior to their singulation from the wafer. Vias through the wafer are formed by mechanically or chemically etching completely through the wafer from the front. A conductor, typically metal, is then formed within the opening through the wafer usually by chemical vapor deposition (CVD) or by reflow of a metal.

[0007] A method which allows for improved processing of a semiconductor device would be desirable.

[0008] The present invention provides a new method which, among other advantages, decreases the number of processing steps required to form a via and to singularize a semiconductor wafer. In accordance with one embodiment of the invention a semiconductor wafer is processed to include a plurality of vias or through-holes partially etched into the wafer, as well as grooves partially etched into the wafer. The grooves are formed in the scribe area of the wafer to define a plurality of individual dice but to leave the wafer intact. The partially etched vias and grooves can be formed into the wafer at the same depth, preferably using a single mask. Finally, the wafer is etched from the back or back ground to the level of the vias and scribes. This results in the simultaneous completion of the vias through the wafer and the singulation of the wafer into a plurality of individual semiconductor dice.

[0009] Additional advantages will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawings attached hereto.

[0019] It should be emphasized that the drawings herein may not be to exact scale unless specified as such and are schematic representations. Except where specified, the drawings are not intended to portray the specific parameters, materials, particular uses, or the structural details of the invention, which can be determined by one of skill in the art by examination of the information herein.

[0020] The term “wafer” is to be understood as a semiconductor-based material including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” in the following description, previous process steps may have been used to form regions or junctions in or over the base semiconductor structure or foundation. Additionally, when reference is made to a “substrate assembly” in the following description, the substrate assembly may include a wafer with layers including dielectrics and conductors, and features such as transistors, formed thereover, depending on the particular stage of processing. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, silicon-on-insulator, silicon-on-sapphire, germanium, or gallium arsenide, among others. Further, in the discussion and claims herein, the term “on” used with respect to two layers, one “on” the other, means at least some contact between the layers, while “over” means the layers are in close proximity, but possibly with one or more additional intervening layers such that contact is possible but not required. Neither “on” nor “over” implies any directionality as used herein.

[0021] A first embodiment of an inventive process to form a semiconductor device is depicted in FIGS. 1–3. FIG. 1 depicts an in-process semiconductor wafer substrate assembly 10 comprising circuitry formed thereon (not depicted) using means known in the art. FIG. 1 further depicts a plurality of grooves 12 which define, but do not singularize, a plurality of individual semiconductor dice 14. In this embodiment, the wafer is between about 500 micrometers (μm) and about 750 μm thick, typically about 725 μm thick, and the grooves are formed to be between about 125 μm and about 175 μm deep (preferably about 150 μm deep) and between about 10 μm and about 100 μm wide (preferably about 50 μm wide). The grooves may be formed using a wafer saw, or may be more preferably formed using an etch which simultaneously defines a plurality of vias or through-holes on the wafer. FIG. 2, which depicts a magnified view of a small portion of the FIG. 1 structure, depicts a portion of the grooves 12 and dice 14 of FIG. 1, and further depicts a plurality of partially etched vias or through-holes 20. The vias are depicted out of scale with the grooves for illustration purposes and will typically be between about 25 μm and about 75 μm in diameter, typically about 50 μm. The location of the through-holes on each die, as well as the number of vias on each die, will depend on their eventual use. In this embodiment the through-holes, when filled with metal or other conductor, will provide an interconnection between the circuit side and the noncircuit side of the semiconductor die. An exemplary use for this includes a means for grounding a feature on the circuit side of the die to a lead frame paddle through a conductor in the via, or for stacking of interconnected die in high-density applications.

[0022] Subsequent to forming the vias and grooves as depicted in FIG. 2, the back (noncircuit) side of the wafer is etched or mechanically ground to thin the wafer to the level of the bottom of the vias and grooves. This singularizes the wafer into individual separate dice and completes the through-hole in each die. In this embodiment, between about 500 μm and about 650 μm of material is removed, depending on the original thickness of the wafer and the depth of the vias and grooves.

[0023] The wafer can be mechanically ground using a Strausbaugh polisher or chemically etched using a plasma, for example one comprising CF₄. A preferred chemical etch comprises the use of SpinEtch® BT from General Chemical Co., which is a mixture of sulfuric acid (H₂SO₄), nitric acid (HNO₃), hydrofluoric acid (HF), and phosphoric acid (H₃PO₄). SpinEtch® BT will remove about 0.5 μm of silicon each second. During removal of material from the back of the wafer, it can be held in place using a Bernoulli chuck on an SEZ SpinEtch® tool. Regardless of the method of thinning the wafer from the back, the wafer is singularized into individual dice while the vias or through-holes are completed to result in the individual semiconductor dice of FIG. 3.

[0024] After dicing the wafer to result in the plurality of individual dice of FIG. 3 a metal layer is formed within the vias. This can be accomplished by various means. For example, the via may be filled by placing a solid metal ball over each via, then reflowing the metal. The metal is drawn into the via by capillarity. Further, chemical vapor deposition (CVD) may be used to fill the vias, or a damascene process may be used. Regardless of the method of via filling, any required wafer processing necessary then continues along with subsequent packaging of the die.

[0025] Another embodiment of the invention is depicted by FIGS. 4–7. In this embodiment, vias 20 are partially etched into the wafer then filled with a conductor 40 such as metal prior to forming the grooves in the wafer. The type of conductor depends on the use of the conductor-filled vias, but may comprise tungsten, copper, silver-filled epoxy, etc. The conductive layer 40 may be formed using chemical vapor deposition (CVD), sputtering, reflow, or other workable processes.

[0026] After forming conductive layer 40 it is removed from over the wafer except from within the vias, for example using chemical mechanical planarization (CMP) or an etch to result in the structure of FIG. 5 having conductor within the vias. Subsequently, grooves 12 which define a plurality of individual dice 14, but do not singularize the wafer, are formed in the wafer as depicted in FIG. 6. As with the first embodiment, the grooves can be formed by etching or using a dicing saw. Subsequently, material is removed from the back of the wafer in a manner similar to the first embodiment to result in the singularized dice of FIG. 7. The removal of material from the back of the wafer exposes the conductor 40 on the back side of the wafer and completes grooves 12 to singularize the wafer into individual semiconductor dice 14. Wafer processing then continues, for example attachment to a lead frame and encapsulation, to further testing for use as a known good die, etc.

[0027] In another embodiment, the vias and grooves can be formed partially into the wafer as depicted in FIG. 2, then the partially formed grooves and vias can be filled with a blanket conductor using a method described for one of the previous embodiments. The metal is removed from the planar surface of the circuit side of the wafer, for example by etching, grinding, or using mechanical polishing or chemical mechanical polishing (CMP). Subsequent to removing the metal from the planar circuit side, the vias are masked off and the metal is etched from the partially formed grooves. After etching the metal from the grooves, the mask is removed and the wafer is back ground according to the previous embodiments to expose the vias from the back of the wafer and to singularize the plurality of dice.

[0028] The etchant used to etch the metal in the previous embodiment (or other embodiments described herein) depends on the metal. For example, to etch copper, ferric chloride (FeCl₃) available from Transcene® Co. of Danvers, Mass. under material no. CE-100 will etch copper at a rate of about 25 μm per minute. For nickel, Type 1 nickel etchant, also available from Transcene® Co., will etch nickel at a rate of about 75 μm per hour.

[0029] In another embodiment, the vias and grooves can be formed partially into the wafer as depicted in FIG. 2, then the partially formed grooves and vias can be filled with a blanket conductor using a method described for one of the previous embodiments. The metal is removed from the planar surface of the circuit side of the wafer, for example by etching, grinding, or using mechanical polishing or chemical mechanical polishing (CMP). Subsequent to removing the metal from the planar circuit side, the wafer is back ground to expose the metal in the vias and in the grooves from the back side of the wafer. At this point, the dice of the wafer are held together by the metal formed within the grooves. The vias are masked off from either the front or back side of the wafer, then the metal is etched from the partially formed grooves from whichever side is masked off to result in singularized semiconductor dice. Wafer processing then continues.

[0030] As depicted in FIG. 8, a semiconductor device 80 formed in accordance with the invention may be attached along with other devices such as a microprocessor 82 to a printed circuit board 84, for example to a computer motherboard or as a part of a memory module used in a personal computer, a minicomputer, or a mainframe 86. FIG. 8 may also represent use of device 80 in other electronic devices comprising a housing 86, for example devices comprising a microprocessor 82, related to telecommunications, the automobile industry, semiconductor test and manufacturing equipment, consumer electronics, or virtually any piece of consumer or industrial electronic equipment.

[0031] The process and structure described herein can be used to manufacture a number of different semiconductor devices which comprise a feature formed using a photolithographic process. FIG. 9, for example, is a simplified block diagram of a memory device such as a dynamic random access memory having digit lines and other features which may be formed on a device which comprises the invention. The general operation of such a device is known to one skilled in the art. FIG. 9 depicts a processor 82 coupled to a memory device 80, and further depicts the following basic sections of a memory integrated circuit: control circuitry 90; row 92 and column 94 address buffers; row 96 and column 98 decoders; sense amplifiers 100; memory array 102; and data input/output 104.

[0032] While this invention has been described with reference to illustrative embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as additional embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.