The present invention relates to the use of a laser to remove surface contamination and oxidation from a ball grid array substrate and to promote adhesion of material for molding operations and other operations. The laser etching can be configured to cover the entire substrate or focused on local areas of the substrate, such as laser etching being pinpointed to the epoxy molding compound/solder resist (EMC/SR) interfaces.
Semiconductor packages are generally fabricated by mounting and electrically connecting the semiconductor die (also known as “semiconductor device”) to a carrier substrate appropriate for the chip type and the subsequent use of the package. For example, ball-grid-array (BGA), chip-on-board (COB), board-on-chip (BOC), chip-scale or leads over chip (LOC) mounting arrangements may be made on printed circuit board strips, tape frames and other carrier substrates known in the art. After mounting the semiconductor die to the substrate, the hybrid combination of the components are electrically connected by wire bonding, conductive adhesives, solder reflow or other connections known in the art. The package is then encapsulated for protection from various atmospheric ailments. Often, the package becomes contaminated or oxidized due to atmospheric contaminants.
During the fabrication of the semiconductor package, a masking material (also known as resist) is used to enhance selectivity on both the semiconductor die and the circuits on the substrate. Resist plays a major role in the lithography process for fabrication of semiconductor devices in which the sizes, as well as the positions of the transistors, resistors and interconnects, are precisely determined on a wafer and fabricated. With the use of a patterned resist, selective etching and impurity doping can be performed. Thus, the resist is not part of the structure itself, but merely a masking material used for either the semiconductor die or the circuitry on the substrate to which the semiconductor die is attached. After the resist has been employed, a removal process is undertaken to remove the resist without damaging the fabricated semiconductor package.
One method of removing a resist layer consists of using reactive plasma etching. The plasma etching method suffers from drawbacks, such as incomplete removal of photoresist and resist popping. As a result, damages occur due to charges, currents, electric-field-induced UV radiation, contamination (such as alkali ions, heavy metals, and particulates), and elevated temperatures. Since plasma etching often leaves residues, a wet strip must follow to complete the stripping process. In many cases, to avoid alkali and heavy metals contamination, the plasma etching is stopped before the endpoint, and the wafer is transferred to a wet bath.
The wet bath also has drawbacks. Disadvantages associated with this method include solution concentrations that change with the number of wafers being stripped, thus affecting stripping quality and throughput; accumulation of contaminants in the baths, which drastically affects yield; and severely corrosive and toxic solutions that impose high handling and disposal costs and create serious safety considerations. Other problems are due to mass transport and surface tension associated with the solutions. For deep submicron technologies, the solutions cannot circulate and tend to accumulate within the patterned structure. This situation is intolerable, as it contaminates the wafer with foreign materials that can lead to drastic yield losses. All of these problems become even more critical for larger wafers. Also, such contaminants are present on the substrates used to mount the semiconductor die for a packaged assembly from the formation of the circuitry thereon using similar type processes.
Lasers may also be used in the manufacture of semiconductor die and substrates to remove resist. Currently, lasers are used in the applications of microelectronic fabrication, such as substrates and resistors. Lasers are widely used for trimming both thick and thin film resistors, for scribing wafers, for hole drilling in substrates, for welding of hermetically sealed packages and for stripping insulation from wires. The marking of silicon wafers with identification numbers has also become well established. In all these applications, lasers have become established production tools, replacing earlier technology for many applications.
A variety of different types of lasers is used in electronic fabrication. The use of the CO2 and the infrared Nd:YAG lasers in electronic processing applications is well established; these lasers have been used for many years for applications such as trimming and drilling. Green and ultraviolet lasers may be focused to a smaller spot than the infrared devices and they may be chosen when a small focal diameter is desired. The use of ultraviolet lasers is relatively new, especially the excimer and frequency-tripled and -quadrupled ND:YAG lasers. These lasers have become more mature and reliable and they now present viable options for electronic processing. They offer the attractive feature of very high absorption in many materials of interest. Lasers have reached production status for a variety of applications in the electronics industry. One of the most significant is the trimming of resistors. This can significantly increase the yield in the processing of resistive elements.
There are numerous teachings relating to removing a resist layer from the surface of a substrate. For example, U.S. Pat. No. 4,789,427 to Fujimura et al., provides a method for removing a resist on a semiconductor device, including the steps of: removing the resist on a layer formed on a semiconductor substrate having a functional region, in a direction of the thickness thereof by a predetermined thickness by applying plasma processing; and removing the remaining resist by applying a chemical process.
In U.S. Pat. No. 5,200,031 to Latchford et al., disclose a process for removing photoresist remaining after a metal etch, which also removes or inactivates a sufficient amount of any remaining chlorine-containing residues, including sidewall residues remaining from the metal etch step, to inhibit corrosion of the remaining metal or metals. The process includes a reducing step using NH3 associated with a plasma followed by a subsequent stripping step using either O2, or a combination of O2 and NH3 gases, and associated with a plasma.
More recent patents have begun to use lasers to remove marks from the substrate. U.S. Pat. No. 5,597,590 to Tanimoto et al. discloses a process in which a substrate such as a wafer is fixed upon a turntable, and then the alignment mark portions are removed with a sensitizing light beam that is projected to the thin film layer. Tanimoto et al. disclose that rotating the substrate has the advantage of causing the flying splinters of the thin film to fly off to the outer side radially due to the centrifugal force and making it difficult to cause the splinters to remain on the substrate surface. It is to be noted that in order to locally remove the resist layer, a photo etching method requiring no post developing operation may be used so that a high-energy ultraviolet light beam, such as an excimer laser, is projected onto the resist layer to break the molecular bond of the resist.
In U.S. Pat. No. 5,686,211 to Motegi et al., a method for removing a thin film layer covering the surface of a substrate, such as a semiconductor wafer, is disclosed. Specifically, Motegi et al. disclose a method wherein a beam of energy, such as an excimer laser, is used to remove the resist material from the alignment marks.
Also, in U.S. Pat. No. 6,009,888 to Ye et al., a wafer is immersed in a liquid bath comprising peroxydisulfate, hydrochloric acid and water and then irradiating the photoresist pattern and polymer layer with a UV laser.
After resist is removed, it is well-known in the art that a critical step in the semiconductor device fabrication process is the encapsulation of semiconductor dice and their interconnections. The encapsulation or “sealing” of a semiconductor die and its wire bond interconnections within a “package” of plastic or other moldable material serves to protect their materials and components from physical and environmental stresses, such as dust, heat, moisture, static electricity, and mechanical shocks.
In a typical encapsulation process for surface-mounted semiconductor dice, a conductive substrate strip, with mounted and wire bonded semiconductor dice placed along the length of the strip, is placed in the lower mold plate of a “split cavity” mold comprising an upper and lower member. The upper and lower members of the mold are frequently referred to a “platens” or “halves.” With the upper mold platen raised, the conductive substrate strip is positioned on the lower mold platen such that the component portions to be encapsulated are in registration with multiple mold cavities formed in the lower mold platen. The mold is closed when the upper platen is lowered onto the lower platen. When the mold is closed, a peripheral portion of the conductive substrate strip is typically compressed between the upper and lower platens to seal the mold cavities in order to prevent leakage of liquified plastic molding compound. The force required to compress the platens together is generally of the order of tons, even for molding machines having only a few mold cavities.
Accordingly, what is needed in the art is a method of cleaning interfaces using a laser. Furthermore, a method of removing a resist layer wherein the substrate can be encapsulated immediately thereafter to prevent contamination or future oxidation is needed.
The present invention envisions a resist removal method comprising a substrate having a surface wherein resist is formed on at least a portion of the surface and a laser is provided to remove the resist from the substrate. The present invention also encompasses a method of fabricating a semiconductor device comprising a substrate having a surface wherein resist is formed on at least a portion of the surface, laser etching the surface of the substrate and encapsulating the substrate in a mold. The present invention also pertains to the cleaning of contaminants on a substrate. Additionally, the present invention teaches a method of enhancing the adhesion of a compound to the substrate surface by roughening the surface of the substrate.
The present invention embodies the use of a laser to remove surface contamination and oxidation from the solder resist layer of a semiconductor system. The laser etching can be performed either alone or as an addition to an automolding system.
Illustrated in drawing
Illustrated in drawing
An Nd:YAG laser may be used in the process of the present invention. However, a CO2 or excimer laser may also be used. Nd:YAG lasers are available in output from a few milliwatts to as high as a kilowatt in power. An advantage of Nd:YAG laser processing is its shorter wavelength; consequently, because of the dependency of the material's emissivity on the wavelength, energy is absorbed by the material more readily than with the CO2 laser, and a lower energy can be used for welding, allowing greater control of the heat input. The wavelength of the Nd:YAG laser can range from 250 nm to 1200 nm. The output of an Nd:YAG laser is most often of 1064 nm wavelength. The active medium is an Nd:YAG laser rod. It is optically pumped by a continuous-pumping lamp and is placed between two external mirrors that form the optical cavity for the laser beam.
The optical cavity of the Nd:YAG laser usually consists of two mirrors mounted separately from the laser rod. Several cavity configurations may be used, but all employ at least one spherical mirror. Both long radius and long radius hemispherical cavities are commonly employed. In some systems, shaping of the beam within the cavity is desirable, and two mirrors with different radii of curvature are used. The HR mirror has a reflectivity of about 99.9% and the output coupler transmission varies from less than one percent on small lasers to about eight percent on larger ones. The optical cavities of Nd:YAG lasers are often equipped with an adjustable or interchangeable aperture for selection of multimode or TEM00 mode operation.
A most critical subsystem of the laser is the cooling system. Without adequate cooling, the laser seals, pumping cavity, lamps, and the rod itself would be quickly destroyed by overheating. Lasing in Nd:YAG is most efficient when the temperature is lowest. Thus, cooling systems are designed to produce the lowest practical system operating temperature.
Another one of the embodiments of the present invention is illustrated in drawing
Excimer lasers are said to be capable of “laser cold cutting.” Normally when CO2 and Nd:YAG lasers are used for material removing, the energy is transformed from optical energy to thermal energy, the material is heated to melt or vaporize, then material changes from solid state to liquid or gaseous state. Excimer lasers can remove material through direct solid-vapor ablation. The incident photon energy is high enough to break the chemical bonds of the target material directly, the material is dissociated into its chemical components, and no liquid phase transition occurs in this process. This chemical dissociation process has greater minimized heat effects, compared with the physical phase change process.
For example, vision systems, such as PRS, can be used to examine structural defects such as broken leads, dendrite growth, solder resist irregularities, oxide contamination, corrosion, etc. In this step, the vision system will typically compare pictures of lead frame fingers, bond pads, and other features on and around the individual semiconductor die sites to a predetermined known good template. Electrical testing can also be accomplished, for example, by using various automated or other test equipment, including curve tracer testing, test probes, RF testing, and the like. Tests screening for intermittent failures, such as high temperature reverse-bias (HTRB) tests, vibration testing, temperature cycling, and mechanical shock testing, etc. are also contemplated by the present invention, as well as tests for solderability, microcorrosion, noise characterization, electromigration stress, electrostatic discharge, plating defects, etc. The results of the different tests are fed into a computer, compiled, and correlated with individual semiconductor die sites on a particular mounting substrate array.
To prevent contamination from particles typically found in the smoke resulting from a conventional laser ablative process, filtered air may be forced over the substrate.
The beam of light may be scanned over the surface of the bare semiconductor die or a partially packaged semiconductor die attached to a substrate in the requisite pattern, or can be directed through a mask, which projects the desired inscriptions onto the desired surface of the bare semiconductor die or partially packaged semiconductor die attached to a substrate. The surface or coating of the bare or packaged semiconductor die thus modified, the laser marking creates a reflectivity difference from the rest of the surface of the bare or packaged semiconductor die.
Preferably, a laser is used to remove contaminants and/or resist from a BGA substrate. Illustrated in drawing
Illustrated in drawing
As can be seen in drawing
Described in drawing
The interposer 30 includes a substantially planar substrate 31 that may be formed from any suitable material, such as resin (e.g., FR-4 resin), plastic, insulator-coated semiconductor material (e.g., silicon oxide-coated silicon), glass, ceramic, or any other suitable, electrically insulative or dielectric-coated material, which may be positioned over the active surface 22 of the semiconductor die 20.
As shown, the interposer 30 includes an aperture or slot 14 formed therethrough for exposing the bond pads 12 of a semiconductor die 20 over which the interposer 30 is to be positioned. Contact areas 15 are carried upon a top side 32 of the interposer 30. Preferably, the contact areas 15 are located proximate to the slot 14 so as to facilitate the positioning of relatively short intermediate conductive elements through the slot 14, between the bond pads 12 of a semiconductor die 20 and the contact areas 15. As illustrated in drawing
While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the invention as disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.
This application is a divisional of application Ser. No. 11/327,817, filed Jan. 6, 2006, pending, which is a divisional of application Ser. No. 10/791,190 filed Mar. 2, 2004, pending, which is a divisional of application Ser. No. 09/863,676, filed May 21, 2001, pending.
Number | Date | Country | |
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Parent | 11327817 | Jan 2006 | US |
Child | 11486288 | Jul 2006 | US |
Parent | 10791190 | Mar 2004 | US |
Child | 11327817 | Jan 2006 | US |
Parent | 09863676 | May 2001 | US |
Child | 10791190 | Mar 2004 | US |