Apparatus for removing contaminants on electronic devices

The invention provides a unique method and apparatus for removing flash or other contaminants from an electronic package such as encapsulated semiconductor device by exposing the device to plasma gas. In a preferred embodiment, a plasma gas cleaner is provided with a reaction chamber used to house the encapsulated device during a deflashing procedure. Plasma gas is supplied to the reaction chamber for reaction on the surfaces of the device. The reaction of the plasma on these surfaces successfully removes excess encasing material and other contaminants. The plasma gas cleaner may be a plasma gas device used for other process steps (e.g., plasma etching) employed during the fabrication and manufacture of the semi conductor device.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention is directed to the field of manufacturing electronic devices.
 More specifically, the invention is directed to cleaning systems and
 methods used to remove foreign material such as flash and other
 contaminants from external leads of the electronic devices.
 2. Description of Related Art
 Intricate electronic devices such as semiconductor integrated circuits
 (ICs) (or "chips") are typically housed in an encasing referred to as a
 "package." The package typically includes a "lead frame" that is
 electrically connected to the IC within the package, and extends outward
 to allow electrical connection of the IC to a circuit board or other
 product. One of the most popular package types used in the art is known as
 the "epoxy molding" or "plastic" package. With this type of package, the
 IC and lead frame are enclosed or encapsulated by a plastic or resin
 material that serves to protect the chip from moisture, contamination, and
 other physical or environmental conditions.
 The basic process flow for the manufacture of a plastic package of a
 semiconductor device starts with the attachment and bonding of the IC die
 to a lead frame containing a number of leads. A preseal inspection often
 is performed to ensure that the die is attached correctly. A plasma
 cleaning step may be included prior to wire bonding to remove any residual
 photoresist, or other organic contaminants on the bonding surfaces of the
 die. The bonding surfaces of the IC die are then respectively connected to
 individual leads on the lead frame with very thin wires during the wire
 bonding step. The lead frames and attached dies are then transferred to a
 molding area.
 In the molding area, the frames are placed on a mold mounted in a transfer
 molding machine. The molding machine in turn injects epoxy or other
 plastic encasing material into the mold around the die on the lead frames,
 thereby forming an individual package around each lead frame leaving only
 external ("outer") leads exposed to the environment. A plating step is
 often used to coat the external leads of the package with a metal finish
 so as to improve the lead solderability, resulting in a more reliable
 electrical connection of the package and the printed circuit. After the
 epoxy sets in the mold, the frames are removed and placed in an oven for
 final curing.
 Often, as a result of the molding step, excess plastic, resin, wax or other
 organic residue material, such as trace oxides or contaminants, can be
 found around the casing of the encapsulated chip, as well as on and
 between the external leads of the chip. As shown in accompanying FIG. 1, a
 typical lead frame 10 is used to provide external electrical connections
 to IC die 20. Once the die 20 is mounted on the lead frame 10 and the
 appropriate wire bonds are made to inner leads 24, 28, the lead frame 10
 is exposed to an encapsulation process step. In this process step, the die
 20 and inner leads 24 (around the boundary indicated by the dashed line
 26) will be encapsulated by a molded plastic casing 30 (FIG. 2).
 During this encapsulation process, the lead frame 10 is inserted into a
 mold cavity while the leads 16 extend outside of the cavity. The mold is
 heated and the plastic is injected into the mold in liquid or semi-liquid
 form under very high pressure. Due to its fluidity, the plastic material
 leaks out of the mold through any crevices where the sealing is imperfect.
 As a result, excess encasing material 36 (FIGS. 2 and 3) "bleeds" out of
 the encapsulated chip package 30 onto and between leads 16. This excess
 encasing material 36 is referred to in the art as "flash." Flash is
 detrimental to the fabrication process in that its presence adversely
 affects the subsequent soldering, trimming and forming operations, in
 addition to the overall electrical characteristics of the device.
 To avoid the problems caused by flash, another process step often referred
 to as "deflash" or "flash removal" is commonly added to the basic process
 flow. Most of the known methods employed to perform this deflash step
 involve exposing the device to chemical solvents or abrasive blasting. The
 flash removal system shown in U.S. Pat. No. 5,318,677, for example,
 performs the deflashing step by dipping the components in a bath of
 glycerol and phosphate. In another example, the cryogenic deflashing
 system of U.S. Pat. No. 5,676,588, attempts to remove flash by exposing
 devices to cryogenic material such as liquid nitrogen (at a temperature of
 about -60.degree. F. or below) and blasting the devices with particulate
 media. Many other variations of these two types of deflashing procedures
 are known in the art.
 The chemical solvent-based deflashing procedures are problematic because of
 the liquid waste that is produced leading to environmental concerns
 regarding the handling and disposing of the used solvents.
 The essential disadvantage of the abrasive-type of flash removal systems is
 that minute quantities of the blasting abrasive become embedded in the
 surface of the electronic part (e.g., lead). These embedded particles must
 be carefully removed before proceeding with other process steps such as
 plating the surface with a metallic (solderable) coating. The abrasive
 deflashing procedure is also often incomplete in regions leaving very thin
 layers of residue that are very difficult to detect upon inspection with
 the naked eye.
 SUMMARY OF THE INVENTION
 The invention provides a unique apparatus for and method of removing
 flash-or other contaminants from electronic packages such as encapsulated
 semiconductor devices by exposing the devices to plasma gas. In a
 preferred embodiment, a plasma cleaner is provided with a reaction chamber
 used to house the devices during a deflashing procedure. Plasma gas is
 supplied to the reaction chamber for reaction on the surfaces of the
 devices. The reaction of the plasma on these surfaces operates to
 successfully remove the excess encasing material and other contaminants
 often found on the devices (particularly on their leads) that may
 interfere with the proper manufacture or operation of the device.
 In another preferred embodiment, the invention makes use in the deflashing
 procedure of the same (or part of the same) plasma gas cleaner used for
 other process steps (e.g., plasma etching) during the fabrication and
 manufacture of the electronic device.
 Among the many advantages derived from the invention include the removal of
 flash without degrading the surface of the leads, without leaving any
 organic residue or other film, and without producing any liquid waste.
 Also, the gaseous waste does not cause environmental concerns (ie, the
 gases released are non-toxic like H.sub.2 O, CO, CO.sub.2, etc). In
 addition, exposing the encasing material (bulk, not the flash) to the
 plasma field could result in chemical changes on the surface producing a
 stronger or tougher package. The plasma could also induce some curing to
 occur on the surface of the bulk of the package.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The present invention will be described in detail as set forth in the
 preferred embodiments illustrated in FIGS. 4 through 7. Although these
 embodiments depict the invention in its preferred application to a
 semiconductor memory device, it should be readily apparent that the
 invention has equal application to any type or configuration of
 semiconductor device (e.g., microprocessor, microcomputer, digital signal
 processor (DSP), programmable logic array (PLA), etc.) in any type of
 molded packaging (e.g., dual in-line (DIP), flatpack (FP), leadless chip
 carrier (LCC), pin-grid array (PGA), etc.), as well as any other
 electronic device that encounters the same or similar problems.
 The invention operates to remove troublesome flash or resin-bleed, shown in
 FIGS. 1-3, by exposing the encapsulated package 30 and its external leads
 16 to a novel deflashing apparatus and method that utilizes plasma gas to
 remove the flash 32, 34, 36 without degrading the surface of the leads,
 without leaving any organic residue or other film, and without producing
 any liquid waste.
 In accordance with a preferred embodiment of the invention, a plasma
 cleaner for performing the flash removal is depicted in block form in FIG.
 4. The plasma cleaner includes a process chamber 44 (also referred to as a
 "reaction chamber") used to temporarily house one or more semiconductor
 packages, represented in FIG. 4 as lead frame 10 and casing 30, during the
 deflashing procedure. A supply of gas 41 is included to provide a source
 of gas to the process chamber 44 from which a reactive plasma can be
 generated, as will be discussed below.
 Preferably, the gas supplied in source 41 is Argon (or, alternatively,
 Oxygen) or Argon combined with any number of plasma forming gases (e.g.,
 O.sub.2, H.sub.2, CF.sub.4, He, air, etc.) known to be used by those of
 ordinary skill in the art. In another preferred embodiment, the plasma
 cleaner used for deflashing in accordance with the invention may be the
 same, or part of the same, plasma cleaner used for other plasma-based
 process steps, e.g., dry etching, etc., used in the fabrication or
 manufacture of the electronic package. The gas from source 41 is
 introduced into the process chamber 44 through gas inlet 40, which may be
 formed integrally with the process chamber 44 itself, or as a separate
 structure, e.g., external tubing, or by any known construct that permits
 controlled communication of gas from source 41 to process chamber 44.
 A power supply 46 is coupled to process chamber 44 to provide a source of
 direct current (DC) voltage or radio frequency (RF) energy to the gas in
 the process chamber 44.
 When supplying RF energy, preferably the power supply 46 is operating in
 the range of 1 KHz to 100 GHz with almost any gas. Although not shown, the
 power supply 46 may apply its output energy through the use of electrodes
 (either external or internal) to the chamber. Alternative mechanisms known
 in the art to generate plasma could also be utilized such as DC abnormal
 glow discharges, parallel plate RF capacitive reactors, flat coil
 inductively coupled RF reactors, electron cyclotron resonance (ECR)
 microwave reactors, etc. The electric field derived from the energy output
 by power supply 46 as applied to the gas in the process chamber 44 is
 effective to convert the gas into a reactive plasma.
 The plasma reacts at the surface of the semiconductor device placed in the
 process chamber 44. The plasma also reacts on the metallic surface of the
 external leads of lead frame 10 to break organic bonds between flash such
 as plastic, resin, wax flash or other contaminants, and the surface of the
 leads. In particular, the energy in the electric field in the chamber 44
 is sufficient to dissociate the reactive gas. In this dissociation
 process, reactive gas molecules are broken into species including free
 radicals, i.e., neutral atoms or collections of atoms with incomplete
 bonding. These free radicals diffuse to the surfaces of the exposed lead
 frame 10 in random directions. Radicals are highly reactive chemically and
 are chiefly responsible for the removal of the organic material on the
 leads.
 A vacuum pump 42, which maintains the pressure inside the process chamber
 44 (usually run at pressures of 150 millitorr to 1500 millitorr), may be
 included in the plasma cleaner to remove the contaminant by-products
 resulting from the plasma reaction at the surface of the leads. In
 accordance with another preferred embodiment of the invention, the plasma
 cleaner may simultaneously accommodate a plurality of packages to allow
 for use in high throughput processes. As shown in FIG. 5a, a plurality of
 encapsulated packages 52 may be arrayed into a plurality of storage units
 54 (e.g., shelves), all housed in a magazine 50. The magazine has vent
 holes 56 that may take the form of any of a variety of shapes, e.g.,
 circles, squares, rectangles, etc. These vent holes 56 permit the plasma
 gas to permeate through the magazine 50 and react with the surfaces of the
 individual packages 52. As shown in FIG. 5b, the reaction chamber 58 may
 house a plurality of magazines 50 for simultaneously performing a plasma
 deflashing process on a host of individual packages 52.
 As a further illustration of the invention, the following example has been
 provided:
 EXAMPLE
 A plurality of semiconductor memories are housed in a magazine and placed
 in a plasma oven for 4 hours. The plasma oven has an internal temperature
 of 175 .degree. C. During this 4 hour period, plasma gas having 90% Argon
 is introduced into the plasma oven. A vacuum is applied to maintain the
 pressure between 300 mTorr-1 Torr. The plasma gas and vacuum are applied
 only between 5 and 20 minutes of the 4 hour period. An RF power supply
 (rated between 250-600 watts) generates an electric field that is also
 applied during this 5-20 minute period to the plasma oven. The purpose of
 keeping the packages in the oven for 4 hours is to cure the parts.
 A cross-sectional view of one implementation of the reaction chamber 58 is
 shown in FIG. 5c, although many other implementations or variations
 developed by those skilled in the art may be utilized. The reaction
 chamber 58 shown in FIG. 5c contains shelving units in the form of powered
 shelf 51a and grounded shelf 51b used to support magazines 50 during the
 deflashing procedure. The powered shelf 51a is also used to conduct the RF
 energy supplied to the reaction chamber 58 through RF feedthru 57, power
 busses 55a, and conductive coupling links 51c. Similarly, grounded shelf
 51b is used to maintain a grounded state through its connection with
 conductive coupling links 51c to ground busses 55b. A vacuum port 53
 permits access to the system's vacuum pump 42 (FIG. 4), for
 implementations that use a vacuum pump.
 As described herein, the invention is operative to perform all or part of
 the flash removal required in the "deflash" step in any process of
 manufacturing electronic devices such as semiconductor memory devices. As
 shown in FIG. 6, the basic process flow for a typical process of
 manufacturing a semiconductor device embodying the invention starts with
 the fabrication step 62, where a plurality of semiconductor dies may be
 fabricated on a wafer. For each individual device, a respective one of the
 plurality of dies is selected and separated from the other dies (step 64).
 The selected die is then attached to the central pad or "island" of a lead
 frame (step 66) using epoxy, polyimide, eutectic or other attaching
 materials known in the art. A visual inspection of the die/lead frame
 combination may be made (step 68) to ensure proper alignment, absence of
 defects, etc.
 A cleaning step 70 is sometimes then utilized to remove any excess adhesive
 or other contaminants that may reside on the surface of the die or on the
 inner leads of the lead frame. It is on these junctures that an electrical
 connection will be made (in step 72) between a bonding pad on the die and
 one of the inner leads of the lead frame. The combined structure is then
 transferred to a molding area where the structure is placed in a mold for
 encapsulation (step 74). Typically, a plastic or other workable resin is
 injected into the mold surrounding the die and the inner leads of the lead
 frame to form a encased package. Only the outer leads of the lead frame
 appear on the exterior of the package.
 As noted in the section above, imperfections in the mold result in an
 excess of encasing material and other contaminants 32, 34, 36 (FIGS. 2 and
 3) appearing on the casing itself 30, as well as on and between the outer
 leads 16 of the lead frame. In accordance with the invention, a "deflash"
 step, represented as step 76 in FIG. 6, utilizing a plasma gas, in the
 manner described above, is effective in removing the excess encasing
 material without leaving any residue or film and without damaging the
 surface material. The package is then sent to an oven or other heat
 chamber for final curing (step 78).
 A lead plating process step is used to coat the external leads with a metal
 layer to enhance the conductivity of the leads (step 80). A number of
 finishing steps (represented by step 82) can then be employed to complete
 the fabrication and manufacture of the semiconductor device, as necessary.
 For example, the finishing steps may include: marking the encapsulated
 package, soldering the encapsulated package to a printed circuit board,
 performing electrical or physical tests of the device, placing the package
 in device tubes for shipment, etc.
 As should be readily apparent, many of the basic process flow steps
 discussed above may vary (e.g., order of steps changed, steps added, steps
 subtracted, steps substituted, etc.) without detracting from the
 invention. For example, the lead plating step 80 may easily be performed
 prior to final curing in step 78. Indeed, in another preferred embodiment,
 the plasma clean step 76 and cure step 78 are implemented simultaneously
 using the same chamber. As shown in FIG. 5b, the plasma cleaner can be
 modified to add a source of heat 59 to provide the heat necessary to cure
 the molded package directly in the reaction chamber 58. The heat output
 from the source 59 may be supplied to reaction chamber 58 in any manner
 known in the art. Moreover, the plasma cleaner used to perform the plasma
 clean step 76 in accordance with the invention may be the same plasma
 cleaner used to perform one or more plasma-based processing steps (e.g.,
 fabrication dry etching step 60 (FIG. 6), plasma-based pre-mold clean step
 70, etc.) utilized in the fabrication and manufacture of the electronic
 device.
 Although the precise manner of implementing the plasma clean step (step 76)
 of the invention may differ, the basic process flow for the preferred
 embodiment of the invention is shown in FIG. 7. In step 77a, a gas is
 introduced into a plasma cleaner. The gas is converted to a reactive
 plasma (step 77b) and caused to react on one or more surfaces to be
 cleaned, e.g., external leads, of the electronic package (step 77c). The
 reaction on the surface caused by the reactive plasma breaks the organic
 bond between the surface and any contaminants residing thereon, and
 produces volatile by-products. These by-products are then removed from the
 plasma cleaner (step 77d) to complete the deflashing procedure.
 A basic in-line block diagram of a manufacturing system that may be used to
 perform the foregoing process of the preferred embodiment is shown in
 accompanying FIG. 8. (Each section of the system shown in block form could
 be implemented in any manner known to those of ordinary skill in the art.
 Any number of different fabrication techniques, for example, known to be
 used at the time by ordinary skilled artisans may be utilized for the
 "fabrication" section 90 of the system. As shown, a fabrication section 90
 may be used to fabricate one or more electronic devices such as
 semiconductor integrated circuits in the form of IC dies on a wafer of
 silicon (or other material). A die selection/separation section 91
 provides a determination of which one of the plurality of IC dies will be
 selected for a particular package currently under process. Typically, the
 selection will be based on the presence of identifying marks on the IC die
 indicating the results of a prior inspection in the fabrication section
 90. A die attachment section 92 is next utilized to join the selected IC
 die and a lead frame, as previously discussed. The die attachment section
 92 may also include an inspection station used to provide an added degree
 of quality assurance. A cleaning section 93 can be used to remove any
 excess epoxy or other material that may appear on the surfaces of the die
 or lead frame as a result of the previous die attachment. The cleaning
 section 93 may utilize a plasma cleaner or some other cleaning apparatus
 known in the art. A wire bonding section 94 follows in the line to provide
 the necessary electrical connections between the bonding pads on the die
 and the inner leads of the lead frame.
 The attached die and lead frame are then transferred to mold section 95
 where the two components are placed in a mold for encapsulation. The
 resulting encapsulated device forms a package which leaves only outer
 leads of the lead frame exposed. The plasma cleaning section 96, as
 discussed above, employs the novel plasma cleaner discussed above to
 remove any flash or other contaminants that may appear on the encasing or
 outer leads of the package. This plasma cleaning section 96, in accordance
 with one preferred embodiment, may be combined with the curing section 97,
 as discussed above. A lead plating section 98 can then be used to coat the
 outer leads with a material to improve solderability or conductivity of
 the package. A variety of miscellaneous operations known in the art (as
 described above) may then be performed in the finishing section 99 to
 complete the manufacture of the semiconductor device, as necessary.
 While the invention has been described in detail in connection with the
 best mode of the invention known at the time, it should be readily
 understood that the invention is not limited to the specified embodiments
 described. Rather, the invention can be modified to incorporate any number
 of variations, alterations, substitutions or equivalent arrangements not
 heretofore described, but that are commensurate with the spirit and scope
 of the invention.
 For example, although the plasma cleaner in the preferred embodiment was
 disclosed (with reference to FIG. 4) utilizing a source of gas 41 supplied
 to the process chamber 44 prior to conversion to a reactive plasma, it
 should be readily understood that the invention could be modified to allow
 for the conversion of the gas in the source 41 prior to introduction into
 the process chamber 44 through the gas inlet 40 (with the appropriate
 modifications, e.g., imposing the electric field in the source 41, etc.).
 Another exemplary alteration which should be readily apparent is the
 elimination of the vacuum pump 42 (FIG. 4) when using, for example,
 atmospheric plasma-based cleaning devices known in the art in which a low
 pressure environment is not necessary to effect generation of plasma.
 In view of the many modifications which can be made, the nature, spirit and
 scope of the invention is not limited by the foregoing descriptions but is
 only be limited by the scope of the claims appended hereto.