Abstract:
A method of tape automated bonding a heater chip of an ink jet printer to a flexible circuit includes the step of attaching an electrically conductive bonding pad to a first portion of a surface of the heater chip. A mask is applied to a second portion of the surface of the heater chip. An exposed surface of the bonding pad is chemical dry etched for a predetermined time period to thereby remove contaminants from the etched exposed surface. The chemical dry etching is terminated at an end of the predetermined time period such that substantially none of the bonding pad is removed. Lastly, the flexible circuit is electrically connected to the etched exposed surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of TAB bonding, and, more particularly, to a method of attaching a beam of a flexible circuit to a bonding pad. 
     2. Description of the Related Art 
     Tape Automated Bonding (TAB) is used within the ink jet printer industry as a way of interconnecting a heater chip and a flexible circuit. The heater chip is a multi-layered device in which ink is heated in order to cause the ink to be jetted out of orifices or nozzles in the heater chip toward a print medium. The flexible circuit carries power to the heater chip in order to selectively energize the ink emitting nozzles. The bonding pads are patterned in a thin film process of fabricating the heater chip, which may include sputtering, chemical vapor deposition, etching and/or photolithography. The flexible circuit includes finger-like beams which are bonded or welded to the aluminum bonding pad on the heater chip. The bonding may be performed with ultrasonic, thermosonic or thermocompression bonding. A problem is that the TAB bond between the flexible circuit and the heater chip is known to fail due to poor bond strength. The poor bond strength can be attributed to many different factors, including contaminated bond pads or non-optimized bonding parameters. Since the TAB circuit is one of the most expensive parts within an ink jet print head, the scrap cost associated with TAB bond failure significantly increases the overall cost of the ink jet printhead. 
     What is needed in the art is an improved method of TAB bonding a flexible circuit to a bonding pad of a heater chip, such that higher bond strength and less bond failures are achieved. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of removing dirt or other contaminants from the surface of a bonding pad before TAB bonding is performed between the flexible circuit and the bonding pad of the heater chip. 
     The invention comprises, in one form thereof, a method of tape automated bonding a heater chip of an ink jet printer to a flexible circuit. The method includes the step of attaching an electrically conductive bonding pad to a first portion of a surface of the heater chip. A mask is applied to a second portion of the surface of the heater chip. An exposed surface of the bonding pad is chemical dry etched for a predetermined time period to thereby remove contaminants from the etched exposed surface. The chemical dry etching is terminated at an end of the predetermined time period such that substantially none of the bonding pad is removed. Lastly, the flexible circuit is electrically connected to the etched exposed surface. 
     An advantage of the present invention is that various contaminants are removed from the surface of the bonding pad which is to be TAB bonded to the flexible circuit, thereby resulting in higher bond strength and higher production yields. 
     Another advantage is that the cleaning process does not remove substantial amounts of material from the surface of the bonding pad, thereby leaving the bonding pad intact for TAB bonding. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawing, which is a side view of one embodiment of the method of the present invention, in which a heater chip is placed inside a reactive ion etching (RIE) chamber. 
    
    
     The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing there is shown a heater chip  10  disposed within a reactive ion etching chamber  12 . Heater chip  10  includes a silicon substrate  14  attached to an aluminum bonding pad  16 . A beam  18  of a flexible circuit is shown schematically in a dotted line and will be placed on an upper surface  20  of bonding pad  16 , as indicated by arrow  22 , in order to TAB bond the flexible circuit to heater chip  10  after heater chip  10  has been withdrawn from RIE chamber  12 . Thus, bonding pad  16  defines the extent of the bonding area on heater chip  10 . Immediately, adjacent to bonding pad  16  on silicon substrate  14  is a mask layer  24  for protecting silicon substrate  14  while substrate  14  is in RIE chamber  12 , as will be discussed in more detail below. Mask  24  may be formed of either silicon nitride, photo resist or polyimide, or another known etch mask or a combination of the above masks. 
     RIE chamber  12  includes a grounded housing  26  containing two planar electrodes, including a grounded electrode  28  and a positive potential electrode  30 . Heater chip  10  is laid flat upon positive electrode  30  such that heater chip  10  is disposed between electrodes  28  and  30 . A radio frequency (RF) power supply  32  applies a voltage potential across electrodes  28  and  30 . A gas inlet  34  carries a gas, i.e., a neutral ionized gas containing a large number of free electrons and charged ions, into RIE chamber  12  between electrodes  28  and  30 . The gas forming the plasma may be oxygen (O 2 ), carbon tetraflouride (CF 4 ) and/or other gases known in the art. RF power supply  32  provides constant energy to the plasma in order to offset the recombination of the charged particles in the plasma, and thereby maintain the plasma in an ionized state. The plasma must also be kept at low pressure in order to reduce the collision rate and thus the recombination rate of the ions. To produce the low pressure, a vacuum pump continuously pumps against the chamber  12  through gas outlet  36 , removing contaminants and residual gases, in order to maintain an equilibrium pressure (not shown). 
     In operation, the applied electrical field F from outlet power supply  32  strips the free electrons, with which plasma formation begins, from gas atoms. Once available, the electrons are accelerated by the applied field F and collide with gas molecules with several effects, such as ionization, dissociation and excitation, as is well known in the art. 
     Since the plasma is an electrical conductor, the interior of the plasma is at a uniform electric potential. Both electrons and ions escape from the plasma and are neutralized on the walls of chamber  12 . However, the electrons escape more easily because they are much smaller and more mobile. Thus, a surplus of positive ions are left in the plasma, resulting in a positive charge. Ions  38  leaving the plasma initially have a rather random direction of movement, as shown by the ions nearest to electrode  28  in the drawing. The electric field F across electrodes  28  and  30  tends to redirect ions  38 , causing them to acquire a velocity substantially parallel to the electric field F. As ions  38  are accelerated along the electric field F, they eventually collide with bonding pad  16  at an angle that is substantially perpendicular to surface  20 . Mask  24  provides a protective layer to prevent ions  38  from damaging silicon substrate  14 . 
     With the method of the present invention, an anisotropic etching is achieved, i.e., the downward etch rate is much larger than the lateral etch rate, resulting in a very evenly distributed etching of upper surface  20  of bonding pad  16 . The perpendicular bombardment of surface  20  of bonding pad  16  as discussed above is what causes the etching to be anisotropic in nature. 
     At sufficiently high energies, the bombarding ions  38  erode surface  20  as they strike it, a process known as “sputtering”. Sputtering is a very unselective process in which chemical bonds are broken due to impact. Ion bombardment at lower energies results in chemical etching by locally heating bonding pad  16  and by loosening chemical bond. 
     Etching that is highly dependent upon such ion imbardment is referred to as reactive ion etching (RIE). In reactive ion etching, an additional potential is applied to electrodes  28  and  30  so that ion bombardment energies exceed the plasma potential. Reactive ion etching is characterized by the substrate electrode  30  being connected to the RF power supply and the other electrodes  28  being grounded. This configuration allows a high potential between bonding pad  16  and the plasma. If this configuration were reversed, a lower bias would be produced, which is characteristic of the plasma mode. 
     A thin layer of contamination  40 , e.g., formed of aluminum oxide, debris, organic matter, etc., is present on upper surface  20  of bonding pad  16 . Heater chip  10  is exposed to reactive ion etching for a time period that is long enough for the bombardment of ions  38  to remove contaminants  40  from surface  20 , but is not long enough to cause any depression to be formed in surface  20 , such as by removing the aluminum of bonding pad  16  itself. Thus, surface  20  remains substantially intact. This time period is approximately between 30 seconds and 10 minutes, and preferably is approximately between 2 minutes and 7 minutes, and more preferably between 4 minutes and 7 minutes. The anisotropic nature of reactive ion etching results in a uniform cleaning of surface  20 , without penetrating into the aluminum of bonding pad  16 . By removing contaminants  40  from bonding pad  16 , a stronger bond may be formed between beam  18  and surface  20  in the TAB bonding process. After the reactive ion etching process removes contaminants  40 , contaminants  40  are carried away by reactions with a stream of oxygen and/or carbon tetraflouride gas within chamber  12 . 
     A throttle valve (not shown) may be fitted on the vacuum pump to allow pressure and gas residence time within chamber  12  to be varied independently. The operating pressure within chamber  12  is set to be between approximately 100 mTorr and 500 mTorr during the reactive ion etching process, and preferably is approximately between 150 mTorr and 200 mTorr. The operating gasses, such as oxygen and carbon tetraflouride, flow through chamber  12  at a rate of approximately between 1 and 250 standard cubic centimeters per minute (sccm). Each gas is regulated separately with a mass flow controller within a range of between 1 and 250 sccm, preferably between 1 and 50 sccm. 
     After the reactive ion etching has taken place and contaminants  40  have been removed, heater chip  10  can be removed from chamber  12 . Mask layer  24  may then be removed by any conventional process, and beam  18  is TAB bonded to surface  20  of bonding pad  16 . (The protective layer may not need to be removed or may already be built into the chip). 
     In the method shown, reactive ion etching is used to remove contaminants  40  from bonding pad  16 . However, it is to be understood that any kind of dry etching, such as chemical dry etching and/or anisotropic dry etching can also be used to remove contaminants  40 . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.