Abstract:
A method for rendering a surface of a contact rough includes submerging the surface of the contact in an electroplating bath having a dissolved metal salt, and pulsing an electric current through the contact and the bath to form a rough metallic structure on the surface of the contact.

Description:
This application is a divisional (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 08/586,232, filed Jan. 12, 1996, now issued as U.S. Pat. No. 5,876,580, Mar. 2, 1999. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. 
    
    
     BACKGROUND 
     This invention relates to rough electrical contact surfaces. For example, rough surfaces may be useful on contact pad surfaces in thin-film membranes used for testing integrated semiconductor devices. The contact pad surfaces make contact with device pads on the surface of semiconductor devices. See co-pending U.S. patent application Ser. No. 08/303,498. Because device pads can develop oxide layers, the membrane contact pads must penetrate oxide layers or other contaminants to make good electrical connections. 
     One method of penetration wears away the oxide layer by force often combined with rubbing motion. Another method of penetration uses contact pads having roughened surfaces that can pierce through the oxide layer. The roughened surface can be made, for example, by depositing already-formed, small, hard (e.g., of rhodium or titanium carbide) particles on the surface of a contact pad, and then electroplating the whole assembly with a thin layer of nickel. 
     SUMMARY 
     In general, in one aspect, the invention features a method for rendering a surface of a contact rough, including submerging the surface of the contact in an electroplating bath having a dissolved metal salt, and pulsing an electric current through the contact and the bath to form a rough metallic structure on the surface of the contact. 
     Implementations of the invention may feature the following features. The dissolved metal salt may include nickel chloride. The electroplating bath may have a concentration of about 150 to about 400 grams of nickel chloride dissolved in a liter of water. The electroplating bath may be heated to about 55 degrees C.±about 5 degrees. The electric current may have a current density of between about 35 amps per square foot to about 75 amps per square foot. The rough metallic structure may be plated with gold. The electric current may be pulsed for a length of time between 0 and about 3 minutes, and may be pulsed with a duty cycle having an on period greater than an off period. The ratio of the on period to the off period may be between about 4:1 and about 8:1. The on period may be between about 0.4 seconds and about 0.8 seconds, and the off period may be about 0.1 seconds. The ratio of the on period to the off period may be about 6:1, and the on period may be about 0.6 seconds and the off period may be about 0.1 seconds. 
     The rough metal structure may be comprised of spikes. The spikes may be substantially conical. The spikes may have a height between about 0.350 microns and about 1.275 microns. The spikes may have a base between about 0.345 microns and about 1.250 microns. A side of each of the spikes may have an angle from normal of between about 10 degrees and about 45 degrees. The spikes may have a density of between about 1 and about 2 spikes per square micrometer. 
     In general, in another aspect, the invention provides a roughened contact including a contact having a surface, and a solid metal layer deposited on the surface of the contact and having spikes protruding away from the contact. 
     In general, in another aspect, the invention provides a roughened contact including a contact having a surface, a solid metal layer deposited on the surface of the contact, wherein the solid metal layer comprises nickel, having spikes protruding away from the contact, wherein the spikes are substantially conical, the spikes have a height between about 0.350 microns and about 1.275 microns, the spikes have a base between about 0.345 microns and about 1.250 microns, wherein a side of each of the spikes has an angle from normal of between about 10 degrees and about 45 degrees, and wherein the spikes have a density of between about 1 and about 2 spikes per square micrometer, and a conductive plating deposited on the surface of the solid metal surface, wherein the conductive plating comprises gold. 
     In general, in another aspect, the invention provides a membrane for use in testing a circuit including a flexible membrane substrate, a contact on the flexible membrane substrate having a surface, and a solid metal layer deposited on the surface of the contact and having spikes protruding away from the contact. 
     Implementations of the invention may feature the following features. The spikes may be substantially conical. The solid metal layer may include nickel. A conductive plating may be deposited on the surface of the solid metal layer. The conductive plating may include gold. The flexible membrane substrate may include a polyimide film. 
     The advantages of the invention may include one or more of the following. Relatively sharp spikes may be created on the surface of a contact. These spikes may be deposited with a relatively uniform height, and dispersed relatively evenly across the contact surface. These spikes do not require the dropping of hardened particles onto the contact surface. Instead, these spikes may be created through a simple, continuous electroplating process. The hard, sharp spikes reduce damage to small and delicate components and form consistently reliable connections. 
     Other features and advantages of the invention will become apparent from the following description and from the claims. 
    
    
     DESCRIPTION 
     FIGS. 1 a  through  1   g  are cross-sections of a substrate during a spike deposition procedure. 
     FIG. 2 is a schematic diagram of spike deposition apparatus. 
     FIG. 3 is a timing diagram of applied current during spike deposition. 
     FIG. 4 is a cross-section of a substrate including particle-less spikes. 
     FIG. 5 is a cross-section of a single spike. 
     FIGS. 6 a  and  6   b  are photomicrographs of a contact pad including particle-less spikes. 
     FIGS. 7 a  through  7   f  are contour plots of spike deposition parameters. 
     FIGS. 8 a  and  8   b  are elevated and cross-section views of a membrane having a contact pad with spikes. 
    
    
     Referring to FIGS. 1 a  through  1   g , a particle-less spike deposition procedure begins with a substrate  10 . Substrate  10  may be a polyimide thin-film membrane, a pad deposited on a membrane, a semiconductor wafer, a printed circuit board, or any other material requiring electrical contacts. Usually a thin metal seed layer  11  is deposited on the upper surface of substrate  10 . Seed layer  11  may be a 100-400 Å layer of chromium, with a 500-4000 Å layer of copper. Seed layer  11  may be used to conduct electric current during electroplating steps. 
     Next, a photoresist layer  12  is deposited on the upper surface of seed layer  11 , as in FIG. 1 b . Photoresist layer  12  is exposed in certain regions and then dissolved away leaving bump areas  14   a ,  14   b , and  14   c . The surface of seed layer  11  in each bump area  14  is then cleaned with agents suitable for the substrate material. If substrate  10  is a contact formed on a polyimide film, its exposed areas may be cleaned by first plasma cleaning the surface, second using an acid cleaner combined with a wetting agent, and third immersing substrate  10  in an acid dip. The cleaned and exposed bump areas  14  are then ready for bump and spike deposition. 
     Using the apparatus of FIG. 2, substrate  10  (with masking photoresist layer  12 ) is placed in a tank  16  containing an electroplating solution  18 . One effective solution uses a concentration of 150 to 400 grams nickel chloride to each liter of deionized water, mixed with 30 to 45 grams of boric acid. Substrate  10  is connected to a current source  20 , whose other terminal connects, through current regulator  22 , to electrode  24  emersed in electroplating solution  18 , forming electrical circuit  25 . 
     Referring to FIG. 1 d , a first solid layer (or base)  26   a ,  26   b ,  26   c  for each bump is deposited within each bump area  14   a ,  14   b , and  14   c . These first layers  26 , e.g., of nickel, may be deposited through a steady application of current through circuit  25 . Particle-less spikes  28  are then deposited on the top surface of each of these first layers  26  by pulsing current through circuit  25 . During spike deposition, the electroplating solution  18  may be heated to approximately 55 degrees Centigrade±5 degrees. Current density may be between about 35 amps per square foot and about 75 amps per square foot. The electroplating solution  18  may have a Ph between 1.1 and 3.0, and may be agitated using percolated N 2  air. 
     The current may be pulsed with a repeated duty cycle as shown in FIG. 3, where period P is broken into two sub-periods: a first, ON period  30  where current I through circuit  25  is high, and a second, OFF period  32 , where current I is zero. Good results have been achieved using a duty cycle period of between 0.5 seconds and 0.9 seconds, where ON period  30  lasts between about 0.4 seconds and 0.8 second, and OFF period  32  last about 0.1 seconds. The current may be pulsed typically between 0 and 3 minutes. The periodic pulsing of current through circuit  25  causes nickel spike formations  28  to grow on the exposed surfaces of each bump  26 . These pointed spikes  28  are grown without use of any deposited hard particles. 
     Through experimentation, good growth parameters include a 55 degree C. temperature for electroplating solution  18 , a current of 35 amps per square foot, and a current duty cycle having an ON period of about 0.6 seconds, and an OFF period of about 0.1 seconds. The current is pulsed for a total of about 3 minutes. 
     Referring to FIG. 1 e , spikes  28  deposited across the exposed surfaces of bumps  26  have a layer of gold  34  deposited over spikes  28 , to ensure good electrical contact (FIG. 1 f ). Depending upon application, gold layer  34  may not be required. Then photoresist layer  12  is stripped, leaving a series of exposed bumps  26  on the surface of substrate  10 , each bump  26  having sharp spikes  28 , coated with gold layer  34 , as shown in FIG. 1 g , and in greater detail in FIG.  4 . The surface of substrate  10 , having such spikes  28 , may be referred to as roughened. 
     Referring to FIG. 5, each spike may be generally characterized by its height H, width W, and angle from normal θ. Measurements of spikes created by the described method had heights varying from about 0.353 microns to about 1.266 microns (mean: 0.718 microns), widths varying from about 0.345 microns to about 1.250 microns (mean: 0.715 microns), and an angle θ from about 10 degrees to about 45 degrees (mean: 28 degrees). Photomicrographs of a contact bump  26  having deposited spikes  28  are shown in FIGS. 6 a  (elevated) and  6   b  (enlarged). The spikes are relatively uniformly distributed across the surface of the contact with a density of 1 to 2 spikes/μm 2 . In addition to the spikes, the surface of contact bump  26  exhibits rolling hills and valleys known as asperities, which do not affect the use of the electrical contact. 
     Each of the deposition parameters may be varied and still generate spikes on the substrate surface. Referring to FIGS. 7 a  through  7   f , contour plots are shown for trading off different deposition parameters. The experimental results shown in contour plot  700   a  of FIG. 7 a  employed a duty cycle of 0.4 seconds ON, 0.1 seconds OFF, a deposition time of 2 minutes, and both current density I and temperature of electroplating solution were varied. Contour lines  710  represent an equivalent end result in terms of spike production, with higher numbers representing qualitatively better spikes (that is, −2 is better than −16). Contour plot  700   b  (FIG. 7 b ) performed the same trade-offs, but with a duty cycle of 0.6 seconds ON, 0.1 seconds OFF and a deposition time of 3 minutes. Contour plot  700   c  (FIG. 7 c ) performed the same trade-offs, but with a duty cycle of 0.8 seconds ON, 0.1 seconds OFF and a deposition time of 3 minutes. 
     Likewise, contour plots  700   d  through  700   f  varied both current density and plating time. Contour plot  700   d  used a duty cycle of 0.8 seconds ON, 0.1 seconds OFF and a temperature of 37.5 degrees C. Contour plot  700   e  used a duty cycle of 0.8 seconds ON, 0.1 seconds OFF and a temperature of 55 degrees C. Contour plot  700   f  used a duty cycle of 0.4 seconds ON, 0.1 seconds OFF and a temperature of 55 degrees C. 
     Again, contour lines  710  represent an equivalent end result in terms of spike production, with higher numbers representing qualitatively better spikes. For example, for a duty cycle of 0.8 seconds ON and 0.1 seconds off, better results are achieved with a temperature of 55 degrees compared with 37.5 degrees (compare FIG. 7 d  with FIG. 7 e ). 
     Referring to FIGS. 8 a  and  8   b , a flexible membrane  40  (e.g., of polyimide film) has a contact pad  10 . The surface of contact pad  10  has bumps  26  having spikes  28 , for making excellent electrical contact with a corresponding device pad on a semiconductor device. 
     Other embodiments are within the scope of the claims. For example, different: metal solutions may be used for electroplating. Also, the spikes and other features may be formed on a variety of different substrates. Further, different time periods, current strengths, duty cycles, temperatures, and other deposition parameters may be employed.