Patent Publication Number: US-2005121327-A1

Title: Electroless-plated deposit process for silicon based dielectric insulating material

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to deposition processes of dielectric insulating material and more particularly to an improved electroless-plated deposition process for silicon based dielectric insulating material such as silicon dioxide (SiO 2 ) or polysiloxanes. The present invention provides new thin film deposition features b y a self-aligned, integrated plating technique based on plasma physics and colloidal-related chemistry to fabricate patterns of ultrathin (&lt;20 nm) Co-based barriers and copper films in a selective manner on silicon based dielectric insulating material such as silicon dioxide (SiO 2 ) or polysiloxanes films by using the all-electrochemical integrated plating process, which consists of selective plasma treatment of the surface of substrate, solution immersion for improved wettability, and electroless plating.  
      2. Description of Related Art  
      A wide variety of silicon or carbon based dielectric insulating materials are employed as deposits on metal films in the microelectronics, optoelectronics, and semiconductor industry. For example, polyamide is widely employed as material in fabricating printed circuit boards (PCBs) in which through-holes of the PCB are formed by electroless-plated nickel (or copper) deposition. Also, silicon dioxide (SiO 2 ) is the major material of optical fibers of which surface has to be coated with a protection thin layer in order to prevent the possible attacks from water, contaminants, or the like through either physical vapor deposit (PVD) or chemical vapor deposit (CVD) process. It is known that silicon (Si) has replaced germanium (Ge) as the widely employed semiconductive material because silicon oxide (SiO 2 ) can be easily grown on the substrate by the thermal oxidation means. Undoped silicate glass (USG) is also a widely employed material as shallow trench isolation in the fabrication of integrated circuit (IC) devices such as metal oxide semiconductors (MOS). Phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG) are also popular pre-metal dielectric materials for manufacturing MOS devices and the like. There are a number of dielectric materials having a low dielectric constant such as SiO 2  and polysiloxanes, which have been widely employed for the insulation of copper or aluminum alloy conductors in conjunction with the IC devices. A metal spacer as diffusion barrier must be deposited between a dielectric layer and a metal conductor for preventing possible interdiffusional behavior and chemical reaction at interfacial regions from occurring therebetween. The Co or Ni based films formed by electrochemical thin layer deposition techniques such as electroless-plating or electroplating have manifested a greater potential to be used as such metal spacers above-mentioned.  
      However, all existing dielectric insulating materials cannot catalyze the growth of electroless-plated metal film. Some techniques including an one-phase active palladium (Pd) seeding, an immersion plating, a grafting, and a radiation decomposition have been proposed to grow seeds (i.e., dies) as a substitute for the sensitization/activation process. Till now the sensitization/activation process is still the dominant art for growing dies in the IC fabrication. The sensitization/activation process involves the steps of acidifying (i.e., surface roughing) a dielectric substrate and growing seed in one-phase or two-phase treatment. For a two-phase treatment, the substrate is firstly exposed to SnCl 2 /HCl acidic solution for sensitization in which Sn 2+  ions to be adsorbed on the surface of substrate. Subsequently the substrate is exposed to PdCl 2 /HCl acidic solution for activation in which Pd 2+  ions can be reduced to be neutral by Sn 2+  ions. For a one-phase treatment, the substrate is immersed in a mixed SnCl 2 /PdCl 2  acidic solution to form Sn—Pd compound ions. Next procedure is followed by acceleration, of which purpose is to remove Sn 2+  ions from the surfaces of dies. However, the conventional electroless-plating method is fairly time consuming. Another drawback thereof is that Pd 2+  ions tend to form agglomerates at the size range of 100˜200 nm. As a result, it is impossible of growing an electroless-plated metal film having a thickness no more than 100 nm. Hence, a novel technique should be pursued to overcome the above drawbacks of the prior art.  
     SUMMARY OF THE INVENTION  
      The objective of the present invention is to provide a process for all-electrochemical integrated plating technique for the deposition of very thin metal film onto a substrate such as silicon based dielectric material being either hydrophilic (water adsorptive) or hydrophobic (water repellent). The all-electrochemical integrated plating process can make Co, Ni, Ag, Pd ions reduced into neutral metal ions which in turn can be served as ultrafine catalyst (2˜4 run) for growing electroless-plated Co, Ni, Cu or Fe film formed of an elemental metal, or a binary or tertiary alloy with the film thickness ranging from 20 nm to several thousand nanometers. The process technique of the present invention consists of the steps of (i) plasma treatment of the surface of the substrate by glow-discharged plasma of H 2 /N 2 , N 2 , H 2 , or O 2 ; (ii) immersing the substrate in a basic aqueous solution; (iii) removing hydrogen from the surface of the substrate; (iv) immersing the substrate in a basic aqueous solution having metal ions adsorbed onto the surface of the substrate; (v) reducing the adsorbed metal ions on the surface of the substrate; and (vi) immersing the substrate in an electroless-plated solution for depositing an electroless-plated metal film. The plasma source of H 2 /N 2  can assure the environmental friendliness and better compatibility for the IC process, as opposed to the chlorofluorocarbon compounds (CFC) gases as dry etchant that pose a potential to public hazard. By utilizing the present invention, the expensive palladium chloride and tin chloride used in the prior sensitization/activation process are eliminated. Furthermore the capability for attaining a self-aligned, integrated plating method of the present invention to fabricate patterns of ultrathin (&lt;20 nm) metal films in a selective manner on dielectric films using electroless plating far exceeds that of the conventional sensitization/activation processes.  
      In one aspect of the present invention, the silicon based dielectric material is selected from silicon dioxide, silicate glass, or polysiloxanes so as to form an active surface on the substrate and form a negatively charged surface on the substrate after performing the step (ii) for adsorbing the metal ions for electroless-plated catalysis.  
      In another aspect of the present invention, the silicon based dielectric material selected from either the hydrophilic silicon dioxide or the hydrophilic silicate glass is immersed in a basic aqueous solution for depositing the electroless-plated metal film irrespective of the plasma treatment.  
      In still another aspect of the present invention, the silicon based dielectric material selected from the hydrophobic polysiloxanes undergoes the step (i) for forming a hydrophilic surface, plasma is formed of one of ionized gas including N 2 /H 2 , N 2 , H 2 , and O 2 , and the basic aqueous solution as a strong oxidizing agent for depositing the electroless-plated metal films.  
      In a further aspect of the present invention, the substrate formed of the hydrophobic polysiloxanes, which is selectively treated by glow-discharged plasma of H 2 /N 2 , N 2 , H 2 , or O 2  in the early processing procedure to remove hydrophobic radicals at the surface, is immersed in a basic aqueous solution of strong oxidizing agent for forming a negatively charged surface and converting it into surface silanol groups (Si—OH). The surface of the Si—OH ‘passivated’ dielectric film turns out to be hydrophilic and can be ion exchanged with divalent metal ions in the metal-containing solution. Consequently, the adsorbed metal ions can then be reduced to metal atoms by chemical reducing agents. Metal atoms dispersed on the surface act as catalytic sites for electroless deposition to form a self-aligned electroless-plated metal film pattern on the surface of the substrate.  
      The above and other objectives, features and advantages of the present invention will become apparent by the following detailed descriptions taken with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flowchart illustrating a film deposition process according to the present invention, which consists of plasma treatment with colloid chemistry, following deprotonation of the plasma-treated (OH— terminated) films and adsorption/reduction of metal ions;  
       FIG. 2  is a side view showing the shape of water drop onto a hydrophobic dielectric substrate before and after plasma treatment, where the improved wettability and reduced contact angle are evident by plasma treatment;  
       FIG. 3  is a flowchart illustrating a self-aligned, selective electroless-plated metal deposition process according to the present invention, which consists of (1) the unselected area is covered by mask; (2) the unmasked area is treated by gaseous plasma (O 2  or H 2 /N 2 ); (3) the selected area becomes hydrophilic after the mask removal; (4) all-electrochemical integrated process using a self-aligned seeding process that can be patterned selectively to induce the sequential growth of electroless barrier and metal films  
       FIG. 4  is a high-resolution X-ray near-edge absorption spectra obtained from as-deposited HOSP™ dielectric layers (a), sequentially treated by Ni 2+  ion adsorption (b), adsorbed Ni 2+  ion reduction (c), and electroless Co plating for 10 min (inset). The minor peak at ˜852 eV in (a) is due to X-ray guiding facility according to the present invention;  
       FIG. 5  depicts TEM bright-field micrographs and selected-area diffraction patterns (with major rings indexed) for (a) the initial dielectric material e.g. as-cured HOSP™ dielectric layers after sequential treatments by (b) Ni 2+  ion adsorption/reduction as illustrated in  FIG. 1  and (c) electroless Co plating for 2 minutes in diffraction pattern taken by TEM.  
       FIG. 6  depicts a SEM micrograph of a Sio 2  substrate through a process as illustrated in  FIG. 1  by electroless-plated Co metal film for about 10 minutes;  
       FIG. 7  shows three-dimensional AFM micrographs for (a) the initial dielectric material e.g. as-cured HOSP™ dielectric substrates after a sequential treatment including (b) Ni 2+  ion reducing and electroless Co plating for (c)  1  and (d) 2 min; and  
       FIG. 8  shows illustration of self-aligned MOS capacitive structure comprising silicon based dielectric layer, Co—W—P barrier, and Cu metal film on plasma-treated areas of HOSP™ dielectric substrates using the all-electrochemical integrated deposition process, which is illustrated in  FIG. 3   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 1  in conjunction with  FIG. 2 , there is shown a thin film deposition process by electroless-plating in accordance with the present invention in which the principles of plasma treatment of the surface of the hydrophobic dielectric substrate and the all-electrochemical integrated deposition process are utilized. The process begins in step  1 , where the plasma treatment on the surface of a dielectric insulating material will form dangling bonds and a chemically active surface. This step is particularly essential for hydrophobic dielectric substrates in a subsequent catalysis for metal ion (i.e., very small particle) adsorption. The plasma treatment retains a strong effect with respect to the hydrophobic radicals (e.g., S 1 —CH 3 , Si—H, etc.) of polysiloxanes dielectric polymer as shown in  FIG. 2 . In the left side of  FIG. 2 , a substrate, not processed by plasma, has a contact angle of about 96 degrees measured from the shape of water drop attached on the substrate. As a result, it is difficult of adsorbing very small metal particles having water component onto the surface of the hydrophobic dielectric substrate. Advantageously, as shown in the right side of  FIG. 2 , a fine water drop can be adsorbed onto the surface of the substrate in a contact angle of about 24 degrees measured with respect to drop shape after processing the substrate with plasma.  
      In step  2 , select a basic aqueous solution having an appropriate pH value and immerse the silicon based dielectric material therein. As a result, a great number of OH radicals are adsorbed onto the active surface as shown in part ( 2   a ) of  FIG. 1 . Next, hydrogen removal (deprotonation) takes place to form a negatively charged surface of the dielectric substrate having Si—O— thereon, as shown in part ( 2   b ) of  FIG. 1 .  
      In step  3 , immerse the dielectric substrate having a negatively charged surface in a basic aqueous solution having metal ions (e.g., Co or Ni) for adsorbing metal ions by Columbic forces and/or exchanging ions at the surface of the substrate. As a result, ultrafine nanoparticles of atomic scaled metal ions are formed on the surface of the substrate.  
      In step  4 , expose the substrate to an annealing environment with reducing atmosphere or immerse the same in a reducing solution for the reduction of positively charged metal ions on the surface into neutral metal particles. This step would provide the catalytic effect for the subsequent electroless-plating process.  
      In step  5 , immerse the dielectric substrate in an electroless-plated solution for depositing a thin film of an elemental metal (e.g., Co, Ni, Cu, etc.), a binary alloy (e.g., Co—P, Ni—P, etc.), or a tertiary alloy (e.g., Co—W—P, Ni—W—P, etc.).  
      Due to the drawbacks associated with growth of palladium catalyst seeds by current sensitization/activation process, the present invention using the all-electrochemical integrated plating technique excels in various aspects in terms of better control of very small film thickness, production cost, environmental friendliness and better compatibility adapted to the current IC process.  FIG. 3  is a flowchart illustrating a self-aligned, selective electroless-plated metal deposition process according to the present invention. The process begins in step  1  where a hard mask or photoresist pattern is used to form a selective masking pattern on the hydrophobic substrate. In step  2 , remove hydrophobic radicals from the surface thereof by performing a plasma treatment as illustrated in step  1  of  FIG. 1 . In step  3 , remove the mask to leave a patterned hydrophilic area. In step  4 , form the catalytic metal particles on the order of nanometer and develop the electroless-plated metal thin film by sequentially performing steps  2  to  5  of  FIG. 1 . Alternatively, perform step  4  prior to removing the mask in step  3 , namely, steps  3  and  4  can be reversed.  
      Following is Table I to depict the differences between the present invention and the prior sensitization/activation process regarding seeding technique.  
               TABLE I                          Comparisons between ultrafine catalytic seeding in the invention and       the sensitization/activation process                             Seeding Techniques                                     Ultrafine   Sensitization/               Catalytic   Activation       Description   Seeding   Seeding   Remarks               Plating solution   Basic   PdCl 2 /SnCl 2                 metal ions       Process cost   Low   High   Pd metal is relatively                   expensive.       Solution stability   High   Low   Solution of                   sensitization/activation                   process should be                   constantly                   replenished.       Environmental   High   Low   HCl and HF solution       Friendliness           are highly corrosive.       Seed feature   2-5 nm   &gt;100 nm   The particles formed                   by                   sensitization/activation                   process are highly                   agglomerated.       Self-aligned   Yes   No   Difference may result       Capability           in hydrophilic species                   and hydrophobic ones       Plating thickness   ≧20 nm   ≧100 nm                  
 
      Following is Table II for illustrating process conditions of plasma treatment, electrochemical solution preparations, and their corresponding process steps.  
               TABLE II                       Process conditions of plasma treatment, electrochemical solution       preparations, and their corresponding process steps.                                                Step   1   2   3               Description   Plasma treatment for the   In the basic   Ni(NO 3 ) 2 /           hydrophobic substrate to   aqueous solution   Co(NO 3 ) 2             become hydrophilic       Process   Oxygen/nitrogen/   T = 25˜80° C.,   T = 25° C.,       Conditions   hydrogen   t = 10 min   t = 10 min           plasma or combination           thereof               Step   4   5   6               Description   Highly reducing   Preparation of   Cleaning/           solution/hydrogen   electroless-plated   drying           undergoing a   solution           reduction reaction           at high-temperature       Process   T = 80° C.,   Based on plating   Ultrasonic       Conditions   t = 60 min   operation   cleaning/               conditions   drying                   by nitrogen                   spraying                  
 
 Surface Treatment by Plasma 
 
      This is an essential process for hydrophobic dielectric material (e.g., polysiloxanes). Plasma is formed of ionized gases such as N 2 /H 2 , N 2 , H 2 , or O 2 . Plasma formed of O 2  is advantageous for having the highest efficiency of removing hydrophobic radicals, however, it is disadvantageous for damaging the ring structure (e.g., ring Si—O) of polysiloxanes due to excessive input energy. In contrast, gaseous plasma N 2 /H 2 , N 2 , or H 2  is advantageous for effectively removing hydrophobic radicals without damaging the ring structure of polysiloxanes. Furthermore, N 2 /H 2  is preferred as compared with N 2  or H 2  plasma gaseous species.  
      Basic Aqueous Solution Preparation and Metal Ion Adsorption  
      Selecting a basic aqueous solution with desired components, types, concentration, and temperature would rely on the surface properties of substrate in order to perform a effective surface activation and thus the proficient adsorption of metal ions on the surface thereof.  
      Metal Ion Reduction  
      In addition to the prior high-temperature hydrogen reduction, a chemical solution reduction is embodied in the present invention. SiO 2  and polysiloxanes as representative silicon based dielectric materials will be described below. Catalytic metal ion adsorption, reduction, and electroless-plated film deposit are described in cooperation with soft X-ray synchrotron radiation absorption spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Finally, an exemplary example is described for the illustration of a self-aligned patterned process.  
      Referring to  FIG. 4 , synchronous radiation peaks L 2  of 845 eV and L 3  of 875 eV in an X-ray absorption spectrum has verified that after undergoing steps in  FIG. 1 , SiO 2  or polysiloxanes dielectric substrate has adsorbed metal ions (e.g., Ni) thereon and has grown electroless-plated Co metal film. Curve (a) represents an initial substrate in which a substantially flat peak is shown and the peak of 852 eV is generated b y an analytic instrument, namely, the X-ray guiding facility. Signal of Ni 2+  ion is shown after performing Ni 2+  ion adsorption. After performing the metal ion reduction, the peak will change into peaks of neutral Ni metal, as indicated in curves (b) and (c) of  FIG. 4 . This means that Ni 2+  ions have been reduced into neutral Ni ions which in turn can be served as catalyst for growing electroless-plated Co, Ni, or Cu metallic film formed of an elemental metal or a binary or tertiary alloy. The inset graph in  FIG. 4  represents a curve to show the existence of electroless-plated Co film.  
      Referring to  FIG. 5 , the speckle feature of the TEM dark-field micrograph in part (a) of  FIG. 5 , along with the diffuse electron diffraction pattern (inset), indicates that the unprocessed dielectric films are amorphous. After the adsorbing/reducing treatment, bright-field images (see part (b) of  FIG. 5 ), together with dark-field images and selected-area diffraction patterns, clearly reveal that a high density of ultrafine (2-4 nm) Ni crystalline precipitates are embedded in the dielectric film. Judging from the indexed diffraction pattern inset in part (b) of  FIG. 5 , these nickel precipitates have a face-centered cubic (fcc) structure and a lattice constant of 3.71 Å, ˜5% larger than that given by the X-ray diffraction source data (3.52 Å). They can provide the platform for the formation of electroless Co-based grains, ultimately coalescing to a very thin (20 nm) continuous barrier of grain size approximately 20 nm, as show in part (c) of  FIG. 5 .  
      Referring to  FIG. 6 , it depicts a SEM micrograph of the cross-section of a SiO 2  substrate being processed by electroless-plated Co deposit for about 10 minutes in which the thickness of electroless-plated Co film is approximately 100 nm.  
      Referring to  FIG. 7 , it shows a series of three-dimensional AFM images for the samples giving TEM micrographs in Referring to  FIG. 5 . The part (a) of  FIG. 7  reveals that the dielectric substrate not being processed is featureless and has a mean roughness (R a ) of only 0.65 nm, concurring with the TEM image depicted in part (a) of  FIG. 5 . The surface morphology and R a  resemble those of part (a) of  FIG. 7  for samples after the sequential treatments of the N 2 /H 2  mixed plasma (R a =0.62 nm) and immersion of a basic aqueous solution (R a =0.57 nm). Subsequently after the Ni 2+  adsorbing (R a =0.97 nm) and reducing steps, part (b) of  FIG. 7  indicates that ultrafine catalytic sites are heavily incorporated on the surface of the dielectric substrate, rendering a sharp rise in R a  (2.4 nm). The ultrafine (2˜4 nm), densely distributed Ni catalytic particles allow 70% coverage of the Co-based grains after plating for only 1 min and result in a significant increase in R a  to 8.1 nm (see part (c) of  FIG. 7 ). Moreover, the thickness for just forming a continuous barrier film (R a  being substantially reduced to 3.69 nm) is only ˜20 nm (see part (d) of  FIG. 7 ), in accordance with the TEM result (see part (c) of  FIG. 5 ). Generally, the seed particles, produced using the sensitization/activation, displacement, or other related processes, tend to form aggregates exceeding 20 nm. Thus, these methods are generally incapable of initiating the plating of electroless barriers of extremely fine thickness (e.g., &lt;20 nm). Given the results of the present invention, clearly, it is possible to fabricate electroless-plated metal film of thickness only 20 m or even less using this all-electrochemical integrated plating technique by properly optimizing the surface activating procedure.  
      There is no need of further plasma treatment for the surface of hydrophilic SiO 2  dielectric material. Simply it is immersed in a known basic aqueous solution for forming a negatively charged surface of the dielectric material having Si—O— thereon (see  FIG. 1 ). However, hydrophobic polysiloxanes substrate has to be processed by plasma treatment to form a hydrophilic surface. Also, the hydrophobic polysiloxanes substrate has to be immersed in a basic aqueous solution having strong oxidizing agent in order to form an OH-terminated surface. Next, a deprotonation process is performed to form a negatively charged surface having Si—O −  thereon. After using a suitable strong oxidizing agent, and performing a selective treatment on the surface by plasma or immersing the hydrophobic polysiloxanes substrate in an oxidized basic aqueous solution, the steps shown in  FIG. 3  can be utilized to form a self-aligned electroless-plated film formed of Co or Cu. The image in  FIG. 8  illustrates a self-aligned MOS capacitive structure comprising silicon based dielectric layer, Co—W—P barrier, and Cu metal film on plasma-treated areas of HOSP™ dielectric substrates using the all-electrochemical integrated deposition process, which is presented in  FIG. 3 .  
      While the present invention herein disclosed has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the present invention set forth in the claims.