Abstract:
A method of forming a thin film on a substrate to fabricate a microelectronic device, a microelectronic device comprising a thin film deposited according to the method, and a system comprising the microelectronic device. The thin film may include on of a low k thin film, a thin film comprising photoresist, and a sacrificial polymer. The method comprises dispersing a precursor preparation into a spray of charged droplets through subjecting the liquid precursor preparation to electrostatic forces; directing the charged droplets to move toward the substrate; and allowing the charged droplets to generate a beam of gas-phase ions as the charged droplets move toward the substrate. The method further includes directing the gas-phase ions to impinge upon the substrate to deposit the thin film thereon to yield a deposited thin film on the substrate.

Description:
TECHNICAL FIELD 
   Embodiments of the present invention relate to the field of silicon processing, particular to a method of forming a thin film on a substrate, to microelectronic device including a substrate and a thin film deposited on the substrate, and to a system incorporating a substrate having a thin film deposited thereon. 
   BACKGROUND INFORMATION 
   Fabrication processes of integrated circuits typically involve various stages for depositing thin films of various materials on the surface of a semiconductor substrate. The preparation of such thin films typically includes such methods as evaporation, chemical vapor deposition (CVD) such as plasma enhanced chemical vapor deposition (PEVCD), sputtering, and spin casting. The above deposition schemes are conventionally used as appropriate to deposit metals, silicon, polysilicon, and dielectrics such as silicon dioxide and silicon nitride on the substrate. Typical PEVCD precursors include DMDMOS (dimethyldimethoxysilane), or TOMCATS (tetramethylcyclotetrasiloxane). PEVCD tends to be limited, however, in allowing engineering latitude with respect to resulting film characteristics and types of precursor materials. 
   Alternatively, thin films may be deposited using a spin-on ultra low-k dielectric material, such as LKD-5109, a methylsilsesquioxane (MSQ) material manufactured by the JSR Corporation. Although high yields are possible using an ultralow-k material, in some cases it has been necessary to use thicker passivation and an additional oxide layer on top of the structure to deliver a more mechanically stable stack, in this way driving up fabrication complexity and cost, and further leading to a deterioration of the dielectric constant of the resulting film stack by virtue of the additional oxide layer. Moreover, similar to PEVCD, the spin-on technique allows limited resulting characteristics (e.g. toughness). 
   Electrospray (ES) ionization, a CVD technology in which a conductive liquid is volatilized in transit to the growth front, is also known as a method for depositing molecules onto relevant surfaces. The deposition of pure complex molecules on semiconductor surfaces under ultra high vacuum conditions using ES to allow an exploration of the interaction of the deposited molecules with the surface and with each other is also known. ES is typically used as an ionization technique for mass spectrometry, especially for the analysis of compounds of biological significance. ES has also been used for the deposition of protein thin films, for the deposition of ceramic thin films, and for the deposition of ferroelectrics. ES has also been disclosed along with a pretreatment of a substrate with radio frequency (RF) plasma. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the manner in which embodiments of the present invention are obtained, a more particular description of the same will be rendered by referring to the appended drawings. The drawings are not necessarily to scale, and are not to be considered to be limiting in scope. In the appended drawings: 
       FIG. 1  is a schematic view of an electrospray thin film deposition device (ETFDD) during deposition according to an embodiment of the present invention showing the device in operation; 
       FIG. 2  is a diagram illustrating a charge residue model and an ion desorption model for the formation of solute ions from charges droplets during electrospray ionization; 
       FIG. 3  is a schematic view of an ETFDD during deposition according to another embodiment of the present invention showing the device in operation; and 
       FIG. 4  is a schematic view of an ETFDD according to yet another embodiment of the present invention showing the device in operation; 
       FIG. 5  is a schematic depiction of a system including a microelectronic device fabricated according to embodiments of the present invention; 
       FIG. 6  is a schematic representation of a package for a microelectronic device, the package including substrate including a deposited thin film thereon according to an embodiment of the present invention; 
       FIG. 7  is a schematic top view of an experimental ETFDD using principles according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention provide among others a novel method of forming a thin film, such as, by way of example and not limitation, a low k thin film, on a semiconductor substrate. In the following description, numerous specific details are set forth such as process steps, materials, dimensions, etc., in order to provide a thorough understanding of embodiments the present invention. However, it will be obvious to one skilled in the area that embodiments of the present invention may be practiced without these specific details. 
   Throughout the instant description, the term substrate includes not only a semiconductor substrate, but also any and all layers and structures fabricated over the semiconductor substrate up to the point of processing under discussion. For example, a “substrate” as referred to herein may include one or more structures such as active elements and passive elements including polysilicon gates, wordlines, source regions, drain regions, bit lines, bases, emitters, collectors, conductive lines, conductive plugs, diffusion regions, quantum dots, squids, etc. 
   In addition, as used in the instant description, “precursor preparation” refers to either a solution and/or suspension containing one or more precursors as applicable. “Precursor dispersion” refers to the dispersion of droplets and of gas-phase ions formed by virtue of the electrospray process. 
   Although the description that follows is focused primarily on low k thin film deposition, embodiments of the present invention encompass within their scope the use of ES or enhanced electrospray (EES) for the deposition of other types of thin films, such as, for example, high k dielectric materials, semiconductive materials, conductive and semiconductive organic materials, photoresists and sacrificial polymers to name just a few. 
   Current PEVCD precursor designs tend to be limited in allowing a deposition of low k dielectric layers that exhibits desirable physical and chemical properties. Low molecular weight (i.e. low P vap ) precursors typically used in PEVCD are limited in their structural complexity, thus allowing limited engineering latitude in PEVCD film characteristics resulting from their application. In particular, because PEVCD requires low molecular weight precursors suitable for vapor-phase processing, PEVCD resulting films have limitations with respect to their mechanical strength as well as to their dielectric constant. 
   Advantageously, embodiments of the present invention allow the use of precursors having larger molecular weights than those typically employed in PEVCD, including organic, inorganic and organometallic molecules and clusters, in this way making available a wider portfolio of precursors for film formation. Higher molecular weight precursors in turn allow the design of more structurally precise and complex thin films, enabling further engineering latitude with respect to the properties of the films to be deposited. 
   Referring now to the figures, in an electrospray thin film deposition device ETFDD according to embodiments of the present invention, such as in ETFDD  10  depicted in  FIG. 1 , a precursor preparation  12  is supplied at a controlled rate to a capillary tube  14  having a small-diameter tip  16  held at high potential by the presence of electrodes  18  positioned at the tip  16 . A counter-electrode  19  in the form of a plate may be held at ground potential and attracts the precursor dispersion at the location of the substrate. Precursor preparation  12  may for example be supplied under a mild positive pressure (that is, a pressure sufficient to induce a flow of between 0.1 and 10 microliters per second) to the orifice of capillary tube  14  using an external pumping system. The high potential at tip  16  creates an electrostatic field that induces a surface charge in the liquid precursor preparation  12  emerging from tube  14 . Electrostatic or “Coulomb” forces disperse the liquid into a fine spray  24  of charged droplets, which are accelerated and become a nanoscopic dispersion beam of gas-phase ions  26  before they reach the substrate  28  on which the low k thin film is to be deposited. The combination of droplets  24  and gas phase ions  26  will collectively be referred in the instant description as “precursor dispersion.” Nanoscopic dispersion beam  26  is directed to impinge upon substrate  28  to form the low k thin film thereon. The ES deposition occurs in a vacuum setting, such as in a vacuum chamber  30  defined by a vacuum chamber housing  32  as would be recognized by a person skilled in the art. Optionally, according to an embodiment of the present invention, a part of capillary tube  14  may be heated with a conventional heat source  34  as shown in broken lines in  FIG. 1 . A heating of tube  14  tends to facilitate droplet evaporation according to application needs. 
   As recognized by a person skilled in the art, electrospray techniques allow the possibility of generating gas-phase ions by spraying a solution from the tip of an electrically charged capillary.  FIG. 2  is a diagram illustrating a charge residue model and an ion desorption model for the formation of solute ions from charges droplets during electrospray ionization, such as, for example, with ETFDD  10  shown in the embodiments of  FIGS. 1 ,  4  and  5  (the embodiments of  FIGS. 4 and 5  being described in further detail in the paragraphs below).  FIG. 2  in fact shows the droplets during various stages of their transformation into gas-phase ions. In describing the features illustrated in  FIG. 2 , the instant description will make reference to ETFDD shown in  FIG. 1 , although it is to be understood that the principles sought to be depicted in the diagram of  FIG. 2  apply to electrospray ionization in general. As shown in  FIG. 2 , to produce the spray, each droplet is charged near the Raleigh limit (at which point electrostatic repulsion overcomes surface tension). Within the capillary tube  14 , the high potential charges the liquid emerging at the capillary tip  16 , such that the liquid is dispersed into fine spray  24  of charged droplets. As solvent evaporates from each droplet, charge density on the droplet surface increases until the Rayleigh limit is reached. The resulting instability causes the droplet to disintegrate into smaller droplets. Thus, the charged liquid in the nozzle becomes unstable as it is forced to hold increasing charge, and blows apart into very small (typically less than 10 microns in diameter), highly charged droplets. These highly charged droplets rapidly shrink as solvent molecules evaporate from their surface, decreasing a distance between the electrical charges in each droplet. The above decrease in distance eventually results in the shrunken droplet to blow apart again. There is no consensus in the art as to the mechanism that leads to the formation of solute ions from the charged droplets. According to what is commonly known in the art as the “Charge Residue Model,” eventually, the shrinkage and breaking apart of the droplets result in solute gas phase ions. According to what is commonly known in the art as the “Ion Desorption Model,” a droplet shrinks until its surface electric field becomes large enough to lift an ion at the surface of the droplet over the energy barrier that prevents its escape. Id. In either case, a gas-phase ion is formed from the charged droplets, thus generating a nanoscopic dispersion beam such as nanoscopic dispersion beam  26  shown in  FIG. 1 . According to embodiments of the present invention, nanoscopic dispersion beam  26  is directed onto substrate  28  and is adsorbed thereon, in this way forming a low k thin film deposited on the substrate. 
   Optionally, according to embodiments of the present invention, the precursor dispersion obtained from ES, including droplets  24  and ions  26 , may undergo enhanced activation, exciting the molecular orbitals of the gas phase molecules, promoting the electrons in the molecules to an excited state. When this state is high enough in energy to overcome bond enthalpy, bond scission occurs, creating a reactive intermediate. The reactive intermediate is then allowed to react with either the substrate  28  and/or with the molecules in the existing precursor dispersion to form new bonds. According to embodiments of the invention, enhanced activation of the precursor dispersion, may, for example, take the form of: (1) plasma activation; and (2) activation by irradiation. For example, species possessing moieties capable of absorbing radiation (such as, for example chromophores) may undergo excitation to reactive intermediates using enhanced activation by irradiation. In the case of plasma activation, the moiety expressed on the precursor species would be susceptible to interaction with plasma, for example, if it possessed bonds matched in energy to the plasma species of exposure. A description of each of the above exemplary forms of enhanced activation of the precursor dispersion is provided below in connection with  FIGS. 1 ,  3  and  4 . 
   An electrospray deposition which makes use of enhanced activation of the precursor dispersion according to embodiments of the present invention can be characterized as enhanced electrospray deposition, hereinafter referred to as “EES deposition.” EES deposition advantageously allows additional control of deposition uniformity and rate, allowing modulation of layer formation. 
   Referring to  FIG. 3 , an ETFDD allowing EES through irradiation is illustrated according to an embodiment of the present invention. In  FIG. 3 , like elements have been indicated with reference numerals identical to those used in the ETFDD of the embodiment of  FIG. 1 . Here, in order to further activate the precursor dispersion made up of droplets  24  and of ions  26 , irradiation  36  from a radiation source  38  is admitted to the chamber and directed toward the precursor dispersion. Preferably, irradiation  36  is admitted into the precursor dispersion region via a transparent window  40 . By way of example, the transparent window could be a window made of quartz for 365-193 nm irradiation. According to embodiments of the present invention, by way of example and not limitation, irradiation types could include broad or narrow band UV (for example, Hg vapor arc, deuterium lamp, and laser), IR, electron beam, ion beam (for example, He, Ar, H, Si), and/or X-ray. The irradiation according to embodiments of the present invention could by way of example be a 337, 248 or 193 nm laser or electron beam delivered at between about 10 to about 10,000 Watts. Pulsed irradiation, such as, for example, a 337 nm, 100 Watt, 5 ns pulsed laser irradiation could also be used. Functionality of precursors subjected to enhanced activation involving irradiation would include groups that would photochemically fragment, such as, for example, triphenylsulfonium, or groups that would form radicals, such as, for example, benzophenone, or groups that form carbenes or nitrenes, such as, for example, groups having diazo or azide functionality. 
   Referring next to  FIG. 5 , an ETFDD allowing EES through plasma activation is illustrated according to an embodiment of the present invention. In  FIG. 4 , like elements have been indicated with reference numerals identical to those used in the ETFDD of the embodiment of  FIG. 1 . Here, in order to further activate the precursor dispersion made up of droplets  24  and of ions  26 , an inductively coupled plasma region  42  may be created through the use of RF coils  44 . Optionally, a collimator  46 , preferably held at ground potential, may be used in the path of the precursor dispersion toward the substrate in order to focus the ionized molecular species and to allow additional control of deposition uniformity and rate. A frequency of plasma excitation for RF according to embodiments of the present invention could range from about 3 MHz to about 10 GHz, the range from about 10 to about 100 MHz being preferred for the excitation of HF plasma, and the range from about 1 GHz to about 10 GHz being preferred for the excitation of microwave plasma. In the alternative, embodiments of the present invention further include within their scope the use of a capacitively coupled plasma region for enhanced activation of the precursor dispersion. 
   Precursor preparations useful in embodiments of the present invention, such as the embodiments depicted in  FIGS. 1 ,  3  and  4  could comprise species dissolved or suspended (such as a micro-emulsion, a colloid, etc) in an electrolyte fluid medium that undergoes ionization in an electrospray nozzle (such as the capillary and tip described above) to form a dispersion of molecular sized fragments. Precursor preparations useful in embodiments of the present invention could include, by way of example and not limitation, alicyclic cage hydrocarbons with silicon functional groups, siloxanes, oligo-siloxanes, silica nanoclusters, carbon nanoclusters (such as, for example, diamondoids and fullerines), and/or about 1% to about 25% by weight of molecular and molecular cluster feedstocks in a solution where the solvent includes alcohol, water, acetonitrille, dimethylformamide, DMSO, NMP and/or mixtures thereof. The alcohol could include, by way of example, methanol, ethanol, propane, or butane. 
   Additional functionality could facilitate binding, such as the functionality provided by groups susceptible to cross-linking (for example olefin or epoxide, aldehyde, sulfide, cyclopropane, ketone, oxetane, cyclobutene, acylsilane, silylhalide, acid halide, nitrille, etc.) Surfactant may be added from 1 ppb-1 ppt to disperse the precursor in the solvent and to provide electrolyte for ES. These surfactants could include hydrocarbon sulfonates, carboxylates and/or ammonium salts. 
   According to embodiments of the present invention, carrier gases used to maintain chamber pressure could include He, Ar, H2, Ne, N2, an H2/N2 mixture, methane, butane, nitrous oxide, and/or NH3. 
   Other process parameters useful in practicing a method according to embodiments of the present invention could include, by way of example and not limitation: a spray voltage (i.e. the potential applied between electrodes  18  and substrate  28  to induce the electrospray effect) of about 1000 to about 10000 Volts, with the rage between about 2000 and about 5000 Volts being preferred; a chamber pressure in the range between about 0.01 to about 10 Torr, with the range between about 0.1 to about 1 Torr being preferred; a chamber temperature between about 0 to about 600° C., with the range between about 20 to about 30° C. being preferred; plasma power, that is, the energy applied to the inductive or capacitive coupling between the deposition chamber and the radio frequency power supply (operational under the plasma enhanced condition) between about 1 to about 1000 Watts, with the range between about 50 to about 100 Watts being preferred; and a discharge tip having a diameter in the range between about 10 to about 500 microns. 
   According to an embodiment of the present invention, the substrate onto which one or more thin films according to embodiments of the present invention could be deposited could be subjected to enhanced activation during thin film deposition using well known plasma and/or radiation techniques similar to the ones described above with respect to enhanced activation of the precursor dispersion. Enhanced activation of the substrate would occur according to a mechanism similar to the one described above with respect to enhanced activation of the precursor dispersion. In this case, the deposited thin film would adsorb the energy from enhanced activation, resulting in the formation of reactive intermediates, which would in turn then react with either the substrate or with further precursors being deposited to form new bonds. The above would further induce film formation and allow control of the film morphology. 
   Referring again to  FIG. 3 , for example, irradiation source  38  could be adjusted to have variable angles of incidence in order to allow the irradiation of the precursor dispersion as shown, of the substrate (as shown by the thicker broken irradiation lines emanating from irradiation source  38  toward the substrate in  FIG. 3 ), or of both the precursor dispersion and of the substrate (again as shown in  FIG. 3 ). Optionally, an additional irradiation source  48  could be disposed in the vacuum chamber housing  32  in order to direct further radiation  50  toward the substrate  28  as shown in  FIG. 3 . The embodiment of ETFDD  10  shown in  FIG. 3  would thus allow an irradiation of the precursor dispersion, an irradiation of the substrate, or an irradiation of both simultaneously during deposition. Optionally, the substrate could be subjected to patterned irradiation to activate pre-selected regions thereof according to application needs. In the alternative, the substrate may be subjected to enhanced activation during deposition using well known plasma techniques. 
   According to one embodiment, a multi-chamber process may be used as part of the electrospray technique mentioned above in order to optimize the characteristics of the deposited layers. For example, electrospray may be performed to deposit several separate layers in respective deposition chambers, the conditions of each chamber being optimized based on the desired characteristics of the layer to be deposited via electrospray within that chamber. In the alternative, a multi-chamber process could include electrospray deposition in one or more chambers as described above, followed by additional processing, such as etching back, in subsequent chambers. The latter is sometimes performed to toughen the resulting film or to render the same more uniform. 
   After deposition of the thin film as described for example with respect to the embodiments of  FIGS. 1 ,  3  and  4 , with or without enhanced activation of the precursor dispersion and/or substrate, various subsequent processing may be performed according to embodiments of the present invention. Optionally, the deposited thin film may undergo post-treatment to induce beneficial changes to the integration and performance of the deposited material. Post-treatment could include, for example, any of the following well-known techniques: (1) a removal of the hydrocarbon functionality of a hydrocarbon substituted silicon-based precursor (or “substituted precursor”) to form a low k thin film with greater porosity and a lower dielectric constant; (2) skin formation and/or passivation that provide a relatively short penetration length of heat and/or radiation to densify a few angstroms of film thickness; and/or (3) backfilling with materials to fill pores, depending on a given application, although this third possible post treatment procedure would tend to reduce effectiveness with respect to the dielectric constant. However, the latter procedure could be useful according to the morphology of the deposited film. 
   According to embodiments of the present invention, removal of the hydrocarbon functionality of a substituted precursor may occur by either of the well known techniques of thermal decomposition of the substituted precursor, selective removal of the substituted precursor using solvents or supercritical carbon dioxide (CO 2 ), exposure to irradiation (electron-beam, X-ray, ultraviolet (UV), infrared (IR), microwave, or the like), or otherwise. 
   Referring next to  FIG. 5 , there is illustrated one of many possible systems in which embodiments of the present invention may be used. According to one embodiment of the present invention, system  90  includes electronic assembly  100  which in turn may include a microprocessor including a die having one or more thin films thereon, such as a low k thin film thereon, deposited according to embodiments of the present invention. In an alternate embodiment, the electronic assembly  100  may include an application specific IC (ASIC). Integrated circuits found in chipsets (e.g., graphics, sound, and control chipsets) may also be packaged in accordance with embodiments of this invention. 
   For the embodiment depicted by  FIG. 5 , the system  90  may also include a main memory  102 , a graphics processor  104 , a mass storage device  106 , and/or an input/output module  108  coupled to each other by way of a bus  110 , as shown. Examples of the memory  102  include but are not limited to static random access memory (SRAM) and dynamic random access memory (DRAM). Examples of the mass storage device  106  include but are not limited to a hard disk drive, a compact disk drive (CD), a digital versatile disk drive (DVD), and so forth. Examples of the input/output module  108  include but are not limited to a keyboard, cursor control arrangements, a display, a network interface, and so forth. Examples of the bus  110  include but are not limited to a peripheral control interface (PCI) bus, and Industry Standard Architecture (ISA) bus, and so forth. In various embodiments, the system  90  may be a wireless mobile phone, a personal digital assistant, a pocket PC, a tablet PC, a notebook PC, a desktop computer, a set-top box, a media-center PC, a DVD player, and a server. It may also be a sub system such as a video graphics card, a memory element (SD, MMC, Memory Stick, etc.), a microcontroller and a risk coprocessor. 
   Referring next to  FIG. 6 , a package  200  for a microelectronic device is shown comprising a substrate  210  and a thin film  220  adsorbed on the substrate according to embodiments of the present invention using ES or EES as described above. The thin film  220 , according to embodiments of the present invention, exhibits a morphology reflecting a precursor preparation comprising at least one of inorganic clusters, organometallic clusters, and precursor molecules having a molecular weight above about 500 Daltons. 
   EXAMPLE 
   Referring to  FIG. 7 , a prototype ETFDD was built and a coarse film of mixed ionic and anionic surfactants deposited on a substrate. The ES injector  300  used included a 10 microliter syringe  310 , a stainless steel needle  320  having an orifice diameter of about 100 microns, a polyethylene sleeve  330  and a brass electrode  340 . Plasma discharge centered around the injection orifice of the needle. The pressure of the glass chamber  360  was adjusted to about 1 Torr of air. Common detergent was feedstock as an aqueous solution of about 1%. Plasma  400  was formed in glass chamber  360  having a capacity of about 4 liters equipped with an aluminum bottom  380  and brass top electrodes energized at about 29 MHz with about 80 Watts of forward power with a standing wave ratio of 1.9:1 with capacitive coupling directly to the interior of the glass chamber. Electrospray was performed at about 2 kV with 100 micron diameter orifice (stainless) 10 microliter syringe interfaced with the chamber  360  through a silicone septum and polyethylene sleeve  330  inserted about the center of the top brass electrode. 
   The above experiment proved successful as long as ES flow rate was kept slow enough, that is, at about 1 microliter per 10 seconds in 0.5 microliter aliquots, to maintain pressure limits required to sustain the plasma. The experiment resulted in a water soluble surfactant film being formed on the glass substrate, positioned in the same manner as glass substrate  28  in  FIGS. 1 ,  3  and  4 . 
   The present invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident to persons having the benefit of this disclosure that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the present invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.