Abstract:
Embodiments of the present invention provide PECVD (plasma enhanced chemical vapor deposition) processes that produce uniform, dense SiO 2  (silicon dioxide) films having a high purity that are suitable for use in IC device fabrication. Advantageously, these processes do not require the use of a DC bias or dual frequency RF power and can use some of the same precursors used to make low-k ILD films.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the production of integrated circuit (IC) device structures and the deposition of silicon dioxide (SiO 2 ) using plasma enhanced chemical vapor deposition (PECVD).  
         [0003]     2. Background Information  
         [0004]     The integration of low-k films or layers into semiconductor devices has presented challenges associated with issues of film porosity, mechanical integrity, and intercomponent reactivity. Low-k films having dielectric constants of about 3 to about 2.7 are typical of current processes. The production of integrated circuit device structures can necessitate placing a silicon dioxide (SiO 2 ) film or layer, or capping layer on the surface of low-k (low dielectric constant) ILD (inter-layer dielectric) films. Typically, the deposition of low-k ILD films occurs in a different PECVD (plasma enhanced chemical vapor deposition) tool and or reaction chamber than the PECVD tool or reaction chamber used to deposit a high quality SiO 2  films or layers.  
         [0005]     An example of a PECVD process typically used for creating a high quality SiO 2  films on semiconductor substrates is shown in  FIG. 1 . As can be seen from  FIG. 1 , silane (SiH 4 ) and nitrous oxide (N 2 O) are reacted in a plasma to deposit a SiO 2  film or layer. In this example, a RF power is applied that has both a high frequency (13.5 MHz) and a low frequency component (typically, about 1 to about 400 KHz) and an optional DC bias.  
         [0006]     Transfer of a semiconductor substrate (a wafer) between process chambers increases the expense involved in IC fabrication due in part to the decrease in fabrication rate and the increase in device failure rate. Further, a transfer between process chambers involving a vacuum break is potentially detrimental to the integrity of the interface between a SiO 2  layer and a low-k ILD layer.  
         [0007]     Additionally, the SiO 2  PECVD processes that use either a DC bias or a low frequency RF component in the plasma may damage the dielectric properties of the low-k layer.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]      FIG. 1  provides a process that can be used to deposit a silicon dioxide film by PECVD on a semiconductor substrate.  
         [0009]      FIG. 2  shows a process according to the invention that can be used to deposit a silicon dioxide film by PECVD on a semiconductor substrate.  
         [0010]      FIG. 3  graphically presents the dependence of deposition rate of a PECVD SiO 2  film on the oxygen gas precursor flow rate (sccm) and also its dependence on RF power (Watts) in a process according to an embodiment of the present invention.  
         [0011]      FIG. 4  shows a Fourier transform infrared (FTIR) spectrum of a SiO 2  film deposited using a process according to the present invention.  
         [0012]      FIGS. 5A and 5B  provide comparisons of density ( FIG. 5A ) and HF etch rate ( FIG. 5B ) between SiO 2  films produced by three different PECVD processes.  
         [0013]      FIG. 6  shows the results of dielectric constant measurements by Mercury probe of a low-k ILD film on which SiO 2  films had been deposited by two different PECVD processes and subsequently removed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     Embodiments of the present invention provide PECVD (plasma enhanced chemical vapor deposition) processes that are compatible with other integrated circuit fabrication processes and that produce SiO 2  films suitable for use in integrated circuit devices. The terms, chip, integrated circuit, monolithic device, semiconductor device, and microelectronic device, are often used interchangeably in this field. The SiO 2  films produced are suitable, for example, as capping layers and can be formed over low-k dielectric films. Typically, low-k films are considered to be any film with a dielectric constant smaller than that of SiO 2  which has a dielectric constant of about 4.0. Preferably the low-k film has a dielectric constant of less than about 3.5 and more preferably, less than about 3.0. Low-k films can be, for example, boron, phosphorous, or carbon doped silicon oxides. Carbon-doped silicon oxides can also be referred to as carbon-doped oxides (CDOs) and organo-silicate glasses (OSGs). Capping layers formed over low-k ILDs are typically a fraction of an ILD layer thick and currently about 5 to about 50 nm would be normal thickness for a capping layer, although other layer thicknesses can be created.  
         [0015]     Referring now to  FIG. 2 , a PECVD process according to an embodiment of the present invention in which a SiO 2  film is formed from a low-k precursor is illustrated schematically. Precursor molecules are capable of supplying silicon atoms to the reactive process that forms the SiO 2  film. In the process shown in  FIG. 2 , a SiO 2  film is deposited using a mixture of a silicon-organic precursor, in this case dimethyldimethoxysilane (DMDMOS, (CH 3 O) 2 Si(CH 3 ) 2 ), oxygen gas (O 2 ) (an oxidant), and nitrogen gas (N 2 ) (a carrier gas). The precursor DMDMOS can also be used to create low-k ILD films, such as for example, Applied Material&#39;s Black Diamond, ASM&#39;s Aurora ULK, and Novellus Systems&#39; Coral films. Silicon-organic precursors are vaporizable molecules that contain silicon, hydrogen, and carbon. Optionally, the silicon-organic precursor may also contain oxygen. Typical silicon-organic precursors for low-k dielectric films include, for example, octamethylcyclotetrasiloxane (OMCTS, ((CH 3 ) 2 SiO) 4 ), dimethylmethoxysilane (DEMS, (CH 3 ) 2 (CH 3 O)SiH), diethyldiethoxysilane (DEDEOS, (C 2 H 5 O) 2 Si(C 2 H 5 ) 2 ), dimethyldimethoxysilane (DEDMOS), trimethyltrimethoxysilane, methyl phenyl dimethoxysilane, diphenyl dimethoxysilane, tetramethylcyclotetrasiloxane (TMCTS, (CH 3 (H)SiO) 4 ), trimethylsilane (3MS, (CH 3 ) 3 SiH), and tetramethylsilane (4MS, (CH 3 ) 4 Si). The nitrogen gas can be used as a background (carrier) gas to dilute the precursor and oxidant gas flows. Other carrier gases could also be used in this process instead of or in addition to the N 2  gas, such as for example, Neon (Ne) gas or Argon (Ar) gas. This process could also be performed with oxidants other than O 2  or in addition to O 2 , such as for example, nitrous oxide gas (N 2 O), ozone (O 3 ), water (H 2 O), or carbon dioxide gas (CO 2 ). Additionally, vaporizable liquid weak oxidizers, such as for example, methyl, ethyl, and isopropyl alcohol, in vapor form, may be used. Advantageously, these alcohols also tend to stabilize the plasma.  
         [0016]     Advantageously, the process shown in  FIG. 2  can be run using an RF power having a single frequency component. Exemplary RF frequencies include frequencies that are harmonics of 13.5 MHz, such as for example, 13.5 MHz, 27 MHz, 40.5 MHz, and 54 MHz. The RF power can be set low enough so that nitrogen gas is not ionized, that is, undetectably low amounts of N 2   +  ions are formed. For example, the RF power can be about 300 to about 1000 Watts, preferably about 400 to about 850 Watts, and preferably about 500 to about 700 Watts. The concentration, or lack thereof, of N 2   +  ions can be verified from the distinct spectral footprint left by N 2   +  using optical emission spectroscopy (OES). Advantageously, nitrogen incorporation into a SiO 2  film produced by this low energy nitrogen plasma is negligible. Further, the process illustrated in  FIG. 2  can be run without the use of a DC bias, thus eliminating a damage mechanism for underlying components, such as, for example, a low-k ILD film.  
         [0017]     In the process generally illustrated in  FIG. 2  for forming a high quality SiO 2  film, the pressure in the reaction chamber is generally about 0.5 to about 3 Torr, preferably about 1 to about 2 Torr. The ratio of the amount of precursor, e.g., DMDMOS, to oxidant, e.g., O 2 , is about 1:7 (pressure of precursor gas to pressure of O 2  gas) and the ratio of the amount of precursor to N 2  is about 1:67 (pressure of precursor gas to pressure of N 2  gas) for the reaction to form SiO 2 . In general, these reactant ratios can range from about 1:5 to about 1:15 for pressure of precursor to pressure of oxidant and about 1:25 to about 1:150 for pressure of precursor to pressure of N 2 . Typical gas flow rates were about 20-50 sccm (standard cubic centimeters per minute) for precursor (DMDMOS), about 50-250 sccm for oxidant (O 2 ), and about 1000-4000 sccm for carrier gas (N 2 ).  
         [0018]     Further, embodiments of the present invention provide PECVD processes that allow for a range of deposition rates for the resulting high quality SiO 2  films.  FIG. 3  graphically presents the dependence of the rate of SiO 2  deposition (in Angstroms per second) on the rate of flow of O 2  gas (in sccm) into the process chamber and on RF power (in Watts). As can be seen from  FIG. 3 , a SiO 2  deposition rate can be obtained that is about 1 nm/s or less. This low deposition rate enables great control over the thickness of the resulting film and thus the use of this SiO 2  film as a capping layer. The thickness as shown in  FIG. 3  was measured using a spectroscopic ellipsometer, and the data was confirmed by X-ray reflectivity measurements. Data was collected on a 10-50 nm film deposited on silicon using conditions as described above.  
         [0019]     Referring now to  FIG. 4 , a Fourier transform infrared (FTIR) spectrum of an embodiment of the invention is presented. The FTIR spectrum in  FIG. 4  shows labeled peaks from an as-deposited PECVD SiO 2  film. As can be seen from the FTIR spectrum, peaks can be assigned to Si—O interactions and peaks from trace carbon, such as for example, signature peaks from —CH 3  end groups, which are a component of DMDMOS-based low-k ILD films, and are discernable at about 1270 cm −1 , are not seen. Similarly, peaks attributable to trace amounts of nitrogen in the SiO 2  film are not discernable in the spectrum, such as for example, no discernable peak was found at 3380 cm −1  which would correspond to a N—H bond, and no peak was discerned at 885 cm −1  which would correspond to a Si—N bond. FTIR data was collected on an Accent QS-3300ME in-fab 300 mm FTIR system in transmission mode on a 150-300 nm film deposited on silicon using process conditions as described above. Lack of nitrogen incorporation into the film was further verified with secondary ion mass spectrometry (SIMS).  
         [0020]     Embodiments of the invention provide SiO 2  films having a carbon content of less than about 0.1% and a nitrogen content of less than about 0.1%. Further, SiO 2  films are provided that have a Si to O ratio of about 1:2 plus or minus 10% (i.e., a Si to O ratio of about 0.9:2 to about 1.1:2) by weight.  
         [0021]     Density and etch rate are factors used to determine the quality of SiO 2  films. In general, a SiO 2  film should have a density that is as close as possible to the density of bulk SiO 2 , about 2.2 g/cm 3 . Measurements of density and etch rate for three films of similar thickness, a target of about 60 nm, deposited on a silicon wafer: an exemplary PECVD SiO 2  embodiment (labeled Film A), a reference high quality PECVD SiO 2  film (created from SiH 4  and N 2 O precursors) (labeled Film B), and a low density low-k ILD film (a DMDMOS-based CDO low-k film deposited on the same platform and in the same chamber as the SiO 2  capping layer) (labeled Film C) having a nominal density of 1.35 g/cm 3 , are provided in  FIGS. 5A and 5B , respectively. The magnitude of the Kiessig thickness fringes in an XRR (X-ray reflectometry) measurement is indicative of the density of the film as compared to Si.  FIG. 5A  shows the results of XRR measurements for Films A-C that yielded densities for Films A and B of 1.8 g/cm 3 .  FIG. 5B  presents results obtained from 200:1 HF (water:HF by weight:weight) etch rate measurements for Films A and B. The XRR measurements were made on a Bede 300 mm X-ray system on films of about 60 nm thickness deposited directly on a silicon substrate. In  FIG. 5B , the etch rates for the total etched thickness of Film A and Film B in 60 seconds in a 200:1 HF solution are very similar, again demonstrating the similarity between these two films. It can also be seen from  FIG. 5B  the etch rate for Film A is more linear than that of Film B, indicating that Film A possesses more through-film structural or compositional uniformity.  
         [0022]     Further evidence of compatibility for the PECVD SiO 2  films of the invention with a process requiring a SiO 2  capping layer on a low-k ILD, was provided by dielectric constant measurements of the low-k ILD film subsequent to the deposition of a PECVD SiO 2  capping layer.  FIG. 6  presents dielectric constant measurements by Mercury probe of a low-k ILD film (a DMDMOS based low-k film, Film C above) on which capping layers comprised of Film A and Film B (previously described) had been deposited and subsequently removed. Measurements were made on an SSM Mercury Probe system operating at a frequency of 100 kHz with a voltage range of −40 to −110 V. The low-k ILD film thickness was about 500 nm. The oxide cap was removed prior to testing. The process of deposition and subsequent removal of the SiO 2  layers (Film A and Film B) consisted of: (1) PECVD SiO 2  deposition (oxide); (2) hard mask (HM) film deposition; (3) 200:1 HF dip; and (4) anneal. The wafers containing the films were pulled at several points in the deposition and removal process to assess the impact of each step. It should be noted, however, that the data reflected in  FIG. 6  also reflects the effects of a hard mask deposition and a 200:1 HF dip. An about 0.6% increase in dielectric constant was found that correlates with the deposition of Film A. Although this increase in dielectric constant is small enough to be considered essentially negligible, it should be noted that, in  FIG. 6 , Film A was half the thickness of Film B and HF has a considerable effect on the low-k ILD in absence of the SiO 2  capping layer. Thus, the observed increase in dielectric constant may be more related to increased exposure of the low-k ILD to HF for the thinner Film A sample than the deposition process for Film A.  
         [0023]     Film thickness uniformity can be quantified by the standard deviation or range of the thickness of film as measured at many sites across the wafer. A useful measurement is provided by the equation: 100*(thickness range)/(mean thickness), wherein the thickness range is defined as the difference between the maximum and minimum value in a set of measurements. It is a metric used to evaluate the largest level of variation observed in a set of experimental data. Processes of the present invention can provide films that have a uniformity of at least less than 10%. The processes discussed herein provided thickness uniformities ranging from about 5 to about 7%.  
         [0024]     In general, the processes of the present invention can be run using a PECVD platform having a PECVD reaction chamber, having a generator, a low pressure control, and a proper gas delivery system for the low-k precursor and the other reactant gases selected. The processes described were run on a 300 mm ASM Eagle platform. However, tools such as, for example, 200 and 300 mm PECVD tools from Novellus Systems, Inc., and Applied Materials, Inc. could also be used.