Patent Application
20090294885
1. Field of the Invention
This invention generally relates to the fabrication of integrated circuit (IC) photodetectors, and more particularly, to a photodetector made from a silicon (Si) nanoparticle embedded insulating film, using a high-density plasma-enhanced chemical vapor deposition process.
2. Description of the Related Art
The fabrication of integrated optical devices involves the deposition of materials with suitable optical characteristics such as absorption, transmission, and spectral response. Thin-film fabrication techniques can produce diverse optical thin films, which are suitable for the production of large area devices at high throughput and yield. Some optical parameters of importance include refractive index and the optical band-gap, which dictate the transmission and reflection characteristics of the thin film.
Typically, bilayer or multilayer stack thin-films are required for the fabrication of optical devices with the desired optical effect. Various combinations of the metal, dielectric, and/or semiconductor layers are also used to form multilayer films with the desired optical characteristics. The selection of the material depends on the target reflection, transmission, and absorption characteristics. While a single layer device would obviously be more desirable, no single thin-film material has been able to provide the wide range of optical dispersion characteristics required to get the desired optical absorption, band-gap, refractive index, reflection, or transmission over a wide optical range extending from ultraviolet (UV) to far infrared (IR) frequencies.
Silicon is the material of choice for the fabrication of optoelectronic devices because of well-developed processing technology. However, the indirect band-gap makes it an inefficient material for optoelectronic devices. Over the years, various R&D efforts have focused on tailoring the optical function of Si to realize Si-based optoelectronics. The achievement of efficient room temperature light emission from the crystalline silicon is a major step towards the achievement of fully Si-based optoelectronics.
At present, the Si thin film-based photodetectors operating at wavelengths shorter than 850 nm are attractive for low cost, highly integrated CMOS devices. Si is an indirect bandgap semiconductor with limited speed-responsivity performance, but it is still useful for detection in UV-VIS (visible)-NIR (near-IR) spectrum. However, the indirect bandgap of Si limits the critical wavelength of Si to 1.12 μm, beyond which its absorption goes to zero, making it insensitive to two primary telecommunication wavelengths of 1.30 and 1.55 μm. An additional issue with Si based photo-detectors is the dark current limiting the signal-to-noise ratio (SNR), and the thermal instability at operating temperatures higher than 50° C.
The fabrication of stable and reliable optoelectronic devices requires Si nanocrystals with high photoluminescence (PL) and electroluminescence (EL) quantum efficiency. One approach that is being actively pursued for integrated optoelectronic devices is the fabrication of SiOx (x≦2) thin films with embedded Si nanocrystals. The luminescence due to recombination of the electron-hole pairs confined in Si nanocrystals depends strongly on the nanocrystal size. The electrical and optical properties of the nanocrystalline Si embedded SiOxNy thin films depend on the size, concentration, and distribution of the Si nanocrystals. Various thin-film deposition techniques such as sputtering and plasma-enhanced chemical vapor deposition (PECVD), employing a capacitively-coupled plasma source, are being investigated for the fabrication of stable and reliable nanocrystalline Si thin films, which are also referred to herein as nanocrystalline Si embedded insulating thin films.
However, conventional PECVD and sputtering techniques have the limitations of low plasma density, inefficient power coupling to the plasma, low ion/neutral ratio, and uncontrolled bulk, and interface damage due to high ion bombardment energy. Therefore, the oxide films formed from a conventional capacitively-coupled plasma (CCP) generated plasma may create reliability issues due to the high bombardment energy of the impinging ionic species. It is important to control or minimize any plasma-induced bulk or interface damage. However, it is not possible to efficiently control the ion energy using the radio frequency (RF) power of CCP generated plasma. Any attempt to enhance the reaction kinetics by increasing the applied power results in increased bombardment of the deposited film, creating a poor quality films with a high defect concentration. Additionally, the low plasma density associated with these types of sources (˜1×108-109 cm−3) leads to limited reaction possibilities in the plasma and on the film surface, inefficient generation of active radicals and ions for enhanced process kinetics, inefficient oxidation, and process and system induced impurities, which limits their usefulness in the fabrication of low-temperature electronic devices.
A deposition process that offers a more extended processing range and enhanced plasma characteristics than conventional plasma-based techniques, such as sputtering, PECVD, etc., is required to generate and control the particle size for PL and electroluminescent (EL) based device development. A process that can enhance plasma density and minimize plasma bombardment will ensure the growth of high quality films without plasma-induced microstructural damage. A process that can offer the possibility of controlling the interface and bulk quality of the films independently will enable the fabrication of high performance and high reliability electronic devices. A plasma process that can efficiently generate the active plasma species, radicals and ions, will enable noble thin film development with controlled process and property control.
For the fabrication of high quality SiOx thin films, the oxidation of a grown film is also critical to ensure high quality insulating layer across the nanocrystalline Si particles. A process that can generate active oxygen radicals at high concentrations will ensure the effective passivation of the Si nanoparticles (nc-Si) in the surrounding oxide matrix. A plasma process that can minimize plasma-induced damage will enable the formation of a high quality interface that is critical for the fabrication of high quality devices. Low thermal budget efficient oxidation and hydrogenation processes are critical and will be significant for the processing of high quality optoelectronic devices. The higher temperature thermal processes can interfere with the other device layers and they are not suitable in terms of efficiency and thermal budget, due to the lower reactivity of the thermally activated species. Additionally, a plasma process which can provide a more complete solution and capability in terms of growth/deposition of novel film structures, oxidation, hydrogenation, particle size creation and control, and independent control of plasma density and ion energy, and large area processing is desired for the development of high performance optoelectronic devices. Also, it is important to correlate the plasma process with the thin film properties as the various plasma parameters dictate the thin film properties and the desired film quality depends on the target application. Some of the key plasma and thin-film characteristics that depend on the target application are deposition rate, substrate temperature, thermal budget, density, microstructure, interface quality, impurities, plasma-induced damage, state of the plasma generated active species (radicals/ions), plasma potential, process and system scaling, and electrical quality and reliability. A correlation among these parameters is critical to evaluate the film quality as the process map will dictate the film quality for the target application. It may not be possible to learn or develop thin-films by just extending the processes developed in low density plasma or other high-density plasma systems, as the plasma energy, composition (radical to ions), plasma potential, electron temperature, and thermal conditions correlate differently depending on the process map.
Low temperatures are generally desirable in liquid crystal display (LCD) manufacture, where large-scale devices are formed on transparent glass, quartz, or plastic substrate. These transparent substrates can be damaged when exposed to temperatures exceeding 650 degrees C. To address this temperature issue, low-temperature Si oxidation processes have been developed. These processes use a high-density plasma source such as an inductively coupled plasma (ICP) source, and are able to form Si oxide with a quality comparable to 1200 degree C. thermal oxidation methods.
It would be advantageous if the benefits realized with high-density plasma Si-containing films could be used in the fabrication of photodetectors made from semiconductor nanoparticle embedded Si insulating films. As used herein, a Si insulating film is an insulating film with Si as one of the constituent elements.
The present invention describes a photodetector made from semiconductor nanoparticles (e.g., nc-Si) embedded Si insulating films, such as SiOxNy thin films. The nc-semiconductor particles embedded in the insulating matrix generate high photo-current at low reverse biases. The high SNR of the nc-semiconductor embedded Si insulating thin films overcome the limitations of conventional Si and wide-band gap semiconductor-based photodetectors. The photoconduction in the nc-semiconductor embedded Si insulating thin films makes possible metal-film-metal (MFM) photodetectors, which offer the unique advantages of high sensitivity-bandwidth product, low capacitance, and ease of integration. The photo-response and the current conduction in nc-semiconductor embedded Si insulating thin films can be controlled over a broad range by varying the particle size and distribution, particle density, inter-particle distance, optical dispersion, and film composition. In fabricating the semiconductor nanoparticles embedded Si insulating films, a low temperature, high density plasma (HDP)-based process is described.
Accordingly, a method is provided for fabricating a semiconductor nanoparticle embedded Si insulating film for photo-detection applications. The method provides a bottom electrode and introduces a semiconductor precursor and hydrogen. A thin-film is deposited overlying the substrate, using a high density (HD) plasma-enhanced chemical vapor deposition (PECVD) process. As a result, a semiconductor nanoparticle embedded Si insulating film is formed, where the Si insulating film includes either N or C elements. For example, the Si insulating film may be a non-stoichiometric SiOXNy thin-film, where (X+Y<2 and Y>0), or SiCX, where X<1. The semiconductor nanoparticles are either Si or Ge.
In one aspect, the method heats the substrate to a temperature of less than about 400° C., and the thin-film HD PECVD process uses a plasma concentration of greater than 1×1011 cm−3, with an electron temperature of less than 10 eV. Following the formation of the semiconductor nanoparticle embedded Si insulating film, an annealing process is performed. In one aspect, a heat source is used having a radiation wavelength of about 200 to 600 nanometers (nm) or 9 to 11 micrometers. In another aspect, an HD plasma treatment is performed in an H2 atmosphere, using a substrate temperature of less than 400° C., hydrogenating the semiconductor nanoparticle embedded Si insulating film.
Additional details of the above-described method and a photodetector employing a semiconductor nanoparticle embedded insulating film are presented below.
The semiconductor nanoparticles embedded in the Si insulating film 104 have a diameter in the range of about 1 to 10 nanometers (nm), and are made from either Si or Ge. The semiconductor nanoparticle embedded Si insulating film 104 exhibits a spectral response in a wavelength range of about 200 nanometers (nm) to about 1600 nm. A transparent electrode 106, such an indium tin oxide (ITO) or a thin metal, overlies the insulating film.
The photo-conduction in nc-semiconductor embedded Si insulator thin films overcomes the major limitations of Si and wide hand gap (WBG) semiconductor-based photo-detectors, using charge generation and conduction by the nc-semiconductor particles in a dielectric matrix. The enhanced performance is due to various control variables which are not available with Si or WBG semiconductor-based photodetector (PD) devices whose characteristics are dominantly defined by the material characteristics.
The photodetector performance, spectral-response, and the electrical conduction of the nc-semiconductor embedded Si insulating thin films can be tuned over a wide range by varying the particle size and distribution, particle density, inter-particle distance, optical dispersion, film composition, and doping. The HDP technique is suitable for the fabrication of high performance thin films at low temperatures due to enhanced plasma characteristics (high plasma density, low plasma potential, and independent control of ion energy and density) compared to conventional plasma based techniques. The present invention describes a method for creating uniform particle distribution across the film thickness, irrespective of the thickness, which is not achievable by other approaches for nc-semiconductor particle formation such as ion implantation. While it is difficult to quantify uniform particle distribution effectively, the uniformity of distribution is a clear advantage associated with the in-situ creation of nc-Si particles, as compared to the Si ion implantation approach for nc-Si creation in an insulating matrix.
As explained in more detail below, and as presented in pending patent application NON-STOICHIOMETRIC SiNxOy OPTICAL FILTERS, invented by Joshi et al., filed Apr. 26, 2007, Ser. No. 11/789,947, Attorney Docket No. SLA8118, which is incorporated herein by reference, HDP plasma processed semiconductor nanoparticles embedded Si insulating thin films show a wide optical dispersion depending on the processing conditions. It is possible to vary the refractive index and the extinction constant of the films. In addition, the HDP plasma process enables the independent control of the n and k values, which can be successfully exploited for the fabrication of devices with wide process margins, and a significant reduction in process complexity and cost.
The selection of the thin films for optoelectronic applications depends on the optical, electrical, mechanical, and chemical properties. The selection of the fabrication technique and deposition process is equally important for the fabrication of high quality thin films. Various thin film characteristics such as microstructure, grain size, composition, density, defects and impurities, structural homogeneity, and interfacial characteristics are strongly influenced by the deposition technique and process parameters.
As used herein, a nc-Si embedded SiOxNy (x+y<2) thin film is also referred to as a non-stoichiometric SiOXNY thin-film, where (X+Y<2 and Y>0). A non-stoichiometric SiOXNY thin-film, as used herein, is understood to be a film with nanocrystalline (nc) Si particles, and may also be referred to as a Si-rich SiOXNY thin-film. The term “non-stoichiometric” as used herein retains the meaning conventionally understood in the art as a chemical compound with an elemental composition that cannot be represented by a ratio of well-defined natural numbers and is, therefore, in violation of the law of definite proportions. Conventionally, a non-stoichiometric compound is a solid that is understood to include random defects, resulting in the deficiency of one element. Since the compound needs to be overall electrically neutral, the missing atom's charge requires compensation in the charge for another atom in the compound, by either changing the oxidation state, or by replacing it with an atom of a different element with a different charge. More particularly, the “defect” in a non-stoichiometric SiOXNY involves nanocrystalline particles.
The HDP technique is suitable for the fabrication of high quality thin films due to high plasma density, low plasma potential, and independent control of plasma energy and density. The HDP technique is also attractive for the fabrication high quality films with minimal process or system induced impurity content. The HDP processed films exhibit superior bulk and interfacial characteristics due to minimal plasma induced structural damage and process-induced impurities, as compared to conventional plasma based deposition techniques such sputtering, ion beam deposition, capacitively-coupled plasma (CCP) source based PECVD, and hot-wire CVD. The present invention describes an HDP process for the creation of nano-semiconductor particles in Si insulating films in the as-deposited state. The nc-semiconductor particle concentration can be further enhanced by post-deposition annealing. The electrical conductivity, photo-response, photoluminescence (PL), and electroluminescence (EL) characteristics can be improved by defect passivation treatments. The HDP processed nc-semiconductor embedded Si insulating films have tunable optical dispersion characteristics which can be exploited for the fabrication of optoelectronic devices.
Another significant aspect of the nc-semiconductor embedded Si insulating films is significant PL emission in the visible part of the spectrum, which can be used for the fabrication of active optical devices exhibiting signal gain and wavelength tuning. The optical characteristics of the HDP processed thin films can be further tuned by doping suitable impurities to control the optical response extending on either side of the visible spectrum, i.e., deep UV to far IR. The HDP technique is also suitable for low temperature and low thermal budget defect passivation of the films for an enhanced electrical and optical response.
One interesting feature of the HDP system is that there are no inductive coils exposed to the plasma, which eliminates any source-induced impurities. The power to the top and bottom electrodes can be controlled independently. There is no need to adjust the system body potential using a variable capacitor, as the electrodes are not exposed to the plasma. That is, there is no crosstalk between the top and bottom electrode powers, and the plasma potential is low, typically less than 20 V. System body potential is a floating type of potential, dependent on the system design and the nature of the power coupling.
The HDP tool is a true high-density plasma process with an electron concentration of greater than 1×1011 cm−3, and the electron temperature is less than 10 eV. There is no need to maintain a bias differential between the capacitor connected to the top electrode and the system body, as in many high-density plasma systems and conventional designs such as capacitively-coupled plasma tools. Alternately stated, both the top and bottom electrodes receive RF and low frequency (LF) powers.
High quality stoichiometric SiOxNy (x+y=2) and nc-Si embedded SiOxNy (x+y<2) thin films can be processed by HDP techniques at process temperatures below 400° C. Some of the substrates that are suitable for integrated optical devices are Si, Ge, glass, quartz, SiC, GaN, SixGe1-x. The HDP processed films can be doped in-situ by adding a dopant source gas or incorporating physical sputtering source in the chamber along with the high-density PECVD setup. The optical properties of the HDP processed films can also be modified by implanting dopant species. Some typical process conditions for the fabrication of stoichiometric SiOxNy (x+y=2) and nc-Si embedded SiOxNy (x+y<2) thin films by HD-PECVD technique are listed in Table 1.
To summarize, single or multilayer structures can be made using the above-described nc-Si embedded SiOxNy (x+y<2) thin films, with control over n, k, and wavelength emission in terms of film composition, annealing treatment, passivation, and nc-particle size control. Active waveguides can be formed capable of wavelength conversion and narrowing down the wavelength spectrum. Group IV, rare earth dopants can be added to the films for wavelength control. Optical gain and birefringence can be exploited for optoelectronic applications. Enhanced optical emission control over the emitted wavelength can be obtained by doping. The nc-Si embedded SiOxNy (x+y<2) thin films can be used with a wide range of other materials. For example, optical wave-guides can be integrated with PIN diode detectors. Also, nc-Si embedded thin films can be integrated with wide band-gap semiconductors or phosphors for enhanced light emission and control. Additional details of the fabrication processes can be found in a related pending patent application entitled, HIGH DENSITY PLASMA STOICHIOMETRIC SiOxNy FILMS, invented by Pooran Joshi et al., Ser. No. 11/698,623, filed on Jan. 26, 2007, Attorney Docket No. SLA8117, which is incorporated herein by reference.
Step 1102 provides a bottom electrode. Step 1104 introduces a semiconductor precursor and hydrogen. Step 1105a heats the substrate to a temperature of less than about 400° C. Optionally, higher temperatures may be used. Step 1106 deposits a thin-film overlying the substrate, using a HD PECVD process. In one aspect, the HD PECVD process uses an inductively coupled plasma (ICP) source. In another aspect, the HD PECVD process uses a plasma concentration of greater than 1×1011 cm−3, with an electron temperature of less than 10 eV. Step 1108 forms a semiconductor nanoparticle embedded Si insulating film including either N or C elements. For example, the semiconductor nanoparticle embedded Si insulating film may be non-stoichiometric SiOXNY thin-film, where (X+Y<2 and Y>0). The optical dispersion characteristics of the non-stoichiometric SiOXNY thin-film films can also be tailored by varying the values of X and Y with respect to the thickness of the thin-film. Alternately, the semiconductor nanoparticle embedded Si insulating film may be SiCX, where X<1. The semiconductor nanoparticles are either Si or Ge.
In one aspect, supplying the semiconductor precursor and hydrogen in Step 1104 includes supplying a precursor selected from a group consisting of SinH2
In another aspect, supplying the semiconductor precursor and hydrogen in Step 1104 includes substeps. Step 1104a supplies power to a top electrode at a frequency in the range of 13.56 to 300 megahertz (MHz), and a power density of less than 10 watts per square centimeter (W/cm2). Step 1104b supplies power to a bottom electrode at a frequency in the range of 50 kilohertz to 13.56 MHz, and a power density of up to 3 W/cm2. Step 1104c uses an atmosphere pressure in the range of 1 to 500 mTorr, and Step 1104d supplies an oxygen source gas. For example, N2O, NO, O2, or O3 may be used. Then, forming the semiconductor nanoparticle embedded Si insulating film in Step 1108 includes forming a SiOXNY thin-film. In a different aspect, Step 1104e supplies an inert noble gas. In another aspect, Step 1104 supplies a nitrogen source gas such as N2 or NH3.
Alternately, if Step 1104d supplies SinH2n+2 and a C source, then Step 1108 forms a SiCX thin-film. The C source may be any suitable hydrocarbon-containing precursor. Some examples of hydrocarbon-containing precursors include alkanes (CnH2n+2), alkenes (CnH2n), alkynes (CnH2n-2), Benzene (C6H6), and Toluene (C7H8).
In one aspect, following the formation of the semiconductor nanoparticle embedded Si insulating film, Step 1110 anneals by heating the substrate to a temperature of greater than about 400° C., for a time duration in the range of about 10 to 300 minutes, and in an atmosphere including oxygen and hydrogen. Optionally, the atmosphere may also include inert gases. Then, Step 1112 modifies the size of the semiconductor nanoparticles in the SiOXNY thin-film in response to the annealing. The annealing process may use a heat source having a radiation wavelength in the range of about 150 to 600 nm, or in the range of about 9 to 11 micrometers.
In addition to, or as an alternative to the annealing process, Step 1114 performs a HD plasma treatment with the semiconductor nanoparticle embedded Si insulating film in an H2 atmosphere, using a substrate temperature of less than 400° C. Step 1116 hydrogenates the semiconductor nanoparticle embedded Si insulating film.
More particularly, Step 1116 may include the following substeps. Step 1116a supplies power to a top electrode at a frequency in the range of 13.56 to 300 MHz, and a power density of up to 10 W/cm2. Step 1116b supplies power to a bottom electrode at a frequency in the range of 50 kilohertz to 13.56 MHz, and a power density of up to 3 W/cm2. Step 1116c uses an atmosphere pressure in the range of 1 to 500 mTorr, and Step 1116d supplies H2 and an inert gas.
In a different aspect, Step 1109 optionally dopes the semiconductor nanoparticle embedded Si insulating film with a Type 3, Type 4, Type 5, or rare earth element dopant prior to a phase separation anneal in Step 1110. Alternately, Step 1113 dopes the semiconductor nanoparticle embedded Si insulating film after the phase separation annealing of Step 1110. The doping step can be executed before or after annealing. Annealing is typically required after doping to activate the dopants. Overall, annealing has two purposes: (1) to induce phase separation, and (2) activate the dopants. In response to doping, Step 1108 forms a semiconductor nanoparticle embedded Si insulating film with (modified) optical absorption or emission characteristics in the range of frequencies from deep ultraviolet (UV) to far infrared (IR). As noted above, the doping may be performed in-situ using a dopant source gas or physical sputtering source.
In addition to, or as an alternative to annealing, following the formation of the SiOXNY thin-film, Step 1120 oxidizes the non-stoichiometric SiOXNY thin-film using either a plasma or thermal oxidation process. Then, Step 1122 modifies the size of semiconductor nanoparticles in the SiOXNY thin-film in response to the oxidation process.
Photodetectors have been described that are made with semiconductor nanoparticles embedded Si insulating films. Specific examples of SiOX1NY1 thin-films and SiOX1NY1 thin-film fabrication details have been presented. Some details of other specific materials and process details have also been used to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art.
