Patent Application
20090243048
Non-volatile nanocrystal transistor memory cells use a transistor floating gate as a charge storage region, transferring charge through a tunneling barrier to nanocrystals. The electrostatic properties of a nanocrystal layer are modified, influencing a subsurface channel between source and drain in a MOS transistor to represent various logical values.
In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments, which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice embodiments of the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.
Metallic nanocrystals are used in various embodiments to replace silicon nanocrystals in nanocrystal floating gate memories. Various methods and resulting devices forming protective shells are used to create patterned nanocrystal device for use in floating gate memories and other devices.
Many different methods may be used to form the metallic nanocrystals 210, such as the use of physical vapor deposition of a thin metal layer 110, followed by a rapid thermal annealing in the 50-1200° C. temperature range. In some embodiments, rapid thermal annealing may be performed in the 200-1000° C. range in the case of Pt and Ni on oxide. In one embodiment, annealing the metal layer 110 results in formation of metallic dots, and forms the metallic nanocrystals 210, which are dispersed about the surface of oxide layer 115 such that they are physically separated from each other.
In one embodiment, the metallic nanocrystals may be formed with Pt. As an example, in the case of Pt nanocrystals with a density above 1012/cm2, the distance between 2 nanocrystals (center to center) is above 4 nm. In one embodiment, the metallic nanocrystals 210 are fairly uniformly distributed about the surface of the oxide layer 115 with a density in the 1010−1014/cm2 range and diameter ranging between 2 and 20 nm in various embodiments. The metallic nanocrystal 210 diameter is a function of the annealing time and thickness of the initial metal layer 110. For example, for Pt dots, the density could be 1012/cm2 and the diameter in the 2-10 nm range. These parameters may be varied significantly in further embodiments.
Depending on the initial thickness of the metal layer 110 (continuous or not), the silanization process can lead to silanized metallic nanocrystals for low thickness of the initial metal layer 110 or a stabilized continuous layer for higher. initial thickness. For example, in the case of Pt, an initial Pt layer with a thickness in the range 1-5 nm leads to separated nanocrystals after annealing around 400° C. and an initial metal layer 110 with a thickness around 100 nm leads to a continuous stabilized layer after annealing in the same conditions.
In one embodiment, the metallic nanocrystals 210 include a metal nobler than silicon according to Ellingham diagrams, which are plots of the free energy of formation of a metal oxide per mole of oxygen (O2) against temperature. Some example metals include but are not limited to Ni, Pt, Ag, and W. Further metals may include Ag and Au.
The metallic nanocrystals 210 are then exposed to a Silicon precursor gas, such as SiH4, Si2H6, etc., at a low temperature, such as less than approximately 450° C. This creates a layer of silicon 410 covering the exposed metallic nanocrystals 210, one of which is shown in
One approach of determining a proper thickness of the silicon layer 410 after silanization involves exposing the metallic nanocrystals 210 to an oxidant, annealing (for example 20% O2 in nitrogen) and observing with MEB that there is no coalescence of the metallic nanocrystals 210. In one embodiment, dewetting may happen at the same time as the silanization.
In one embodiment, the silanized metallic nanocrystals 210 are exposed to an oxidizing environment, resulting in oxidation of the silicon layer 410 resulting in a silicon dioxide (SiO2,) protective shell 510 as shown in cross section in
In one embodiment, the protective shell 510 is formed by exposing the metallic nanocrystals 210 to a silicon precursor gas at a temperature less than approximately 450° C. For Pt metallic nanocrystals 210, an exposition to a SiH4 flow at a temperature around 200° C. leads to the formation of a Si protective shell 510 around the metallic nanocrystals 210.
In one embodiment, the protective shell 510 may include a metal oxide having a metal similar or different to the metal used to form the metallic nanocrystals 210. The exposition of silanized metallic nanocrystals 210 formed of Ni (obtained using an exposition to a Silicon precursor at 200° C. for example) to an oxidant atmosphere at temperature above 200° C. could give a protective shell 510 formed by both Nickel oxide and Silicon oxide.
In some embodiments, the metals used for the nanocrystals are nobler than silicon or other material used to form the protective shell 510. This facilitates the oxidation of silanized nanocrystals, leading to formation of the protective shell 510 of oxide. In further embodiments, protection of the metallic nanocrystals 210 may be provided by other dielectric materials, such as nitrides or silicon with nitride for example.
In one embodiment, a device 600 illustrated in
The formation of the control oxide may be performed at a high temperature, in the 150-950° C. range (greater than 700° C. for HTO oxide deposition), and may also include oxidant precursors. The formation of the control oxide 625 may require thermal conditions which are not compatible with stability of high density and small size unprotected metallic nanocrystals 615. Without the process of embodiments of the invention, such temperatures may adversely affect non-encapsulated metallic nanocrystals, and may cause coalescence of the metallic nanocrystals 110, degrading their ability to hold a charge. When using the process of embodiments of the invention, such a temperature results in an oxide that helps maintain overall device 600 integrity and performance characteristics. The protective shells 510 serve to ensure that the metallic nanocrystals 615 maintain their integrity during formation of the control oxide, and function as desired to hold a charge.
A tunnel oxide 630 separates the patterned plurality of metallic nanocrystals 615 from substrate 610, which includes a transistor channel 635 formed in the substrate 610 opposite the tunnel oxide 630, patterned metallic nanocrystals 615 and gate 620 such that a charge on the metallic nanocrystals 615 affects the conductive properties of the transistor channel 635. Tunnel oxide 630 may have an equivalent oxide thickness in the 1-10 nm range in one embodiment and may be varied significantly in further embodiments. Especially for SiO2, the thickness may be in the 30-60 nm range. Typical materials for tunnel oxide 630 include but are not limited to SiO2, SiON, HfA1O, and HfO2. Other materials may also be used.
In one method of forming a device that includes metallic nanocrystals, the silanization or passivation (passivation includes silanization followed by reoxidation) of the metallic nanocrystals 615 before deposition of the control dielectric helps block metallic nanocrystal diffusion on the tunnel dielectric surface. The passivation in one embodiment begins with a selective deposition of silicon on the metallic nanocrystals. The deposition method may be a chemical vapor deposition with a silicon precursor such as silane (SiH4), disilane (Si2H6), trisilane (Si3H8) or other gaseous precursor of silicon. The temperature of the deposition may be selected to avoid diffusion of metal on the tunnel dielectric surface and to allow catalytic reaction between the silicon precursor and the metal. It is a compromise between temperature and pressure. One such temperature range may be above 25° C. and less than approximately 450° C. for SiH4 used on Pt dots.
In one embodiment, selective silicon deposition is performed on the metallic nanocrystals without inter-diffusion between metal and silicon. In a further embodiment, a selective silanization of the metallic nanocrystals results in the formation of a metal-Si compound by reaction between the silicon of the gaseous precursor and the metal of the nanocrystals.
Next, a selective oxidation of the silicon part present on the metallic nanocrystals encapsulates the metallic nanocrystals in a protective shell of oxide. The selective oxidation occurs when the metal of the nanocrystals is nobler than the material to be oxidized to form the shell. In the case of silicon, a silicon oxide shell is formed and may be thermodynamically stable around the metallic nanocrystals.
Several different oxidation processes may be used, such as natural air oxidation, an annealing under an oxidant atmosphere such as O2, NO2, NO, etc., or a chemical oxidation using an oxidant liquid solution that is aqueous or organic. At this point, the nanocrystals are passivated and ready to be encapsulated with a control dielectric. The control dielectric may be deposited at high temperature to form a high quality control oxide. In various embodiments, the temperature used for dielectric process may range from 150 to 900° C. For HTO, the temperature may be around or above 700° C.
In a further embodiment, a metal for the metallic nanocrystals is selected that oxidizes in ambient air (especially Ni). This leads to ensuring that no exposure to ambient air is allowed between the metallic nanocrystal formation and the beginning of silanization or passivation. In a further embodiment, the metallic nanocrystals may be formed with several different metal alloys, such as PtNi. In such a case, the metallic nanocrystal is formed with a core of one metal such as Pt, and is then surrounded by a shell of an oxide of the second metal, such as NiO.
The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.
This application is related to U.S. application Ser. No. ______, entitled “METALLIC NANOCRYSTAL PATTERING” (Attorney Docket No. 2800.005US1), filed ______, which application is incorporated herein by reference.
