Patent Application
20060148260
1. Field of the Invention
Embodiments of the present invention relate to, but are not limited to, electronic devices and, in particular, to the field of electronic device manufacturing.
2. Description of Related Art
In the current state of electronics, horizontal die shrinkage has proceeded at a relentless pace, fueled by, in some instances, roughly ⅓ pitch reduction per generation. By contrast, vertical shrinkage has progressed much more slowly due to various reasons. One obstacle that has impeded vertical shrinkage is the shortcomings associated with various fabrication operations, for example, polishing operations.
For instance, in the field of flash memory devices, several polishing operations may be performed during the fabrication process. Each of the polishing operations is typically directed to planarizing or smoothing specific component or components on the silicon substrate. Unfortunately, because the silicon substrates are often characterized by a widely varied topography that includes different components made of different materials at varying concentrations, efficiently polishing these surfaces is often difficult.
For example, the silicon substrates used to form the foundation for these flash memory devices will typically be embedded with numerous isolation trenches (e.g., shallow trench isolation) that dot the landscape of the substrate at varying concentrations. The isolation trenches are typically made of oxides. The concentration level of the isolation trenches in a particular region will depend on whether the region is dedicated to, for example, flash memory arrays or other structures. The silicon substrate may also include other components of varying concentration including poly components such as poly-1 or floating gates for the flash memory cells. The combination of having multiple components made of different materials at varying concentrations on the substrate surfaces make polishing of these surfaces difficult to control.
Other factors that prevent some of the polishing operations from being an efficient means of planarizing the surfaces of the silicon substrates are the limitations of one or more of the polishing processes. For example, in flash memory fabrication, two of the polishing operations performed may include shallow trench isolation (STI) and self-aligned poly (SAP) polish operations. These polishing operations are primarily performed in order to polish or planarize specific components. For example, the general goal of the STI polish is to polish oxide pillars used to form shallow trench isolation (STI) while the general goal of the SAP polish is to polish poly-1 (floating gate).
Although each of the polishing processes is intended to polish specific components (e.g., STI polish for polishing oxide and SAP for polishing poly-1 or floating gates), the limited selectivity of one or more of these conventional polishing processes may prevent the processes from polishing only their intended component. To illustrate, in a typical SAP polishing process, both poly as well as oxide materials are typically removed.
As a result of these various factors, the ability to produce high quality poly components, such as poly-1, with a high yield is often difficult, particularly as the size of the devices (e.g., flash memory devices) continues to shrink. If the polish of poly-1 is overdone with a conventional process, the poly film and the oxide pillar may be too thin to protect the device from pitting. On the other hand, if the polish of poly-1 is insufficient, there may be excess poly remaining on top of oxide, which may result in shorting or leakage of the device.
Forming poly components with a smooth surface is generally an important factor in forming a defect free thin vertical profile memory device. For instance, when conventional polishing methods (e.g., conventional SAP polish) are used for polishing poly components, they will produce poly components with surface roughness having root mean square (RMS) values of about 5 Å or greater. Because of this surface roughness, the ability to form defect free components on top of the rough surface (e.g., thin silicon oxide/nitride and poly-2 on top of the poly-1) will be compromised particularly as device sizes continue to shrink.
In addition to the problem of surface roughness, other limitations of conventional polish processes include, for example, the presence of microscratches and polish residue on the surface of the silicon substrate after these processes have been completed. The presence of these defects may further limit the manufacturing yields of the overall fabrication process.
The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which:
In the following description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the disclosed embodiments of the present invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the disclosed embodiments of the present invention. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure the disclosed embodiments of the present invention.
According to various embodiments, a highly selective polish process may be performed on a substrate such as a silicon wafer in order to polish a first component without substantially polishing or removing a second component. For these embodiments, the highly selective polish may be a selective poly polish that polishes poly components without substantially removing oxide components. In some embodiments, a selective slurry may be employed during the selective poly polish process. The selective slurry may facilitate the polishing of poly components such as, for example, the poly-1 or floating gate of a flash memory cell, at a substantially higher rate than oxide components such as those that make up, for example, shallow trench isolation (STI). For purposes of this description, this selective poly polish process may be referred to as high selective slurry (HSS) self-aligned poly polish or HSS SAP. In various embodiments, the poly/oxide selectivity of the HSS SAP polish may be greater than or equal to 5 to 1 (5:1). In one embodiment, the poly/oxide selectivity may be equal to or greater than 50 to 1 (50:1).
In addition to the HSS SAP polish, additional processes may be performed. For example, in some embodiments, a buff operation may be performed in combination with the HSS SAP polish in order to increase surface smoothness, reduce microscratches and to remove slurry residue from the poly components and/or the wafer surface.
In some embodiments the selective slurry may include a conventional slurry such as silicon dioxide in a solution of basic pH. In addition, an oxide retarding additive such as tetra-methyl-ammonium hydroxide (TMAH), benzyltrimethylammonium hydroxide (BTMAH), EDP solution [ethylenediamine (NH2(CH2)2NH2)+pyrocatechol] (C6H4(OH)2)+water+pyrazine (C4H4N2)], KOH/IPA [KOH+isopropyl alcohol], and/or other oxide-retarding additive may be added to the slurry.
The oxide pillars 202 may be formed in the nitride layer 206 using a process similar to a damascene process. In this process, a thin layer of pad oxide (not shown) may first be formed on a clean silicon surface (e.g., silicon substrate 204). A thick layer of chemical-vapor deposition (CVD) nitride (e.g., nitride layer 206) may then be deposited on top of the pad oxide. The nitride layer 206 is then patterned and trenches are anisotropically etched into the silicon substrate 204. After resist stripping, the trench sidewalls may be passivated with a thin layer of thermal oxide. The trenches are then filled with a thick CVD oxide and subsequently planarized using shallow trench isolation (STI) polish process such as chemical-mechanical polishing (CMP). The STI polish may result in the planarization and the removal of some nitride and oxide materials from the top of the nitride layer 206 and the oxide pillar 202 as depicted in
The use of a subsequent HSS SAP polishing process (e.g., see block 106 in
Once the oxide pillars 202 are formed, the nitride layer 206 and the oxide pad may be removed. After removing the nitride layer 206, a poly layer 210 such as those used to form, for example, poly-1 or floating gate, may be deposited onto the silicon substrate 204 at block 104 (see
After depositing the poly layer 210, a highly selective poly polish may be performed at block 106 that results in a silicon substrate 204 with a poly layer 210 having a very smooth surface (see
The poly/oxide selectivity of the highly selective poly polish process in various embodiments may be greater than or equal to 5 to 1 (5:1). This means that the removal rate per unit of time of the poly material as opposed to the oxide material using, for example, a CMP polishing pad, may be equal to or greater than 5 to 1 (5:1). In one embodiment, the selectivity is greater than or equal to about 50 to 1 (50:1). During the poly polishing operation, the HSS polish process may self-stop once the underlying oxide (e.g., oxide pillar 202) is exposed, such as in regions of high oxide density (e.g., those regions containing flash memory arrays). For larger poly structures, such as those found in low density regions, the HSS SAP polishing process will still be slowed down due to the support from neighboring exposed oxides. In some embodiments, control over the HSS SAP polishing process may be achieved by using an in-situ optical end point detector.
By employing a highly selective poly polishing process such as HSS SAP polish, several beneficial results may be obtained. For example, depending upon the process conditions including rotational speed, pressure, and the property of the polish pads used, these benefits include, lower within-wafer and within-die nonuniformity in terms of thickness of poly (e.g., poly-1 or floating gate surface). The high selectivity to oxide may also result in a lower level of defects on the poly film and oxide pillars.
A buffing operation may be performed at block 108 in accordance with some embodiments. The buffing may be performed in order to, among other things, reduce surface roughness, reduce microscratches and to facilitate the removal of slurry residue. The buff operation may involve another slurry (in basic pH) and a softer pad than the one used during the HSS SAP polishing operation. The slurry composition and chemistry may be engineered such that the removal rate of poly material (e.g., poly-1) is relatively low such as 200 Å/minute as measured on blanket poly while the oxide removal rate may be made close to zero. In some embodiments, an organic additive in the buff slurry may keep the particles dispersed in the slurry and to reduce the removal rate of the oxide and/or poly materials. As a result of the buffing, the poly surface may be made very smooth, with a surface roughness close to that of virgin single crystal silicon. Microscratches and slurry residue that may cause electrical shorts and/or opens may also be reduced. All of these defects (e.g., surface roughness, microscratches and slurry residue) may be detrimental by forming small crevices that may lead to non-uniform subsequent deposition of dielectric silicon oxide/nitride and poly-2, and by acting as bridging defects that can lead to electrical shorts or opens in later processing.
A chemical rinse process at block 110 may be performed by dipping the silicon substrate 204 in, for example, a tank of basic solution that includes hydrogen peroxide containing cleaning chemical such as KOH+H2O2+H2O or NH4OH+H2O2+H2O in accordance with some embodiments. The chemical rinse may help to remove the organic residue from the water surface remaining from a previous operation and to convert the poly surface from hydrophobic to a hydrophilic surface. The tank may be heated to an elevated temperature and have megasonic power applied to it. The chemical rinse process may be performed to further improve the cleanliness of the silicon substrate surface.
A scrubbing operation may be performed at block 112 in accordance with some embodiments. The scrubbing may clean and dry the silicon substrate surface in double-sided brush scrubber. An ammonium hydroxide solution may be used to facilitate the scrubbing operation.
As a result of the process 100, a silicon substrate containing both poly and oxide components can be fabricated having a well controlled thickness, a very smooth surface, and with low amounts of defects as depicted in
Note that although the illustrated method depicted above relates to the fabrication of devices with flash memory arrays, in other embodiments, the process 100 may be employed for fabricating other types of devices. For example, the process 100 may be used for fabricating any device that contains both poly and oxide components such as DRAM, logic, and other devices.
Note further that in various embodiments, one or more of the blocks 102 to 112 of
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the embodiments of the present invention. Therefore, it is manifestly intended that embodiments of this invention be limited only by the claims.
