1. Field of the Invention
Embodiments of the invention relate to a method of fabricating a flash memory device. More particularly, embodiments of the invention relate to a method of fabricating a flash memory device having a U-shape floating gate.
This application claims the benefit of Korean Patent Application No. 10-2005-0034914, filed on Apr. 27, 2005, the subject matter of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
In many contemporary applications, flash memory is used as the main memory in a host device because of the excellent data integrity provided by its nonvolatile nature, as compared with the former use of DRAM. Flash memory is also routinely incorporated into host devices and related peripheral devices to provide bulk storage of data—thereby replacing existing hard disk and floppy disk data storage elements—because of its high data storage capacity and ease of integration.
The cell transistor used in conventional flash memory is formed on a substrate and is characterized by a stacked structure including a tunnel oxide layer, a floating gate, an inter-gate insulating layer, and a control gate. A programming operation (i.e., a data value storage operation) for a flash memory device having this gate structure is accomplished by the application of a positive voltage to the control gate, which is coupled to the floating gate. With application of this positive voltage, electrons migrate from the substrate to the floating gate through the tunnel oxide layer. This electron migration phenomenon may be the result of the so-called Fowler-Nordheim tunneling effect or hot carrier injection.
Both of these effects are related to the generation of a sufficiently high electric field. In order to develop this high electric field relative to the tunnel oxide layer at a low control gate input voltage, a high coupling ratio must exist between the control gate and the floating gate. The ratio between the voltage induced at the floating gate and the voltage applied to the control gate is termed a “coupling ratio.” The coupling ratio may also be described as a ratio between the capacitance of the inter-gate insulating layer and the sum of the capacitances of the tunnel oxide layer and the inter-gate insulating layer.
Figure (FIG.) 1 is a sectional view schematically illustrating the cell transistor of a conventional flash memory.
Referring to
In the flash memory device having this conventional structure, the surface area of inter-gate insulating layer 6, as formed on the upper surface and sidewalls of floating gate 4, influences a coupling ratio. However, increasing the surface area of the inter-gate insulating layer 6 in this conventional cell area structure also increases the size of floating gate 4.
In order to minimize the coupling capacitance between adjacent flash memory cells, a structure such as the one shown in
However, if the design rule for the constituent flash memory device is 60 nm or lower—which is common in contemporary and emerging flash devices—the distance between neighboring cells may be 40 nm or less. This remarkably small (and shrinking) separation distance causes the inter-cell coupling capacitance to increase accordingly. In an attempt to reduce or minimize the inter-cell coupling capacitance, the height of floating gate 4 has been reduced in some conventional designs. However, reducing the height of floating gate 4 also reduces the coupling ratio, and at some point the reduced coupling ration may cause the flash memory device to fail. Thus, the foregoing conventional remedies find increasingly limited application in the design and fabrication of flash memory having smaller and smaller design rules.
In order to overcome the foregoing limitations which often operate at cross purposes in relation to proposed remedies, a U-shape floating gate structure 5, such as that shown in
In order to form the U-shaped floating gate structures 5 in a flash memory device, node separation is accomplished with a process that makes use of a polishing sacrificial layer (e.g., an oxide layer). In many ways this process is similar to conventionally-known processes used to fabricate cylindrical-shaped capacitor structures. In this process, the recessing of the floating gate and the isolation layer begins in a state in which the polishing sacrificial layer is disposed within the floating gate. Thus, the polishing sacrificial layer within the floating gate is etched during the recessing of the isolation layer so that the floating gate is exposed. In this manner, the floating gate may be recessed. Assuming a case where the floating gate is initially formed with a thickness of about 100 Å, the floating gate may be recessed to a thickness of about 70 Å during this process. Thus, it is difficult to realize the formation of a U-shape floating gate structure having a uniform thickness without layer loss and the resulting failures. Further, if portions of the polishing sacrificial layer remain after this process, a diluted HF solution must often be used to remove the residual portions. Unfortunately, this cleaning process may undesirably etch or recess isolation regions formed in a peripheral circuit region of the flash memory device.
Embodiments of the invention provide a method of fabricating flash memory having a U-shape floating gate structure with a uniform thickness. This structure provides a high coupling ratio and avoids one or more the problems associated with the conventional devices. Additionally, embodiments of the invention provide a method of fabricating a flash memory during which isolation regions in a cell region of a substrate are effectively recessed, but associated isolation regions in a peripheral circuit region of the substrate are not similarly recessed.
The above and other features and advantages of the present invention will become more apparent upon consideration of several exemplary embodiments thereof with reference to the attached drawings in which:
The present invention will now be described with reference to the accompanying drawings in the context of several embodiments. The invention may, however, be embodied in many different forms and should not be construed as being limited to only the embodiments set forth herein. Rather, these embodiments are provided as teaching examples. Throughout this description, like numbers refer to like or similar elements.
Referring to
Before or after forming pad oxide layer 115, an ion implantation process may be performed on substrate 110 to form an ion layer adapted to define the impurity concentration of a well and to control a threshold voltage. Pad oxide layer 115 is formed to suppress crystal defects on the upper surface of substrate 110, or for surface treatment. Pad oxide layer 115 may be formed using a dry oxygen-based oxidation process or wet H2O-based oxidation process at a temperature ranging between about 700° C. to 950° C. Pad oxide layer 115 may be formed to a thickness ranging from between about 50 Å to 250 Å using one of several types of conventionally available furnaces or processing chambers.
Pad nitride layer 120 may be deposited using, for example, a low pressure chemical vapor deposition (CVD) method that reacts SiH4 and NH3 at a temperature ranging from about 500° C. to 850° C. Pad nitride layer 120 may be formed to a thickness as high as the anticipated upper surface of the completed structure and on both sides of an isolation layer. In one embodiment, this thickness may range from about 500 Å to 3000 Å.
As illustrated in
The insulating layer in-filling trenches 125 may be formed from a high density plasma (HDP) oxide layer, a middle temperature oxide (MTO), such as plasma enhanced-tetraethylorthosilicate (PE-TEOS), an undoped silicate glass (USG) oxide layer, or a composite layer formed from two or more of these materials or their like. Following its formation, the density of the insulating layer is enhanced using a thermal treatment process conducted at a temperature of about 800° C. to 1100° C., for example, in an the atmosphere of N2, O2, H2O, or the like to remove moisture from the insulating layer and hardened it against potential damage from subsequently applied etching processes. As a result, the structural density of the insulating layer (e.g., one formed from a MTO) may be made equal to that of a thermal oxide layer. However, such densification processes may be optionally or selectively employed in view of the material actually used to form the insulating layer. A chemical mechanical polishing (CMP) process or an etch-back process using pad nitride layer 120 as a planarizing stop point may be used to planarize the insulating layer in-filling trenches 125. For example, one CMP process adapted to this application uses a slurry including a ceria (CeO2) group of abrasive particles.
Referring to
Referring to
Following formation of tunnel oxide layer 140, a conductive layer 145 is formed from a doped polysilicon layer, for example, having a thickness sufficiently thin (e.g., 100 Å) so as not to completely in-fill the gap portions between adjacent ones of isolation layers 130 and 131. The doped polysilicon layer may be formed, for example, by depositing undoped silicon using a low pressure CVD (LPCVD) at a temperature ranging from 500° C. to 700° C. After the deposition of this undoped layer, selected impurities, such as arsenic (As) or phosphorus (P), may be ion implanted into the polysilicon layer. Alternatively, a doped polysilicon layer may be formed by introducing the impurities using in an in-situ manner during the deposition process. A doping density for the polysilicon layer may range, for example, to 1E21 or higher.
Referring to
In a case where conductive layer 145 is formed from a doped polysilicon layer, a polysilicon germanium layer may be formed by the deposition of silicon germanium. The silane group of gases such as SiH4, Si2H6, SiH2Cl2 or the like, and a gas such as GeH4, GeF4 or the like may be used as a source gases for the deposition of the silicon germanium. The amount of germanium in a sacrificial layer 150 formed from a silicon germanium layer may be controlled by the flow ratio of a germanium source gas into the formation process in view of the desired materials characteristics of layer being formed. In order to control the flow ratio precisely, it is possible to dilute GeH4 or the like as Ge source with hydrogen or nitrogen and supply. As the amount of germanium in the silicon germanium layer is high, the silicon germanium layer may be etched faster than the polysilicon layer. Because of this result, it is preferable in some embodiments of the invention to increase the amount of germanium to as high as reasonably possible in order to develop an etch selectively for sacrificial layer 150 relative to conductive layer 145. In one embodiment, a desired etch rate cannot be achieved for an amount of germanium of 10% or less by weight based on a total weight of the silicon germanium layer. Thus, the amount of germanium in a sacrificial layer 150 formed from a silicon germanium layer may range from about 10% to 100%.
Referring to
Sacrificial layer 150 and conductive layer 145 may be removed, for example, using a CMP. In one embodiment, sacrificial layer 150 is formed from a silicon germanium layer and conductive layer 145 is formed from a doped polysilicon layer. The silicon germanium and polysilicon layers have similar material properties. Thus, sacrificial layer 150 and conductive layer 145 can be effectively removed using a CMP without significant selectivity between these two layer types. In one embodiment each floating gate 145a is formed to a height of about 600 Å.
Referring to
In one embodiment, sacrificial layer pattern 150a is formed from a silicon germanium layer, and isolation layer 130 is formed from a MTO, such as USG. Since a selectivity of silicon germanium with respect to this type of oxide is 10 or higher, sacrificial layer pattern 150a is not etched away, but remains throughout the process of recessing isolation layer 130 and in this manner protects substantial portions (e.g., the inner sidewalls and inner bottom surface) of floating gate 145a. Thus, embodiments of the invention provide a U-shaped floating gate structure 145a that is not significantly etched (or non-uniformly thinned) during the process of recessing isolation layer 130.
Referring to
Alternatively, one embodiment of the invention proposes use of a silicon germanium etchant containing peroxyacetic acid in order to achieve a relative selectivity of 30 or higher. One acceptable etchant of this type comprises peroxyacetic acid, fluoride, and solvent. The amount of peroxyacetic acid in such an etchant may range from between 1 to 50% by weight based on the total weight of the etchant. In one embodiment, the fluoride is fluoric acid, and the solvent is acetic acid. In such an embodiment, the amount of peroxyacetic acid ranges from 1 and 50% by weight based on the total weight of the etchant, the amount of fluoric acid ranges from about of 0.1 to 30% by weight, and the amount of acetic acid ranges from about 10 to 50% by weight. Deionized water may also be added to the etchant solution in an amount that ranges from 10 to 40% by weight based on the total weight of the etchant. In addition, a surfactant or similar substance may be added to the etchant solution, as desired.
As the peroxyacetic acid has an excellent oxidative property, the silicon germanium can be removed with a high etch selectivity of 30:1 or more without causing damage to the polysilicon layer in the foregoing example. If an etchant including peroxyacetic acid is used, a sacrificial layer pattern 150a formed from silicon germanium may be selectively removed with respect to a polysilicon floating gate 145a without recessing (or thinning) floating gate 145a. Thus, floating gate 145a may be formed with a uniform thickness.
Further, as described above, since the selectivity of silicon germanium with respect to the oxide is 10 or more, isolation layer 131 of the peripheral circuit region P is not recessed while removing sacrificial layer pattern 150a.
Referring to
Control gate 160 may be formed from polysilicon, silicide, polycide, and/or a metal layer. A constituent silicide may be formed from tungsten silicide, cobalt silicide, and/or titanium silicide, for example. A cobalt silicide and titanium silicide may be formed by first depositing polysilicon, then depositing cobalt or titanium, and thereafter performing one or more rapid thermal annealing (RTA) process(es).
In a case where cobalt silicide is used, a first RTA process may be performed at a temperature of between 400° C. to 500° C. in a nitrogen atmosphere for about 50 seconds to cause reaction between the polysilicon and cobalt in order to form a CoSi layer. Then, a second RTA process may be performed on the resultant structure at a temperature of between 800° C. to 900° C. in a nitrogen atmosphere for about 30 seconds in order to form a low resistance layer of CoSi2.
In the case where nickel silicide is used, a first annealing process may be performed at low temperature to form a NiSi layer. A tungsten silicide may thereafter be directly deposited using CVD. The resulting polycide includes a stacked structure of polysilicon and silicide.
Referring to
Referring to
An oxide layer 147 is then formed on silicon germanium layer 146 to completely fill the gap between adjacent ones of isolation layers 130 and 131. Oxide layer 147 may be formed from a phosphorus silicate glass (PSG), a boron phosphorus silicate glass (BPSG), spin on glass (SOG), or the like.
A sacrificial layer 150′ well adapted to the subsequently performed polishing process may then be formed from silicon germanium layer 146 and oxide layer 147.
Referring to
Referring to
Referring to
Referring to
In the illustrated examples, the process of removing a sacrificial layer pattern 150a formed from silicon germanium, or removing the silicon germanium layer pattern 146a of sacrificial layer pattern 150′a with respect to the floating gate 145a with a selectivity is an important feature. For this reason, a silicon germanium etchant comprising peroxyacetic acid has been found to be useful in some embodiments of the invention. Consider, for example an experimental example further illustrating the usefulness of this type of etchant within the context of embodiments of the invention.
Referring to
Since the etch rate increases with the amount of germanium, greater etch qualities per unit time may be obtained by adjusting the germanium content. If the silicon germanium layer is composed of polysilicon germanium rather than being formed epitaxially, the etchant may penetrate along crystal boundaries more efficiently and thus, an increased etch rate may be obtained.
As shown from
As described above, and in accordance with embodiments of the invention, by forming the upper surface of the floating gate in a U shape, the surface area of the associated inter-gate insulating layer may be increased without increasing the cell size so as to increase the corresponding coupling ratio. Therefore, the efficiency of data storage and erase operations for a flash memory cell incorporating the U-shaped floating gate may be improved without adversely affecting the degree of integration.
When the U-shape floating gate is formed, use of a silicon germanium layer as a sacrificial layer is proposed. This type of layer supports the floating gate during node separation using a conventional CMP, or the like, and protects the floating gate during a subsequently applied process adapted to recess the isolation layer. Further, the sacrificial layer may be selectively removed from the floating gate using an etchant including peroxyacetic acid.
Therefore, according to embodiments of the invention, the floating gate may be protected from damage during a process adapted to recess the isolation layer, and the floating gate may be further protected from damage during a process adapted to remove the sacrificial layer or residual portions thereof. Therefore, the floating gate may be maintained with a uniform thickness and deterioration of the electrical characteristics of the flash memory cell due to damage of the floating gate may be avoided. Further, since in some embodiments the etchant including peroxyacetic acid provides a selectivity of silicon germanium with respect to an oxide, the isolation layer in the peripheral circuit region is not recessed when the sacrificial layer is selectively removed from the floating gate.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the following claims.
Number | Date | Country | Kind |
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10-2005-0034914 | Apr 2005 | KR | national |