This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-145533, filed on Jun. 18, 2009; the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to a method for manufacturing a semiconductor device.
Proposals have been made for a memory device with a stacked structure in which a plurality of conductive layers functioning as word electrodes or control gates are alternately stacked with insulating layers. For instance, JP-A 2007-266143 discloses a technique for three-dimensionally arranging memory cells by forming through holes (memory holes) in the aforementioned stacked structure, forming a charge storage layer on the inner wall of the hole, and then burying a silicon pillar in the hole. JP-A 2007-266143 also discloses formation of contact holes for connecting upper wirings to respective conductive layers in a single etching process by forming the end portion of the conductive layers in a staircase structure and using its step difference.
A possible method for forming the aforementioned staircase structure portion is, for instance, to form a resist on the stacked structure of the conductive layers and insulating layers and repeat, a plurality of times, resist slimming for reducing the planar size of this resist and etching of the conductive layers and insulating layers using the resist as a mask. It is desirable that these processes be continuously performed in the same processing chamber in view of processing efficiency. However, in that case, there is concern that the slimming width of the resist may vary for each process of resist slimming. JP-A 2007-266143 does not specifically describe such a method for repeating resist slimming and etching, and the associated variation of resist slimming width.
In one embodiment, a method is disclosed for manufacturing a semiconductor device. The method can include forming a resist on a subject layer containing silicon. The method can etch the subject layer using the resist as a mask and with a gas containing a halogen element, which is introduced into a processing chamber. After the etching of the subject layer, the method can slim a planner size of the resist with oxygen gas and a gas containing a halogen element, which are introduced into the same processing chamber.
Embodiments will now be described with reference to the drawings. Although the semiconductor is illustratively silicon in the following embodiments, semiconductors other than silicon may also be used.
The semiconductor device according to an embodiment includes a memory cell array with a plurality of memory cells three-dimensionally arranged therein, and a peripheral circuit formed around the memory cell array.
In
In this specification, an XYZ orthogonal coordinate system is introduced for convenience of description. In this coordinate system, the two directions parallel to the major surface of the substrate and orthogonal to each other are referred to as an X direction and a Y direction, and the direction orthogonal to both the X direction and the Y direction, that is, the stacking direction of a plurality of conductive layers WL1 to WL4, is referred to as a Z direction.
As shown in
On the insulating layer 14 is provided a stacked body in which a plurality of insulating layers 17 and a plurality of conductive layers WL1 to WL4 are alternately stacked. The number of conductive layers WL1 to WL4 is arbitrary, and illustratively four in this embodiment. The insulating layer 17 contains silicon oxide. Each of the conductive layers WL1 to WL4 is a silicon layer doped with impurity and having conductivity.
A stopper layer (e.g., SiN layer) 24 is provided on the uppermost insulating layer 17 in the aforementioned stacked body. An upper select gate USG is provided above the stopper layer 24 via an insulating layer 25. An insulating layer 27 is provided on the upper select gate USG. The insulating layers 25 and 27 are layers containing silicon oxide or silicon nitride, and the upper select gate USG is a silicon layer doped with impurity and having conductivity.
As shown in
A plurality of memory holes aligning in the Z direction are formed in the aforementioned stacked body on the substrate 11. The memory holes are arranged in a matrix illustratively along the X direction and the Y direction.
As shown in
The silicon pillars 15, 19, and 32 are formed from polycrystalline silicon or amorphous silicon. The silicon pillars 15, 19, and 32 are shaped like a pillar, such as a cylinder, aligning in the Z direction. The lower end of the silicon pillar 15 is connected to the cell source 12. The lower end of the silicon pillar 19 is connected to the silicon pillar 15, and the upper end of the silicon pillar 19 is connected to the silicon pillar 32.
An insulating layer 29 is provided on the insulating layer 27 on the upper select gate USG, and a plurality of bit lines BL aligning in the Y direction are provided on the insulating layer 29. Each of the bit lines BL is arranged so as to pass immediately above a corresponding sequence of the silicon pillars 32 arranged along the Y direction and is connected to the upper end of the silicon pillar 32 via a contact electrode 30 provided through the insulating layer 29.
As shown in
As shown in
The insulating film 20 has a structure in which a charge storage layer 22 is sandwiched between a first insulating film 21 and a second insulating film 23. The silicon pillar 19 is provided inside the second insulating film 23, and the second insulating film 23 is in contact with the silicon pillar 19. The first insulating film 21 is provided in contact with the conductive layers WL1 to WL4, and the charge storage layer 22 is provided between the first insulating film 21 and the second insulating film 23.
The silicon pillar 19 provided in the stacked body of the conductive layers WL1 to WL4 and the insulating layers 17 functions as a channel, the conductive layers WL1 to WL4 function as a control gate, and the charge storage layer 22 functions as a data storage layer for storing charge injected from the silicon pillar 19. That is, a memory cell having a structure in which the channel is surrounded by the control gate is formed at the intersection between the silicon pillar 19 and each of the conductive layers WL1 to WL4.
This memory cell has a charge trap structure. The charge storage layer 22 includes numerous traps operable to confine charges (electrons), and is illustratively made of silicon nitride film. The second insulating film 23 is illustratively made of silicon oxide film and serves as a potential barrier when a charge is injected from the silicon pillar 19 into the charge storage layer 22 or when a charge stored in the charge storage layer 22 diffuses into the silicon pillar 19. The first insulating film 21 is illustratively made of silicon oxide film and prevents charges stored in the charge storage layer 22 from diffusing into the conductive layers WL1 to WL4.
As shown in
Referring to
Furthermore, on the inner wall of the hole formed in the stacked body composed of the stopper layer 24, the upper select gate USG, and the overlying and underlying insulating layers 25 and 27, a gate insulating film 33 is formed in a tubular shape, and the silicon pillar 32 is buried inside it. Thus, this stacked body includes an upper select transistor UST with the silicon pillar 32 serving as a channel and the upper select gate USG therearound serving as a gate electrode.
A peripheral circuit, not shown, is formed on the same substrate 11 around the memory cell array described above. The peripheral circuit illustratively includes a driver circuit for applying a potential to the upper end portion of the silicon pillar 32 via the bit line BL, a driver circuit for applying a potential to the lower end portion of the silicon pillar 15 via the cell source wiring CSL and the cell source 12, a driver circuit for applying a potential to the upper select gate USG via the upper select gate wiring USL, a driver circuit for applying a potential to the lower select gate LSG via the lower select gate wiring LSL, and a driver circuit for applying a potential to each of the conductive layers WL1 to WL4 via the word line WLL.
The semiconductor device according to this embodiment is a nonvolatile semiconductor memory device allowing data to be erased and written electrically and freely and being capable of retaining its memory content even when powered off.
The X coordinate of the memory cell is selected by selecting the bit line BL, the Y coordinate of the memory cell is selected by selecting the upper select gate USG to turn the upper select transistor UST to the conducting or non-conducting state, and the Z coordinate of the memory cell is selected by selecting a word line WLL, or conductive layers WL1 to WL4. Then, data is stored by injecting electrons into the charge storage layer 22 of the selected memory cell. The data stored in the memory cell is read by passing a sense current in the silicon pillar 19, which passes through the memory cell.
In the semiconductor device of this embodiment, as shown in
In the following, a method for forming the staircase structure portion of the conductive layers WL1 to WL4 in the semiconductor device according to this embodiment is described with reference to
It is assumed that the lower select transistor LST, the transistors of the peripheral circuit and the like have already been formed on the substrate 11. A plurality of insulating layers 17 and a plurality of conductive layers WL1 to WL4 are alternately stacked on the insulating layer 14 on the lower select transistor LST illustratively by the chemical vapor deposition (CVD) process. The insulating layer 17 is a layer containing silicon oxide, and each of the conductive layers WL1 to WL4 is a silicon layer.
After the stacked body of the insulating layers 17 and the conductive layers WL1 to WL4 is formed, a process for forming the memory holes MH, the insulating film 20 including a charge storage layer, the silicon pillar 19 and the like shown in
Subsequently, on the aforementioned stacked body, a resist 41 is formed as shown in
First, the resist 41 is subjected to lithography and development using a mask, not shown, and patterned so that the end of the resist 41 is located at a desired position as shown in
Next, the resist 41 is used as a mask to perform reactive ion etching (RIE) to remove the portion of the first insulating layer 17 from the top and the conductive layer WL4 therebelow exposed from the resist 41 as shown in
Specifically, the wafer with the aforementioned stacked body formed thereon is placed in a processing chamber. CHF3 gas and BCl3 gas, for instance, are first introduced into the processing chamber and then turned into plasma to etch the first insulating layer 17. Subsequently, HBr gas and Cl2 gas, for instance, are introduced into the same processing chamber and then turned into plasma to etch the conductive layer WL4.
Subsequently, oxygen gas and a gas containing a halogen element are introduced into the same processing chamber and then turned into plasma to perform resist slimming for reducing the planar size of the resist 41 as shown in
Subsequently, the slimmed resist 41 is used as a mask to perform RIE in the same processing chamber. As shown in
Also in this process, CHF3 gas and BCl3 gas, for instance, are first introduced into the processing chamber and then turned into plasma to etch the insulating layers 17. Subsequently, HBr gas and Cl2 gas, for instance, are introduced into the same processing chamber and then turned into plasma to etch the conductive layers WL3 and WL4.
After the process of
Subsequently, the slimmed resist 41 is used as a mask to perform RIE in the same processing chamber. As shown in
Also in this process, CHF3 gas and BCl3 gas, for instance, are first introduced into the processing chamber and then turned into plasma to etch the insulating layer 17. Subsequently, HBr gas and Cl2 gas, for instance, are introduced into the same processing chamber and then turned into plasma to etch the conductive layers WL2, WL3, and WL4.
Subsequently, the resist 41 is entirely removed, which results in the structure shown in
The process of etching the insulating layers 17 and the conductive layers WL2 to WL4 and the process of slimming the resist 41 described above are continuously performed in the same processing chamber by switching gas species and the like introduced therein. That is, in the aforementioned sequence of processes, the wafer remains in the processing chamber, and a desired reduced-pressure atmosphere of a desired gas is maintained in the processing chamber without opening to the atmosphere. Thus, efficient processing can be performed.
In general, oxygen gas is used to remove a resist containing an organic material. This is based on the so-called ashing phenomenon in which oxygen gas is turned into plasma to oxidize and remove the resist. However, when the sequence of processes for processing the aforementioned staircase structure portion is performed in the same processing chamber using oxygen gas alone, there is a problem of variation in the reduction width (slimming width) of the planar size of the resist. Variation in the slimming width of the resist causes variation in the width of each step processed by using the resist as a mask and may affect the subsequent process and product quality.
The inventors have investigated the above problem and found that one of the causes is considered to be the fact that halogen elements contained in the gas used in etching the conductive layers WL3 to WL4 and the insulating layers 17 in the previous process remain in the processing chamber also at resist slimming. That is, at resist slimming, ashing by oxygen is dominant, but the resist may also be removed by the action of residual halogen elements activated or ionized by the plasma at resist slimming. In fact, the residual amount of halogen elements used in the previous process and existing in the processing chamber at resist slimming is considered infinitesimal. However, the residual amount is not intentionally controlled but variable, which may vary the resist slimming width.
Thus, in this embodiment, at resist slimming, a gas containing a halogen element is used in addition to oxygen gas as described above. The amount of oxygen introduced into the processing chamber is larger than that of the halogen element, and ashing by oxygen is dominant in the resist slimming.
The residual amount of halogen elements in the processing chamber at resist slimming is considered infinitesimal. A halogen element in a larger amount than this residual amount is introduced into the processing chamber at resist slimming. By desirably controlling the amount of the halogen element introduced at resist slimming, the resist slimming width due to the effect of halogen elements can be controlled. That is, the halogen element introduced in an intentionally controlled amount suppresses the effect of residual halogen elements remaining in an uncertain amount and improves the controllability of the resist slimming width.
In other words, in this embodiment, by resist slimming using a mixed gas of oxygen gas and a gas containing a halogen element, the resist slimming width can be stabilized, which serves to reduce variation in the width of each process of the staircase structure portion processed by using the slimmed resist 41 as a mask.
The resist slimming process in the aforementioned sequence of processes was performed under the following condition using a mixed gas of O2 and SF6, for instance. Then, stabilization of the resist slimming width was confirmed.
O2 gas and SF6 gas were introduced into the processing chamber at a flow rate of 200 sccm and 8 sccm, respectively, and the processing chamber pressure due to the mixed gas was maintained at 50 mTorr. An electromagnetic wave was generated by applying radio frequency power to transformer coupled plasma (TCP) electrodes provided outside the processing chamber and introduced into the processing chamber to excite the above mixed gas into plasma. The TCP electrodes were subjected to a radio frequency power of 1000 W. The wafer holder was grounded, and the wafer side was not biased. Furthermore, the temperature of the wafer was controlled at 60° C. by a temperature controlling mechanism, such as a heater, provided in the wafer holder.
As shown in the result of
Here, O2 gas is introduced at a flow rate of 200 sccm. That is, for 200 sccm of O2 gas, the appropriate flow rate of SF6 gas is 7 to 9 sccm. Hence, by setting the flow rate ratio of SF6 gas in the mixed gas of O2 gas and SF6 gas to 3.4 to 4.3%, the effect of residual halogen elements can be suppressed, and the resist slimming width can be stabilized.
Furthermore, as shown in the result of
The relationship between the flow rate of a gas containing fluorine introduced into a processing chamber and the etching rate of a resist as shown in
In this graph of
The bold solid line represents the etching rate when using SF6 gas, the dash line represents the etching rate when using CF4 gas, and the dashed-dotted line represents the etching rate when using NF3 gas, respectively.
As a result of
In this embodiment, the gas introduced at resist slimming is not limited to SF6, but may be other fluorine-containing gases, or those containing a halogen element other than fluorine. For instance, NF3 was used as a gas containing a halogen element and added to O2, and it was confirmed that the resist slimming width can be controlled by introducing NF3, just like SF6.
In the case of NF3 gas as well, an appropriate flow rate can be derived on the basis of the result of
The flow rate of SF6 gas is about 4 sccm when the resist etching rate indicates its peak. In contrast, the flow rate of NF3 gas is about double the flow rate of SF6 gas when the resist etching rate indicates its peak. It can be considered that six F atoms are dissociated from one molecule of the compound SF6 in plasma and three F atoms are dissociated from one molecule of the compound NF3 in plasma. Therefore, the same effect as the case of SF6 gas can be realized by setting the flow rate of NF3 gas about double the flow rate of SF6 gas. Hence, it is desirable to set the flow rate ratio of NF3 gas in the mixed gas of O2 gas and NF3 gas introduced into the processing chamber to 2.8 to 8.6%.
It is confirmed that the resist slimming width can be controlled similarly by introducing CF4 when using CF4 added to O2 as a gas containing halogen elements.
In the case of CF4 gas as well, an appropriate flow rate can be derived on the basis of the result of
The flow rate of SF6 gas is about 4 sccm when the resist etching rate indicates its peak. In contrast, the flow rate of CF4 gas is about six times the flow rate of SF6 gas when the resist etching rate indicates its peak. Therefore, the same effect as the case of SF6 gas can be realized by setting the flow rate of CF4 gas about six times the flow rate of SF6 gas. Hence, it is desirable to set the flow rate ratio of CF4 gas in the mixed gas of O2 gas and CF4 gas introduced into the processing chamber to 8.4 to 25.8%. In the case of CF4 gas, the range of the flow rate that obtains the same effect as the case of SF6 gas is not simply the ratio, i.e., 6/4 times, which makes the number of F (fluorine) atoms equal. It is considered that this is because of the effect of the deposition of C (carbon).
As described above, after the staircase structure portion shown in
After the stopper layer 24 and the interlayer insulating layer 43 are formed, a plurality of contact holes punched through the interlayer insulating layer 43, the stopper layer 24, and the insulating layer 17 below the stopper layer 24 and reaching the corresponding conductive layers WL1 to WL4 are collectively formed. After these contact holes are formed, a conductive material, such as tungsten, is buried in each of the contact holes to form a contact electrode 63 as shown in
Each of the conductive layers WL1 to WL4 is electrically connected to the upper word line WLL shown in
Next, another example of the method for forming the aforementioned staircase structure portion is described with reference to
During RIE of the insulating layers 17 and the conductive layers WL1 to WL4, a reaction product resulting from the constituent element of the insulating layers 17 and the conductive layers WL1 to WL4, such as silicon, may be generated and attached to the upper surface and sidewall of the resist 41. The product is relatively resistant to oxygen gas serving primarily for resist removal in the slimming of the resist 41, and functions as an interference layer 42 interfering with the progress of etching of the resist 41.
Depending on the etching apparatus used, due to its evacuation characteristics, the film thickness of the interference layer 42 formed in the center portion of the wafer tends to be larger than the film thickness of the interference layer 42 formed in the edge portion. Hence, at resist slimming, the interference layer 42 in the edge portion of the wafer vanishes earlier than the interference layer 42 in the center portion, and resist slimming proceeds in the edge portion, while the interference layer 42 still remains in the center portion of the wafer. Consequently, in the wafer surface, the slimming width of the resist 41 may vary between the center portion and the edge portion and cause the width of each step of the staircase structure portion to vary in the wafer surface. Also, the consumption of the film thickness of the resist in the longitudinal direction is large due to the interference layer 42 attached to the side wall of the resist 41 when performing a desired slimming, and therefore, a lack of the film thickness of the resist 41 may occur in the case where multiple steps are patterned.
Thus, in the example described below, after etching the insulating layers 17 and the conductive layers WL1 to WL4 and before slimming the resist 41, the process of removing the interference layer 42 is performed. The difference between the etching rate of the resist 41 and the etching rate of the interference layer 42 under the etching condition for removing the interference layer 42 is small as compared to the etching condition for resist slimming.
After the insulating layer 17 and the conductive layer WL4 are etched, O2 gas and a fluorine-containing gas are introduced into the processing chamber. For instance, O2 gas and NF3 gas are introduced into the processing chamber at a flow rate of 200 sccm and 30 sccm, respectively, and the processing chamber pressure due to the mixed gas is maintained at 50 mTorr. The condition except the flow rate of the fluorine-containing gas is the same as that at resist slimming. By this plasma etching, the interference layer 42 is removed (
Subsequently, O2 gas and NF3 gas are introduced into the processing chamber at a flow rate of 200 sccm and 10 sccm, respectively, and resist slimming is performed (
Subsequently, RIE of the stacked body using the slimmed resist 41 as a mask, removal of the interference layer 42, and resist slimming are repeated a necessary number of times.
A graph of
As shown in the graph of
Hence, at resist slimming, the flow rate of NF3 gas is relatively decreased to increase the etching rate of the resist 41 to enhance the processing efficiency. On the other hand, at the removal of the interference layer 42, the flow rate of NF3 gas is relatively increased to suppress the etching of the resist 41 to efficiently remove the interference layer 42.
In other words, the flow rate (e.g., 30 sccm) of NF3 gas set at the removal of the interference layer 42 makes the etching rate of the resist 41 lower than the flow rate (e.g., 10 sccm) of NF3 gas set at the slimming of the resist 41.
For efficiently removing the interference layer 42 while suppressing the etching of the resist 41, it is desirable to set the flow rate of NF3 gas introduced into the processing chamber at the removal of the interference layer 42, for instance, three times or more the flow rate of NF3 gas introduces into the processing chamber at the slimming of the resist 41.
If a gas used at the removal of the interference layer 42 and a gas used at the slimming of the resist 41 are the same gases, the number of gas species to be prepared is decreased, and the cost can be reduced.
By slimming the resist 41 after removing the interference layer 42, variation in resist slimming width due to variation in the thickness of the interference layer 42 can be suppressed. Consequently, the width of each step of the aforementioned staircase structure portion can be suppressed from varying between the center portion and the edge portion in the wafer surface.
As a comparative example, without removing the interference layer 42, O2 gas and NF3 gas were introduced into the processing chamber at a flow rate of 200 sccm and 10 sccm, respectively, to perform slimming of the resist 41. Then, there occurred a difference of step width of approximately 100 nm between the center portion and the edge portion in the wafer. In contrast, after removing the interference layer 42, O2 gas and NF3 gas were introduced into the processing chamber at a flow rate of 200 sccm and 30 sccm, respectively, with the other conditions being the same as at resist slimming, to perform resist slimming under the same condition as the above comparative example. Then, the difference of step width between the center portion and the edge portion in the wafer was reduced to approximately 20 nm.
Also in the case of SF6 gas as shown in
Hence, at resist slimming, the flow rate of SF6 gas is relatively decreased to increase the etching rate of the resist 41 to enhance the processing efficiency. On the other hand, at the removal of the interference layer 42, the flow rate of SF6 gas is relatively increased to suppress the etching of the resist 41 to efficiently remove the interference layer 42.
In other words, the flow rate of SF6 gas set at the removal of the interference layer 42 makes the etching rate of the resist 41 lower than the flow rate of SF6 gas set at the slimming of the resist 41.
For efficiently removing the interference layer 42 while suppressing the etching of the resist 41, it is desirable to set the flow rate of SF6 gas introduced into the processing chamber at the removal of the interference layer 42, for instance, three times or more the flow rate of SF6 gas introduced into the processing chamber at the slimming of the resist 41.
Also in the case of CF4 gas as shown in
Hence, at resist slimming, the flow rate of CF4 gas is relatively decreased to increase the etching rate of the resist 41 to enhance the processing efficiency. On the other hand, at the removal of the interference layer 42, the flow rate of CF4 gas is relatively increased to suppress the etching of the resist 41 to efficiently remove the interference layer 42.
In other words, the flow rate of CF4 gas set at the removal of the interference layer 42 makes the etching rate of the resist 41 lower than the flow rate of CF4 gas set at the slimming of the resist 41.
For efficiently removing the interference layer 42 while suppressing the etching of the resist 41, it is desirable to set the flow rate of CF4 gas introduced into the processing chamber at the removal of the interference layer 42, for instance, three times or more the flow rate of CF4 gas introduces into the processing chamber at the slimming of the resist 41.
The shape of the silicon pillar in the memory cell array is not limited to a cylinder, but may be a prism. Furthermore, the invention is not limited to burying a silicon pillar entirely in the memory hole. As an alternative structure, a silicon film may be formed in a tubular shape only at the portion in contact with the insulating film including the charge storage layer, and an insulator may be buried inside it. Furthermore, the insulating film structure between the conductive layer and the silicon pillar is not limited to the oxide-nitride-oxide (ONO) structure, but may be a two-layer structure of a charge storage layer and a gate insulating film, for instance.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and devices described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and devices described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modification as would fall within the scope and spirit of the inventions.
Number | Date | Country | Kind |
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2009-145533 | Jun 2009 | JP | national |