[0001] Aspects disclosed herein relate to methods of manufacturing arrays of substantially oxide-free structures, such as titanium nanotubes or microwel!s.

Description of the Related Art

[0002] Titanium oxide (TiO _x ) is useful for various physical and chemical functions, including for use as a gas, ion, or biological species-sensing material. Accordingly, TiO _x is being used for various biometric sensing applications, such as in a phosphate sensor for DNA sequencing. Conventionally, porous TiO _x films are formed on a substrate by anodization methods, such as anodic oxidation of a titanium sheet in an aqueous solution containing hydrofluoric (HF) acid.

[0003] One problem with conventionally manufactured TiO _x structure arrays, however, is that the formed structures have unwanted oxides, such as native oxides or otherwise uncontrolled oxides, on the surfaces thereof. The oxides are generally the result of anodization or other process steps. The native oxides negatively affect device uniformity and performance, for example, by increasing a signai-to-noise ratio in a biological species-sensing device. Additionally, conventionally manufactured arrays are not highly ordered.

[0004] Therefore, there is a need in the art for methods of reducing or eliminating oxides on the arrays of TiO _x structures, such as nanotubes or microweils, or other TiO _x films. SUMMARY

[0005] Methods are disclosed to provide arrays of substantially oxide-free structures, such as titanium nanotubes or microweils. In one aspect, the method includes plasma treating the structure having an oxide layer thereon to weaken the bonds in the oxide layer and then bombarding the oxide layer having weakened bonds with hydrogen radicals to reduce the oxide layer to a titanium layer. The cyclic plasma treatment and hydrogen radical exposure processes are generally repeated until the oxide layer is removed from the structure. Arrays of titanium structures manufactured according to the described methods are well controlled and have improved device performance since the oxide layer has been removed and the signai-to-noise ratio of the device has been optimized for improved sensing.

[0008] in one aspect, a method for reducing oxides is disclosed. The method includes positioning a substrate having an array of metal structures formed thereon in a process chamber, the array of metal structures having an oxide layer formed thereon, plasma treating the oxide layer to form a plasma-treated oxide layer, and exposing the plasma-treated oxide layer to hydrogen or argon/hydrogen radicals to remove the plasma-treated oxide layer.

[0007] in another aspect, a method for reducing oxides is disclosed. The method includes positioning a substrate having an array of titanium oxide structures formed thereon in a first process chamber, the array of titanium oxide structures having an oxide layer formed thereon, plasma treating the oxide layer to form a plasma-treated oxide layer having weakened titanium-oxygen bonds in the first process chamber, transferring the substrate to a second process chamber, and exposing the plasma-treated oxide layer to hydrogen or argon/hydrogen radicals to remove the plasma-treated oxide layer in the second process chamber. [0008] In yet another aspect, a titanium oxide structure is disclosed. The titanium oxide structure includes a complementary metal oxide stack layer having a sensor therein, a titanium nitride layer disposed over the complementary metal oxide stack layer, a titanium layer disposed over the titanium nitride layer, and a plasma-treated oxide layer disposed over the titanium layer, the plasma-treated oxide layer having weakened titanium-oxygen bonds therein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to aspects, some of which are illustrated in the appended drawings, if is to be noted, however, that the appended drawings illustrate only exemplary aspects and are therefore not to be considered limiting of its scope. The present disclosure may admit to other equally effective aspects.

[0010] Figure 1 is a perspective side view of a portion of a substrate having an array of titanium structures formed thereon.

[0011] Figure 2 is a top view of the array of titanium structures of Figure 1 ,

[0012] Figure 3 is a process flow for reducing oxides from a TiO _x structure.

[0013] Figures 4A-4C depict cross-sectional views of a titanium structure formed according to a process flow disclosed herein.

[0014] Figure 5 is a substrate processing system that may be used to perform a process flow disclosed herein.

[0015] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures, it is contemplated that elements and features of one aspect may be beneficially incorporated in other aspects without further recitation. DETAILED DESCRIPTION

[0016] Methods are disclosed to provide arrays of substantially oxide-free structures, such as titanium nanotubes or microweils, in one aspect, the method includes plasma treating the structure having an oxide layer thereon to weaken the bonds in the oxide layer and then bombarding the oxide layer having weakened bonds with hydrogen radicals to remove the oxide layer. The cyclic plasma treatment and hydrogen radical exposure processes are generally repeated until the oxide layer is removed from the structure. Arrays of titanium structures manufactured according to the described methods are well controlled and have improved device performance since the oxide layer has been removed and the signai-to-noise ratio of the device has been optimized for improved sensing.

[0017] Methods described herein will refer to reduction of a TiO _x native oxide layer of titanium nanotubes or microweils as an example. However, it is also contemplated that the described methods are useful to reduce native oxides on any structures, such as other metal structures or carbon structures. The described methods are also useful for reducing native oxides on films, such as high quality titanium dioxide (Ti0 ₂ ) films deposited by atomic layer deposition (ALD). Additionally, the described methods are useful to remove any uncontrolled or otherwise unwanted oxides.

[0018] Figure 1 is a perspective side view of a portion of a substrate 00 having an array of titanium structures 104 formed thereon. Figure 2 is a top view of the array of titanium structures 104 of Figure 1. As shown in Figure 1 , a TiO _x barrier layer 102 is disposed over the substrate 100. An array of titanium structures 104 is formed on the TiO _x barrier layer 102. The TiO _x barrier layer 102 provides a bottom surface of the titanium structures 104.

[0019] The substrate 100 is generally any substrate having a porous layer thereon. A porous layer is generally any layer having natural pores thereon. In one aspect, the substrate 100 has a porous titanium layer thereon. Some examples of porous titanium, which is useful for gas and biological-species sensing, include uniform pore structures such as nanotubes and microwelis, bimodal pore structures, gradient pore structures, honeycomb structures, and closed-pore structures. As shown in Figure 1 , the titanium structures 104 are nanotubes.

[0020] Figure 3 is a process flow 300 for reducing oxides from a TiO _x structure. Figures 4A-4C depict cross-sectional views of a titanium structure 104 formed according to process flows disclosed herein, such as at various operations of the process flow 300.

[0021] The process flow 300 begins at operation 310 by positioning a substrate 100 having an array of titanium structures 104, shown as TiO _x structures as an example, formed thereon in a process chamber.

[0022] Prior to operation 310, the substrate 100 having the array of titanium structures 104 formed thereon is generally formed by any suitable method. In one aspect, an array of titanium structures 104 is formed by depositing a titanium layer, such as titanium thin film, over the substrate 100 and anodizing the titanium layer in an HF acid solution to form the titanium structures 104, such as TiOx structures. As shown in Figure 4A, the formed titanium structure 104 is a microwell, which includes a native oxide layer 450 on the surface thereof, as an example. However, the process flow 300 is useful for removing generally any oxides from a substrate surface, in some aspects, the formed titanium structure 104 also includes various additional layers, including but not limited to, a complementary metal-oxide semiconductor (CMOS) stack 456 having a biometric sensor 459 therein, a titanium nitride (TiN) layer 458 and a titanium (Ti) layer 454. In one aspect, the TiN layer 458 is disposed over the CMOS stack 456 and the Ti layer 454 is disposed over the TiN layer 458. In another aspect, for example, the TiN layer 458 is disposed on an in contact with the CMOS stack 456, and the Ti layer 454 is disposed on and in contact with the TIN layer 458. As discussed above, the native oxide layer 450 affects device uniformity and performance.

[0023] At operation 320, the substrate 100 is exposed to a plasma treatment process to weaken the bonds in the native oxide layer 450, such as titanium- oxygen (Ti-O) bonds in a TiO _x layer, to form plasma-treated oxide layer 452, as shown in Figure 4B. In one aspect, weakening the bonds includes physically damaging the native oxide layer 450 such that a lower-energy, for example between about 1 and about 3 electron volts (eV), is required for subsequent breaking the bonds during subsequent exposure to hydrogen radicals. The plasma treatment physically damages or otherwise weakens the Ti-0 bonds of the native oxide layer 450 so that the plasma-treated oxide layer 452 is prepared for subsequent reduction and removal.

[0024] At operation 330, the substrate is exposed to, or bombarded with, hydrogen radicals to remove the plasma-treated oxide layer 452. Since the plasma-treated oxide layer 452 has already been plasma-treated to weaken the Ti-0 bonds of the layer, a low-energy hydrogen exposure can be used to remove the plasma-treated oxide layer 452 by reacting the hydrogen radicals with the weakly bonded Ti-0 molecules. More specifically, the hydrogen radicals react with the oxide in the plasma-treated oxide layer 452 and cause an oxide reduction and formation of products, such as water (H ₂ 0) and titanium hydrides. Using low-energy hydrogen selectively removes the plasma-treated oxide layer 452 and thus reduces the potential for damage to the other layers on the substrate.

[0025] in some aspects, only a portion of the plasma-treated oxide layer 452 will be removed during operation 330. Accordingly, operation 320 and operation 330 are generally repeated any number of times until the native oxide layer 450 is removed and titanium structures 104, which are substantially oxide free, are formed, as shown in Figure 4C. Optionally, the substrate 100 is cooled down after operation 330 and prior to operation 320 being repeated.

[0026] in one aspect, plasma treating the substrate 100 at operation 320 includes a low energy plasma treatment at a plasma power of 13,56 Megahertz (MHz), The plasma precursor is generally an unreactive gas, including but not limited to an inert gas, such as argon (Ar) and/or helium (He), The flow rate of the plasma precursor is between about 10 standard cubic centimeters per minute (seem) and about 50 seem. The radio frequency (RF) power is between about 200 watts (W) and about 700 W, The process chamber pressure is between about 5 mii!itorr (mTorr) and about 60 mTorr. The substrate is generally at a low temperature, for example, about room temperature (e.g., between about 20 degrees Celsius (°C) and about 25°C).

[0027] in one aspect, exposing the substrate 100 to hydrogen radicals to remove the plasma-treated oxide layer 452 at operation 330 includes a hot wire chemical vapor deposition (HWCVD) process. The HWCVD process generally includes providing hydrogen (H ₂ ) gas into a process chamber, such as an HWCVD chamber, at any suitable flow rate, heating one or more filaments disposed in the process chamber to a temperature sufficient to dissociate the H ₂ gas and provide at least a portion of the energy for facilitating subsequent removal of at least a portion of the plasma-treated oxide layer 452, and exposing the substrate 100 to the dissociated H gas to remove at least some of the plasma-treated oxide layer 452. The one or more filaments are generally heated to a temperature between about 1 ,200°C and about 1 ,700°C. The temperature of a substrate heater is generally low, for example, between about 200°C and about 400°C. The flow rate of the H ₂ gas is generally between about 100 seem and about 500 seem, for example, about 400 seem. The process chamber pressure is generally between about 0.1 torr (T) and about 1 ,0 T, for example, about 0.5 T. The duration of the HWCVD process is generally between about 50 seconds and about 4 hours, for example between about 100 seconds and about 200 seconds, such as about 120 seconds.

[0028] In another aspect, exposing the substrate 100 to hydrogen radicals to remove the plasma-treated oxide layer 452 at operation 330 includes introducing hydrogen radicals to a process chamber from a remote microwave or radiofrequency (RF) plasma source (RPS).

[0029] In one aspect, the substrate 100 is optionally cooled down for between about 50 seconds and about 200 seconds, such as 120 seconds, after operation 330 and prior to operation 320 being repeated,

[0030] Figure 5 is a schematic top view of a substrate processing system 500 that may be used to perform a process flow disclosed herein, such as a system available from Applied Materials, Inc., Santa Clara, California. The substrate processing system 500 generally includes a plurality of processing chambers 560a-560i, a transfer chamber 562, a load lock chamber 564, and a factory interface 566. The transfer chamber 562 is coupled to the load lock chamber 564 and the processing chambers 560a-i. The load lock chamber 564 is coupled between the factory interface 566 and the transfer chamber 562.

[0031] The factory interface 566 is maintained at a substantially atmospheric pressure and includes one or more robots 572 for transferring substrates between cassettes 570 coupled to the factory interface 566 and the load lock chamber 564. Robots 572 are configured to transfer substrates into the substrate processing chambers 560 for processing. A third chamber 574, such as a buffer chamber with substrate cooling stations 576, is positioned between the robots 572.

[0032] As illustrated in Figure 5, the substrate processing system 500 includes nine processing chamber 560a-i. However, nine processing chambers 560a-i is for illustrative purposes. The substrate processing system 500 generally includes any suitable number of processing chambers. Each substrate processing chamber 560a-i can be outfitted to perform a substrate processing operation such as dry etch processes, cyclical layer deposition (CLD), ALD, chemical vapor deposition (CVD), HWCVD, physical vapor deposition (PVD), pre-clean, substrate degas, substrate orientation, and other substrate processes. Examples of such process chambers are available from Applied Materials, inc., Santa Clara, California.

[0033] According to aspects described herein, the system 500 generally includes at least three processing chambers. For example, a first substrate processing chamber 560a is a pre-clean chamber configured to plasma treat the substrate 100 as described above at operation 320, and a second process chamber 560f is an HWCVD chamber configured to expose the substrate 100 to hydrogen radicals as described above at operation 330. Additionally, the third chamber 574 is a buffer chamber having substrate cooling stations therein for cooling the substrate 100 between repetitions of operations 320 and 330.

[0034] As shown in Figures 1 and 2, the array of titanium structures 104 formed by process flows described herein is well ordered. Weil ordered generally means that the titanium structures 104 of the array are arranged in an orderly and substantially uniform way. In the example shown in Figure 4C, the formed titanium structure 104 is a microwell having a sensor 459, such as a phosphate sensor for DNA sequencing. The formed titanium structure 104 is useful for applications, such as photocatalysis, solar cells, electrochromic devices, biomedical coatings, drug delivery, gas sensing, biological species sensing, and other biomedical applications. Since the native oxide layer 450 has been removed from the microwell, the signal-to-noise ratio is optimized, for example improved by 30%, thus increasing device sensitivity, and the ability of the sensor 459 to be used for the various applications. [0035] Benefits of the present disclosure include formation of well-ordered arrays of titanium structures, such as nanotubes or microwelis, which provide improved performance because the oxides on surfaces thereof have been removed to form a titanium layer. Additionally, the cyclic process of plasma treatment and exposure to hydrogen radicals provides a method of removing oxides at lower temperatures, which can be achieved in existing process chambers, such as pre-clean chambers and HWCVD chambers, without having to retrofit the chambers for higher temperature components.

[0036] While the foregoing is directed to aspects of the present disclosure, other and further aspects of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.