Abstract:
A method for improving the lithium cobalt oxide (LiCoO2) film (such as films in thin film batteries) morphology includes using oxygen (O2) and argon (Ar) gases during sputtering deposition of the LiCoO2 film. This may allow for the manufacturing of thicker LiCoO2 films. Such a method may also significantly reduce or eliminate cracking and obvious columnar structures within the resulting LiCoO2 film layer. Sputtering using a mixture of O2 and Ar also may produce a LiCoO2 film layer that requires lower annealing temperatures to reach good utilization and has higher lithium diffusion rates.

Description:
[0001]    This application claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/780,169, filed Mar. 13, 2013, entitled “Method to Improve LiCoO 2  Morphology in Thin Film Batteries,” the entirety of which is incorporated herein by reference as if fully cited herein 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to batteries, and more specifically to the LiCoO2 film layer in lithium ion thin film batteries. 
       BACKGROUND 
       [0003]    Many electronic devices, such as laptops, tablet computers, smartphones, and the like, use rechargeable batteries to provide power to one or more electronic components. A number of electronic devices use thin film batteries as the power source because of the many advantages it has over other batteries. For example, lithium ion thin film batteries which have a potential high energy density while maintaining a relatively compact configuration. 
         [0004]    A lithium thin film battery&#39;s performance highly depends on the film of the cathode, among other layers. Current lithium thin film batteries available have a capacity of 2 mAh or less, which is often dictated by the thickness of the cathode file. In many batteries, a LiCoO2 cathode film has a thickness of 10 microns or thinner. One way to increase battery capacity, and thus increase the number of applications the battery can be used for, is to increase the thickness of the LiCoO2 film. 
         [0005]    Many disadvantages are associated with thicker films. For example, thicker films may result in film cracking and undesirable crystal structures due to the required high deposition rate for thick films. Film cracking and/or undesirable crystal structure may cause low energy utilization and lithium diffusion rates, which may result in poor battery performance. Such disadvantages significantly limit the film thickness in lithium thin film batteries and consequently, confine the application of thin film batteries in electronic devices. 
       SUMMARY 
       [0006]    Some embodiments described herein include methods for improving the LiCoO2 layer morphology in lithium thin film batteries. Sputtering methods using a mixture of argon (Ar) and oxygen (O2) gases may be used to deposit the LiCoO2 film layer. The LiCoO2 layer may then be annealed at a predetermined temperature to achieve the desired resulting film layer with generally no film cracking. The method also allows for producing thicker LiCoO2 film layers with very little or no film cracking and a desirable crystalline structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of an electronic device incorporating the thin film battery. 
           [0008]      FIG. 2  is a cross-sectional view of the stacked layers within a battery device. 
           [0009]      FIG. 3A  is an exemplary top view of a thin film layer deposited with Ar only. 
           [0010]      FIG. 3B  is an exemplary top view of a thin film layer deposited with a mixture of Ar and O2. 
           [0011]      FIG. 4A  is an exemplary cross-sectional view of a thin film layer deposited with Ar only. 
           [0012]      FIG. 4B  is an exemplary cross-sectional view of a thin film layer deposited with a mixture of Ar and O2. 
           [0013]      FIG. 5A  is a simplified view of a (003) crystallized structure. 
           [0014]      FIG. 5B  is a simplified view of a (101) crystallized structure. 
           [0015]      FIG. 5C  is a simplified view of a (104) crystallized structure. 
           [0016]      FIG. 6  is an illustrative chart of results from exemplary testing of annealing temperatures conducted on a film layer deposited with Ar only and a film layer deposited with a mixture of Ar and O2. 
           [0017]      FIG. 7  is a simplified cross-sectional view of a multilayer thin film with alternating layers in some exemplary embodiments. 
           [0018]      FIG. 8  is a cross-sectional view of a multilayer thin film with alternating layers in another embodiment. 
           [0019]      FIG. 9  is a simplified flow diagram of a method of manufacturing the multilayer thin film of  FIG. 8 . 
       
    
    
     SPECIFICATION 
     Overview 
       [0020]    In some embodiments, a thin film battery and method for improving the lithium cobalt oxide (LiCoO2) film morphology are disclosed. In some embodiments of the present disclosure, oxygen (O2) and argon (Ar) gases may be used during sputtering deposition of the LiCoO2 film which allows for the manufacturing of thicker LiCoO2 films. Such method may also significantly reduce or eliminate cracking and obvious columnar structures within the resulting LiCoO2 film layer. Sputtering using a mixture of O2 and Ar also produces a LiCoO2 film layer that requires lower annealing temperatures to reach good utilization and has higher lithium diffusion rates. These advantages will be further described in more detail below. 
         [0021]    In some embodiments of the current disclosure, the LiCoO2 may be a multilayer film. The film may include alternating layers in which one layer is sputtered with Ar only, and another layer is sputtered with an Ar and O2 mixture. 
         [0022]    In some embodiments, the LiCoO2 may be a multilayer film stack with film layers that alternate in composition. For example, in some embodiments, one layer may be a standard layer deposited with Ar only and using conventional sputtering methods, and a second layer may be have a different composition than the first. The two layers may alternate to form a multilayer stack. 
         [0023]    The crystalline structure of the film, in some embodiments, may also be modulated by depositing a seed layer on a substrate, and subsequently depositing the LiCoO2 film layer. The seed layer may facilitate in producing a LiCoO2 film layer having a desirable crystalline structure. 
         [0024]    In some embodiments, a substrate with a designed texture surface may be used. The LiCoO2 may be deposited on the substrate having a designed texture surface which may assist in the a desired crystalline structure of the LiCoO2 film. 
         [0025]    The film morphology and microcrystalline structure may, in some embodiments, be modulated by having a multilayer structure with the layers being prepared with substrates under different process conditions. 
       DETAILED DESCRIPTION 
       [0026]    Turning to the figures, an illustrative lithium ion battery will be discussed in further detail.  FIG. 1  is an illustrative electronic device  100  incorporating one or more batteries  102  (shown in  FIG. 2 ). Although the illustrative electronic device  100  in  FIG. 1 , 
         [0027]      FIG. 2  illustrates a cross-sectional view of the thin film battery  102  with a battery core  104  having a stacked layer configuration. The battery core  104  can have an anode current collector layer  106 , an anode layer  108 , an electrolyte  110 , a cathode layer  112 , a cathode current collector layer  114 , and a substrate  116 . The battery  102  may further include an encapsulation  118  or housing around the battery core  104  to provide some protection and structure for the battery  102 . It should be noted that although the battery  102  is illustrated in  FIG. 2  as being generally rectangular, many other dimensions and shapes are envisioned, such as but not limited to, geometric, non-geometric, or the like. 
         [0028]    A positive terminal  122  and a negative terminal  120  may extend through the encapsulation  118 , or may otherwise be configured such that the terminals  120 ,  122  are in communication with the battery core  102  and with one or more external components (e.g., components of the electronic devices  100 ). The terminals  120 ,  122  may transfer current from the battery core  104  to one or more components of the electronic device  100  and also may transfer current to the battery core  104  from an external power source (e.g., charging the battery  102 ). 
         [0029]    The cathode current collector  114  may be in communication with the positive terminal  122 , and the anode current collector  106  may be in communication with the negative terminal  124 . The cathode current collector  114  and anode current collector  106  may be made from a material that has a high electric conductivity (low resistivity), corrosion resistant, and is stable at high temperatures (i.e., no alloy formation at high temperatures, such as at 700° C.). The cathode current collector  114  may be positioned on a substrate  116 , or otherwise may form the substrate and base on which the cathode layer  112  can be positioned. 
         [0000]    Sputtering with O2 and Ar 
         [0030]    A method of manufacturing the film layer (e.g., cathode layer  112 ) will now be discussed in further detail. In some embodiments, a thicker LiCoO2 film may be produced using a method of sputtering deposition using a well-controlled amount of O2 mixed with Ar. Conventional sputtering methods for manufacturing a LiCoO2 film layer use only Ar to deposit the LiCoO2. Using only Ar, however, may create film cracking and distinct columnar structures after annealing, particularly when manufacturing thicker films. Mixing O2 with Ar during the sputtering process allows for the production of a thicker LiCoO2 while also resulting in a better film morphology and battery performance which will be discussed in further detail below. 
         [0031]    Using O2 may affect the sputtering plasma ionization, sputter rates, and film nucleation and growth. In some embodiments, the amount of O2 compared to Ar may vary based on the battery design and desired crystallization structure. For example, the amount of O2 mixed with the Ar may be enough to achieve a LiCoO2 with a better morphology without causing the film to be lithium (Li) deficient. 
         [0032]    The advantages to manufacturing the LiCoO2 film with a mixture of Ar and O2 will now be discussed. First, depositing LiCoO2 with a mixture of O2 and Ar may eliminate film cracking.  FIG. 3A  illustrates an exemplary film layer in which LiCoO2 was deposited with only Ar. In contrast,  FIG. 3B  illustrates the same film in which LiCoO2 was deposited with Ar mixed with O2. As shown in  FIG. 3A , using only Ar produces film cracking as evident by cracks  124 . 
         [0033]    Second, depositing the LiCoO2 film with a mixture of Ar and O2 may also eliminate columnar structures within the film.  FIG. 4A  illustrates an exemplary film layer having distinct columnar structures  126 . Such columnar structures  126  may be susceptible to cracking which limits lithium diffusion. The columnar structures  126  may also result in a film having a lower density which limits lithium utilization.  FIG. 4B  illustrates the same film in which LiCoO2 was deposited with Ar mixed with O2. When LiCoO2 is deposited with a mixture of Ar and O2, the columnar structures  126  may be eliminated, and thus, a denser film structure having increased Li diffusion rates and increased Li utilization may be achieved after annealing. 
         [0034]    In some embodiments, sputtering the LiCoO2 with a mixture of Ar and O2 may also produce a more desirable crystal structure. Typically, LiCoO2 exhibits four types of crystallization structures: (003), (102), (101), and (104).  FIGS. 5A-5C  illustrate exemplary crystallization structures of (003), (101), and (104) of LiCoO2 having O2  128 , Li  130 , and Co  132 . 
         [0035]    Using conventional methods of film sputtering with only Ar typically results in (003) and (102) crystallization structures. The planes of a (003) crystallization structure are generally parallel to the substrate or film surface  134 . The Li layer is shown as  130  is parallel to the film surface  134 . This structure may increase the difficulty of the Li ions diffusing through the film surface  134 , thereby resulting in a slower diffusion rate. The Li ions instead rely on grain boundary diffusion in (003) crystallization structures. After annealing, the (003) and (102) structure may transition to a (101) crystallization structure, but not a (104) structure, the preferred structure, even after annealing at high temperatures (such as at approximately 700° C.). 
         [0036]    In contrast, sputtering with a mix of O2 and Ar may result in a denser film with a preferred (104) structure after annealing. A (104) crystallization structure is most preferred structure because the predominant phase of a (104) crystallization structure has planes parallel to the film surface  134 . In addition, the (104) crystallization structure has open channels for the Li ions to diffuse to the film surface  134 , has lower resistance, and has a better rate capability during discharge. Because Li ions may be used to carry charge, the open channels may provide for lower impedance compared to a film layer sputtered with only Ar, and thus, may provide for a higher lithium ion diffusion rate. Sputtering with a mix of O2 and Ar may also result in a structure with a Li diffusion coefficient of LiCoO2 that is higher than sputtering with only Ar. 
         [0037]    Further, less stringent annealing is required with a LiCoO2 film deposited with the Ar and O2 mixture as described herein. To reach an acceptable utilization, lower temperatures are required with film deposited with the O2 and Ar mixture compared to film deposited with Ar only. For example, film deposited with Ar may require 600° C. to 700° C. or higher to reach acceptable utilization. In contrast, to achieve a generally equivalent utilization, a LiCoO2 film deposited with a mixture of Ar and O2 may be annealed at much lower temperature (e.g., at least approximately 300° C.). Less astringent annealing (e.g., annealing at a lower temperature) may also allow for a larger substrate selection and a reduction to the cost of product. For example, lower annealing temperature may allow for cheaper substrates, such as polymer substrates, which may be more economical. 
         [0038]    Exemplary testing have been conducted that compared annealing temperatures of a film deposited with Ar only and with a mixture of Ar and O2. As shown in the illustrative chart of  FIG. 6 , results from the exemplary testing showed that there is a large difference between the diffusion coefficient of film deposited with only Ar and film deposited with a mixture of Ar and O2. For example, at 300° C., 400° C., and 500° C. annealing temperatures, the diffusion coefficient of the Ar and O2 mixture is much higher than that of the film deposited with only Ar. 
         [0039]    Thus, film deposited with a mixture of Ar and O2 not only allows for annealing at lower temperatures, but may also provide for higher utilization at lower temperatures and higher Li diffusion rates which may lead to better battery performance and may reduce the cost of producing the battery  104 . 
         [0000]    Multilayer Film with Alternating Ar and Ar/O2 Sputter 
         [0040]    In some embodiments, the film may be a multilayer film stack that may prevent crack propagation. A multilayer film stack may be beneficial for tall structures, which may be more susceptible to cracking than shorter structures. The LiCoO2, in some embodiments, may be deposited using alternating methods, and subsequently, the multilayer film stack may be composed of different layers.  FIG. 7  illustrates an exemplary multilayer film stack with a first film layer  138  deposited with only Ar, which has a high deposition rate and cracks, and a second film layer  136  deposited with a mixture of Ar and O2, which has a low deposition rate and no cracks. The cracks in the film layer  138  deposited with Ar only may be eliminated by the film layer  136  deposited with the mixture of Ar and O2. Additionally, in some embodiments, the multilayer film may be configured such that the film layer  136  deposited with Ar and O2 occupies only a small portion of the multilayer film compared to the film layer  138  deposited with Ar only. Such configuration may maintain the high average deposition rate and also significant reduces or eliminates crack propagation. It should be noted that the multilayer film with alternating Ar film layer  138  and Ar and O2 film layer  136  may be combined with other embodiments described herein. Although  FIG. 7  illustrates a four layer stack, it is appreciated that the number of layers and the order in which the layers are deposited with respect to each other may vary depending on the design of the battery. 
         [0000]    Multilayer Film with Alternating Composition 
         [0041]    Another method of manufacturing the LiCoO2 film layer will now be described. In some embodiments, the film layer may be a multilayer film with layers having alternating composition. The deposition of the multilayer film may be done in a multistep process. For example,  FIG. 8  illustrates an exemplary multilayer film with a standard layer  140  and an alternative film layer  142 . Although  FIG. 8  depicts a six layer film, it is appreciated that the number of layers, the number of different layer types, and the order in which the layers are deposited with respect to each other may vary depending on the design of the battery. 
         [0042]      FIG. 9  is an illustrative flow chart of the method for manufacturing the multilayer film with an alternating composition. A film may first be deposited with a standard layer  140  using conventional sputtering methods under typical conditions with a high deposition rate (such as depositing with Ar only) (step  200  of  FIG. 9 ). In some embodiments, the thickness of the standard layer  140  may be maintained at a level where no cracking occurs. An alternative layer  142  may be deposited on the standard layer  140  having a different composition (step  202  of  FIG. 9 ). This different composition may be a material by design that enhances film performance such as a Li-rich, O2-rich, or pure Li layer. The thickness of this layer  142  may be less than the thickness of the standard layer  140 . Another standard layer  140  may be deposited on the alternative layer  142  under typical conditions with a high deposition rate (such as with Ar) (step  204  of  FIG. 9 ). This alternative layering process may continue until a desired thickness is reached (step  206  of  FIG. 9 ). 
         [0043]    Typically, the higher the deposition rate, the poorer the film quality. The method of manufacturing a multilayer film with alternating composition as described herein provides for a film where a majority of the film may have a high deposition rate (e.g., the standard layer  140 ) but still maintains a good film quality (such a stable film). The multilayer film with alternating composition as described herein may also be more stable because it may disrupt the continuous columnar structures that may appear in the standard layer  140 . The columnar structures may be susceptible to cracking which results in slowing or terminating Li ion diffusion. Additionally, because of the alternative layer  142 , the multilayer film may have improved cathode utilization, better mechanical properties, lower resistance, and/or better discharge or charge performance. 
         [0044]    It should be noted that the multilayer film with alternating composition may be combined with other embodiments described herein. 
         [0000]    Substrate with a Seed Layer 
         [0045]    In some embodiments, the thin film crystalline structure may be modulated by first depositing a seed layer on a substrate. The seed layer may be sputter deposited evaporated, wet coated, printed on, or other suitable methods. The LiCoO2 may be deposited on the seeded layer, and the seeded layer may assist with modulating the LiCoO2 film layer to have a desired crystalline structure. It should be noted that having a substrate with a seed layer may be combined with other embodiments described herein. 
         [0000]    Substrate with a Designed Texture 
         [0046]    Another method of modifying the thin film crystalline structure will now be described. The thin film crystalline structure may be modulated by making a desired texture on the substrate. The texture may be made by wet etching, sputter etching, mechanical roughing, or other suitable ways during the manufacturing of the substrate. The LiCoO2 may then be deposited on the texture, and the textured surface may assist with modulating the crystalline structure of the LiCoO2 film layer. For example, the texture may prevent straight columnar structures in the film layer. Additionally, the textured surface may improve adhesion of the film layer, particularly in thick films, because of the increased surface area of the substrate. It should be noted that having a substrate with a designed texture may be combined with other embodiments described herein. 
         [0000]    Multilayer Film with Alternating Process Condition 
         [0047]    In some embodiments, the film morphology and microcrystalline structure may be modulated by having a multilayer structure in which the layers are prepared under different process conditions. For example, the film layers may be prepared with different substrate temperature, bias (e.g., a power supply with a predetermined voltage being applied to the substrate), or process gas pressure. Having a film with selected layers prepared under different processes may prevent voids or crack formation through the film stack during film depositions. 
         [0048]    It should be noted that this method may be combined with other embodiments described herein. 
       CONCLUSION 
       [0049]    The foregoing description has broad application. For example, while examples disclosed herein may focus on discrete embodiments, it should be appreciated that the concepts disclosed herein may be combined together and implemented in a single structure. Additionally, although the various embodiments may be discussed with respect to improving LiCoO2 morphology in thin film batteries found in smartphones, the techniques and structures may be implemented in any type of electronic devices using thin film lithium ion batteries. Accordingly, the discussion of any embodiment is meant only to be an example and is not intended to suggest that the scope of the disclosure, including the claims, is limited to these examples.