Patent Publication Number: US-9418832-B2

Title: Method of forming a dielectric film

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor fabrication and, more particularly, to methods of forming a dielectric film. 
     BACKGROUND 
     In order to provide integrated circuits (ICs) with increased performance, the characteristic dimensions of devices and spacing on the ICs continue to be decreased. Fabrication of such devices often requires the deposition of dielectric materials into features patterned into layers of material on silicon substrates. In most cases, it is important that the dielectric material completely fill such features. Filling such narrow features, so-called gap filling, places stringent requirements on materials used, for example, for pre-metal dielectric (PMD) applications. The pre-metal dielectric layer on an integrated circuit isolates structures electrically from metal interconnect layers and isolates them electrically from contaminant mobile ions that degrade electrical performance. PMD layers may require filling narrow gaps having aspect ratios (that is the ratio of depth to width), of five or greater. Dielectric films play an important role in the fabrication of semiconductor devices. It is therefore desirable to have improved processes for the deposition of dielectric films. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a method of forming a dielectric film comprising: depositing a flowable oxide on a semiconductor structure in a first phase having a first oxygen flow rate for a first time interval; depositing a flowable oxide on a semiconductor structure in a second phase having a second oxygen flow rate for a second time interval; depositing a flowable oxide on a semiconductor structure in a third phase having a third oxygen flow rate for a third time interval, wherein the second oxygen flow rate is greater than the first oxygen flow rate, and wherein the third oxygen flow rate is greater than the second oxygen flow rate. 
     In a second aspect, the present invention provides a method of forming a dielectric film comprising: depositing a flowable oxide on a semiconductor structure in a first phase having a first oxygen flow rate for a first time interval; depositing a flowable oxide on a semiconductor structure in a second phase having a second oxygen flow rate for a second time interval; depositing a flowable oxide on a semiconductor structure in a third phase having a third oxygen flow rate for a third time interval, wherein the first oxygen flow rate is zero, and wherein the second oxygen flow rate and the third oxygen flow rate increase based on a piecewise linear function. 
     In a third aspect, the present invention provides a method of forming a dielectric film comprising: depositing a flowable oxide on a semiconductor structure, wherein oxygen gas is flowed at a monotonically increasing flow rate up to an upper flow limit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain elements in some of the figures may be omitted, or illustrated not-to-scale, for illustrative clarity. The cross-sectional views may be in the form of “slices”, or “near-sighted” cross-sectional views, omitting certain background lines which would otherwise be visible in a “true” cross-sectional view, for illustrative clarity. Furthermore, for clarity, some reference numbers may be omitted in certain drawings. 
       Features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a semiconductor structure including a dielectric film with a gap; 
         FIG. 2  is a semiconductor structure including a dielectric film with dishing; 
         FIG. 3  is a flowchart indicating process steps for illustrative embodiments; 
         FIG. 4  is an oxygen flow graph indicating three phases of deposition in accordance with illustrative embodiments; 
         FIG. 5  is an oxygen flow graph indicating a piecewise linear flow profile in accordance with illustrative embodiments; 
         FIG. 6  is an oxygen flow graph indicating a non-linear flow profile in accordance with illustrative embodiments; 
         FIG. 7  is an oxygen flow graph indicating a linear flow profile in accordance with illustrative embodiments; and 
         FIG. 8  is a system for implementing illustrative embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments will now be described more fully herein with reference to the accompanying drawings, in which exemplary embodiments are shown. Embodiments of the present invention provide an improved method for flowable oxide deposition. An oxygen source gas is increased as a function of time or film depth to change the flowable oxide properties such that the deposited film is optimized for gap fill near a substrate surface where high aspect ratio shapes are present. The oxygen gas flow rate increases as the film depth increases, such that the deposited film is optimized for planarization quality at the upper regions of the deposited film. 
     It will be appreciated that this disclosure may be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this disclosure to those skilled in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure. For example, as used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms “a”, “an”, etc., do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. It will be further understood that the terms “comprises” and/or “comprising”, or “includes” and/or “including”, when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” “embodiments,” “exemplary embodiments,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” “in embodiments” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     The terms “overlying” or “atop”, “positioned on” or “positioned atop”, “underlying”, “beneath” or “below” mean that a first element, such as a first structure (e.g., a first layer), is present on a second element, such as a second structure (e.g. a second layer), wherein intervening elements, such as an interface structure (e.g. interface layer), may be present between the first element and the second element. 
       FIG. 1  shows an example of a semiconductor structure  100  including a dielectric film  108  with poor gap fill quality. Semiconductor structure  100  comprises a substrate  102 , which may be a bulk substrate such as a silicon wafer. Structures  104  and  106  are formed on the substrate  102 . Structures  104  and  106  may be transistor gates, or part of other elements such as resistors or diodes. A dielectric film  108  is deposited on the structure  100  to a depth D, after which point, a planarization process, such as chemical mechanical polish (CMP) may be performed. However, due to the narrow gap between structure  104  and structure  106 , a gap  110  is formed between structure  104  and  106 . This gap is undesirable for a variety of reasons, which include device variability and reduced product yield. 
       FIG. 2  shows an example of a semiconductor structure  200  including a dielectric film  208  with good gap fill quality. Dielectric film  200  may be formed from a flowable oxide (FOX). Flowable oxide may be used as a dielectric film due to its good gap fill properties. Generally, the flowable oxide is deposited using chemical vapor deposition (CVD) such as plasma enhanced CVD (PECVD). The flowable oxide may include, but is not limited to, boron-doped silicon oxide, phosphorus-doped silicon oxide, boron phosphorus silicon glass (BPSG), phosphorus silicon glass (PSG), fluorinated silicate glass (FSG), and combinations thereof. 
     As used herein, the term “flowable oxide” is a flowable doped or undoped silicon oxide film having flow characteristics that provide consistent fill of a gap. The flowable oxide film may also be described as a soft jelly-like film, a gel having liquid flow characteristics, a liquid film, or a flowable film. The flowable oxide deposition methods described herein are not limited to a particular reaction mechanism (e.g., the reaction mechanism may involve an adsorption reaction, a hydrolysis reaction, a condensation reaction, a polymerization reaction, a vapor-phase reaction producing a vapor-phase product that condenses, condensation of one or more of the reactants prior to reaction, or a combination of these). The substrate is exposed to the process gases for a period sufficient to deposit a flowable film to fill at some of the gaps. The deposition process typically forms soft jelly-like film with good flow characteristics, providing consistent fill. However, flowable oxide films may suffer from poor planarization quality (PQ). After deposition of the film is complete, it is often desirable to planarize the film to a desired depth. Therefore, the flowable oxide film may be prone to “dishing” such as shown by region  211  of  FIG. 2 , where the top surface of dielectric  208  is not planar. This dishing is also undesirable because it can induce device variability and reduce product yield. In current processes, multiple depositions of various processes, with multiple planarization processes in between the depositions, may be performed in order to have a region of dielectric layers with good gap fill quality and also have acceptable PQ. 
     Embodiments of the present invention provide a dielectric formation method that utilizes flowable oxide in a single deposition process with a single planarization process. The method provides a flowable oxide that has good gap fill properties, and also has good planarization quality. The gap fill quality is most critical near the base of a high-aspect ratio feature, where gap fill may be difficult. Reducing or omitting oxygen gas flow into the deposition tool reaction chamber during the initial stages of deposition allows the flowable oxide to retain the good gap fill capabilities. As the depth of the flowable oxide increases, and exceeds the height of the high aspect ratio features, the planarization quality (PQ) becomes more important. At a higher depth, the flow of oxygen gas is increased. The effect of increased oxygen gas on the flowable oxide decreases the gap fill quality, but increases the PQ. However, with the depth of the flowable oxide exceeding the height where deep gaps exist, the gap fill properties are not as important, while the planarization quality becomes more important. Hence, embodiments of the present invention mitigate the tradeoff between gap fill quality and planarization quality, allowing a single deposition process to be used for a pre-metal dielectric film, saving considerable manufacturing cost over prior art methods. 
       FIG. 3  is a flowchart  300  showing process steps for embodiments of the present invention. In process step  350 , flowable oxide is deposited in a first phase. The first phase includes first oxygen flow rate for a first time interval. In some embodiments, the first oxygen flow rate is zero. In some embodiments, the first time interval ranges from about 5 seconds to about 15 seconds. In process step  352 , flowable oxide is deposited in a second phase. The second phase includes a second oxygen flow rate for a second time interval. In some embodiments, the second oxygen flow rate ranges from about 50 standard cubic centimeters per minute (sccm) to about 90 sccm. In some embodiments, the second time interval ranges from about 20 seconds to about 30 seconds. In process step  354 , flowable oxide is deposited in a third phase. The third phase includes a third oxygen flow rate for a third time interval. In some embodiments, the third oxygen flow rate ranges from about 110 standard cubic centimeters per minute (sccm) to about 160 sccm. In some embodiments, the third time interval ranges from about 20 seconds to about 30 seconds. In process step  356 , a planarization process is performed on the deposited flowable oxide. The planarization process may include chemical mechanical polish (CMP). 
       FIG. 4  is an oxygen flow graph  400  indicating three phases of deposition in accordance with illustrative embodiments. The horizontal (X) axis  422  represents time. The vertical (Y) axis  424  represents oxygen gas flow rate. In embodiments, flowable oxide deposition starts at time  0  and proceeds to time t 1  with no oxygen gas flowing. In some embodiments, time t 1  ranges from about 5 seconds to about 15 seconds, and, in some particular embodiments, may be about 10 seconds. Then at time t 1 , the oxygen gas flow ramps up to flow rate f 1 . In some embodiments, flow rate f 1  ranges from about 50 standard cubic centimeters per minute (sccm) to about 90 sccm, and in other embodiments may range from about 85 sccm to about 95 sccm. Curve  426  shows the flow rate of oxygen gas as a function of time. The ramp portion  430  of the curve  426  shows the transition from zero flow rate to flow rate f 1 . In embodiments, the transition time from zero flow rate to flow rate f 1  may be about 1 second. The first step portion  432  of the curve  426  shows the oxygen gas continuing at flow rate f 1  up until time t 2 , at which point the flow increases to flow rate f 2 . In embodiments, the duration between time t 1  and time t 2  may range from about 20 seconds to about 30 seconds. The ramp portion  434  of curve  426  shows the transition from flow rate f 1  to flow rate f 2 . In embodiments, the transition time from flow rate f 1  to flow rate f 2  may be about 1 second. The second step portion  436  of the curve  426  shows the oxygen gas continuing at flow rate f 2  up until time t 3 , at which point, the deposition process is complete. In embodiments, the duration between time t 2  and time t 3  may range from about 20 seconds to about 30 seconds. In embodiments, flow rate f 2  may range from about 110 standard cubic centimeters per minute (sccm) to about 160 sccm. In other embodiments, flow rate f 2  may range from about 140 sccm to about 180 sccm. 
     Increasing the oxygen gas flow creates a more dense SiO 2  film, hence changing the properties of the flowable oxide. In particular, the viscosity of the flowable oxide is increased, which increases the planarization quality, while decreasing the gap fill quality. Embodiments of the present invention exploit this relationship to use little or no oxygen initially, to have good gap fill quality near the substrate surface, where high aspect ratio shapes are present, and increase the flow of oxygen as the deposition film height increases, where there are no high aspect ratio shapes, but the planarization quality becomes important. 
       FIG. 5  is an oxygen flow graph  500  indicating a piecewise linear flow profile in accordance with illustrative embodiments, where the oxygen flow rate increases based on a piecewise linear function as shown by curve  526 . The horizontal (X) axis  522  represents time. The vertical (Y) axis  524  represents oxygen gas flow rate. In embodiments, flowable oxide deposition starts at time  0  and proceeds to time t 1  with no oxygen gas flowing. In some embodiments, time t 1  ranges from about 5 seconds to about 15 seconds, and, in some particular embodiments, may be about 10 seconds. Then at time t 1 , the oxygen gas flow gradually increases at a first rate of increase, as shown by curve portion  532  of curve  526 . In some embodiments, the first rate of increase ranges from about 4 sccm per second to about 6 sccm per second, and the duration between time t 1  and t 2  is about 20 seconds, such that at time t 2 , the current flow rate f 1  is about 100 sccm. The curve portion  534  of curve  526  represents a second rate of increase up to a flow rate of f 2 . In some embodiments, the second rate of increase ranges from about 7 sccm per second to about 10 sccm per second. In some embodiments, the flow rate may reach a maximum level and then remain constant at flow level f 2 . In some embodiments, flow level f 2  may range from about 160 sccm to about 200 sccm. In some embodiments, the duration between time t 1  and time t 2  may range from about 20 seconds to about 40 seconds. 
       FIG. 6  is an oxygen flow graph  600  indicating a non-linear flow profile in accordance with illustrative embodiments. The horizontal (X) axis  622  represents time. The vertical (Y) axis  624  represents oxygen gas flow rate. In embodiments, flowable oxide deposition starts at time  0  and proceeds to time t 1  with no oxygen gas flowing. In this embodiment, oxygen gas is flowed at a monotonically increasing flow rate up to an upper flow limit f 2  as shown by curve  626 . In some embodiments, upper flow limit f 2  ranges from about 150 sccm to about 200 sccm. In some embodiments, time t 1  ranges from about 5 seconds to about 15 seconds, and, in some particular embodiments, may be about 10 seconds. At time t 1 , the flow of oxygen gas starts increasing in a non-linear manner. In some embodiments, the flow rate of oxygen gas may be a function of time. In some embodiments, the flow rate of oxygen gas may be described by a function of the form:
 
 f=K (1+ t−t 1) 2  
 
Where f is the flow rate, and K is a constant, and t is the current time in seconds. In some embodiments, the constant K may range from about 0.2 to about 0.3. In some embodiments, the difference between time t 1  and time t 2  may be about 15 seconds, and the difference between time t 2  and time t 3  may be about 10 seconds. In one embodiment, the value of K is 0.25, and the difference between time t 2  and t 1  is 10 seconds, and the difference between time t 3  and t 1  is 25 seconds, such that, at time t 2 , the current flow rate of oxygen gas f 1  may be derived by:
 
 f 1=0.25(11) 2 =30.25 sccm
 
And at time t 3 , the current flow rate of oxygen gas f 2  may be derived by:
 
 f 2=0.25(26) 2 =169 sccm
 
       FIG. 7  is an oxygen flow graph  700  indicating a linear flow profile in accordance with illustrative embodiments. The horizontal (X) axis  738  represents depth. The vertical (Y) axis  724  represents oxygen gas flow rate. In this embodiment, the oxygen gas flow rate is a function of deposition depth. In embodiments, an in-situ metrology tool may be used to monitor the flowable oxide film depth in real time, and adjust the flow of oxygen gas accordingly. In embodiments, flowable oxide deposition starts at depth  0  and proceeds to depth d 1  with no oxygen gas flowing. In embodiments, depth d 1  may range from about 200 angstroms to about 1000 angstroms. Then, at depth d 1 , the flow rate increases linearly as shown by curve  726 , to a depth of d 2 . In some embodiments, depth d 2  may range from about 2500 angstroms to about 3500 angstroms. 
       FIG. 8  is a system  800  for implementing illustrative embodiments. System  800  includes a controller  818 . Controller  818  may be a computer comprising memory  820 , and a processor  822  which is coupled to memory  820 , such that the processor  822  may be configured to read and write memory  820 . In some embodiments, multiple processors or cores may be used. The memory  820  may be a non-transitory computer-readable medium, such as flash, ROM, non-volatile static ram, or other non-transitory memory. The memory  820  contains instructions that, when executed by processor  822 , control the various subsystems to operate system  800 . Controller  818  may also include a display  824  and a user interface  826  for interacting with the system  800 . The user interface  826  may include a keyboard, touch screen, mouse, or the like. 
     The controller  818  may be coupled to a deposition tool  830  which may be a chemical vapor deposition tool (CVD) suitable for deposition of flowable oxide. The controller may receive input data  810 . Input data  810  may include recipe parameters for depositing flowable oxide in accordance with embodiments of the present invention. Embodiments of the present invention may further include a computer program product embodied in a non-transitory computer-readable medium that implements the multiple phases of deposition as illustrated in  FIGS. 4-7 . 
     The controller  818  may also generate output data  814 . The generated output data  814  may include deposition simulation data, such as computed flow rates and time durations of each phase to achieve a desired dielectric film quality and thickness under certain process conditions. 
     While the invention has been particularly shown and described in conjunction with exemplary embodiments, it will be appreciated that variations and modifications will occur to those skilled in the art. For example, although the illustrative embodiments are described herein as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events unless specifically stated. Some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein, in accordance with the invention. In addition, not all illustrated steps may be required to implement a methodology in accordance with the present invention. Furthermore, the methods according to the present invention may be implemented in association with the formation and/or processing of structures illustrated and described herein as well as in association with other structures not illustrated. Therefore, it is to be understood that the appended claims are intended to cover all such modifications and changes that fall within the true spirit of the invention.