Patent Publication Number: US-2022216104-A1

Title: Gold through silicon mask plating

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 62/805,604, to Chua et al, entitled “Gold Through Silicon Mask Plating” filed on Feb. 14, 2019, which is incorporated by reference herein in its entirety. 
    
    
     FIELD 
     The present disclosure relates generally to gold through silicon mask (TSM) plating, particularly for substrate processing in semiconductor manufacturing. 
     BACKGROUND 
     The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Forming metal structures at a micron scale is most commonly done by plating operations performed by using one or more pre-patterned sacrificial masks. One method employs an organic photoresist mask. This method is commonly referred to as through-resist plating. Some manufacturers in the Wafer Level Packaging (WLP) industry widely use such methods to form copper (Cu) and gold (Au) redistribution layers (RDL), flip-chip-bump interconnects of Cu, nickel (Ni) and tin (Sn) alloy solder, and to perform Au bumping. Even more complex structures such as those found in Micro-Electro-Mechanical Systems (MEMS) applications also utilize through-resist plating technology. 
     While providing another option, TSM plating with Au also presents some formidable difficulties. The Au metal readily deposits spuriously on an Si mask, and even more so at via openings. Such undesired Au depositions on Si continue to grow, often more intensely at a via entrance which may attenuate desired Au via fill plating patterns. Eventually, the via entrance or at least parts of the via entrance become substantially closed off. This may prematurely disallow the Au via from being filled altogether. These defective partially-filled via are colloquially termed voids and depending on the nature of the close-off these voids can be of various shapes, sizes and location. 
     SUMMARY 
     The present disclosure seeks to address at least some of the difficulties discussed above. The disclosure provides example methods for gold (Au) TSM plating. ISM here is distinguished from Through Silicon Via (TSV) methods in that in TSM the Si is usually a sacrificial layer disposed above a metal seed layer and as such the deposition is intended to occur only in a via. A significant amount of gold deposition still occurs however on the Si field, typically occurring more severely at the via entrance. This spurious deposition is undesirable and, in some instances, a close-off of a via entrance may interrupt an Au via filling process. This interruption may cause or leave behind defective structures or partial fills and/or voids, and a highly non-uniform via filling. These and other issues are addressed in some examples by a cyclical deposition-etch treatment process, also referred to herein as an Etch Assisted Gold Through Silicon Mask (EAG-ISM). This process seeks in some examples to minimize Si field plating and maintain via entrance integrity, thus enabling complete via fill and the elimination of close-off voids, especially in high aspect ratio structures. 
     Thus, in some examples, a method for etch assisted gold (Au) through silicon mask plating (EAG-TSM) comprises: providing a seed layer on a substrate; providing a silicon mask on at least a portion of the seed layer on the substrate, the silicon mask including one or more via to be filled with Au; subjecting the masked substrate to at least one processing cycle, each processing cycle including an Au plating sub-step and an etch treatment sub-step; and repeating the at least one processing cycle until a selected via fill thickness is achieved. 
     In some examples, residual Au deposited adjacent a via by the plating sub-step is removed by the etch treatment sub-step. In some examples, a via-fill efficiency is based on a degree of via fill versus a degree of residual Au. 
     Some examples further comprise adjusting the via-fill efficiency based on an inclusion of Sulphur trioxide (SO3) a chemistry of the Au plating solution in the plating sub-step. Some examples further comprise adjusting the via-fill efficiency based on an inclusion of cyanide (CN) in a chemistry of the Au plating solution in the plating sub-step. 
     In some examples, an etchant in the etch treatment sub-step includes I-/I2 in a 6:1 molar ratio. In some examples, an etchant in the etch treatment sub-step includes aqua-regia in a 1:3 molar ratio. 
     In some examples, a cycle time of a first cycle in the at least one cycle is equal to a cycle time of a second cycle in the at least one cycle. In some examples, a cycle time of a first cycle in the at least one cycle is not equal to a cycle time of a second cycle in the at least one cycle. 
     In some examples, a frequency of applying the etch treatment sub-step during a first period of subjecting the masked substrate to the at least one processing cycle is different to a frequency of applying the etch treatment sub-step during a second period of subjecting the masked substrate to the at least one processing cycle. 
     In some examples, a plating system for a method of etch assisted gold (Au) through silicon mask plating (EAG-TSM) is provided. IN some examples, the plating system comprises: a cathode; an anode; an in-situ substrate processing module configured to perform alternating sub-steps of at least one processing cycle, the sub-steps including an Au plating sub-step and an etch treatment sub-step, the processing module common to each sub-step operation; a robot to receive or transfer a substrate to the processing module, the substrate including a seed layer and a silicon mask on at least a portion of the seed layer on the substrate, the silicon mask including one or more via to be filled with Au by the plating system; a substrate holder; and an etchant delivery means operable to deliver etchant during the etch treatment sub-step. 
     In some examples, the substrate holder is operable to lower the substrate into an Au plating solution in the processing module during the Au plating sub-step and withdraw the substrate from the plating solution after the Au plating sub-step. In some examples, the substrate holder is further operable to hold the substrate in the path of an etchant during the etch treatment sub-step while the substrate holder is in the withdrawn position. 
     In some examples, the processing module is configured to repeat the at least one processing cycle until a selected via fill thickness is achieved. 
     In some examples, the substrate holder is configured to apply a selected substrate rotation speed during at least the etch treatment sub-step. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Some embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings: 
         FIGS. 1A-1B  are schematic diagrams illustrating example aspects of methods involving Through Silicon Mask (TSM) and a Through Silicon Via (TSV) techniques. 
         FIGS. 2A-2F  illustrates schematic views certain field deposition irregularities, according to some examples. 
         FIGS. 3A-3C  depict sectional views of some example void defects. 
         FIG. 4  illustrates example operations in an EAG-TSM method, according to example embodiments. 
         FIGS. 5A-5C  illustrate aspects of example formations created by an example EAG-TSM process. 
         FIG. 6  shows sectional views of a processing module, according to an example embodiment. 
     
    
    
     DESCRIPTION 
     The description that follows includes systems, methods, and techniques that embody illustrative embodiments of the present invention. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of example embodiments. It will be evident, however, to one skilled in the art that the present inventive subject matter may be practiced without these specific details. 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. The following notice applies to the software and data as described below and in the drawings that form a part of this document: Lam Research Corporation 2019-2020, All Rights Reserved. 
     There are certain difficulties when seeking, to form metal structures at a very small scale. As discussed above, forming structures at a micron level for example is most commonly done by plating operations using one or more pre-patterned sacrificial masks. One method employs an organic photoresist mask and is commonly referred to as through-resist plating. The WLP industry also widely uses such methods to form Cu and Au RDL, flip-chip-bump interconnects of Cu, Ni and Sn alloy solder, and Au bumping. Even more complex structures such as those found in MEMS applications also utilize through-resist plating technology. 
     In some more specialized applications, there is a need to form large high aspect ratio (HAR) structures from gold metal. The dimensions of a large HAR structure may range roughly 2 to 50 μm (micron) laterally by 10 to 200 μm in height. The term ‘via’ is used herein to refer to empty spaces in a substrate or layer to be filled with metal, and the term ‘via filling’ is used herein to refer to metal deposition in these via. It is noted that such via are not necessarily round cylindrical in shape and may take on more complex shapes. Such large HAR dimensions can however prevent the use of more traditional organic photoresist masks. An alternate method includes the use of Si as a sacrificial mask.  FIGS. 1A-1B  depict respective schematic diagrams of example aspects of methods involving TSM and TSV techniques. TSM processes may be distinguished clearly from TSV processes in that integration schemes and plating deposition processes may be very different. 
     TSM plating with Au may present significant difficulties. The gold metal readily deposits spuriously on an Si mask, particularly at via openings.  FIGS. 28-2F  show some examples of problematic field deposition formations and possible issues relating thereto when performing Au TSM on a masked substrate of  FIG. 2A . Such undesired Au depositions on Si continue to grow, sometimes even more intensely at the via entrance which interferes with the desired Au via fill plating. Eventually the via entrance, or parts of the via entrance, are substantially closed off (see  FIG. 3C  for example), prematurely interrupting the Au via filling altogether. Such defective partially-filled via are colloquially termed voids and depending on the nature of the close-off these can be of various shapes, sizes and location.  FIGS. 3A-3C  depict examples of some of these defects (voids). 
     Taming to present examples that seek to address these issues, one approach for gold-based TSM includes a cyclical sequential plating-etch treatment method also termed Etch Assisted Gold TSM (EAG-TSM) herein. Example operations in such a method  400  are shown in  FIG. 4 . As illustrated, the method  400  may include phases or cycles  402  (for example, EAG cycle-I through EAG cycle-n, as shown). In some examples, each EAG cycle  402  in the EAG-TSM method  400  may include a plating sub-step  404  and an etch treatment sub-step  406  until a desired Au fill thickness  408  is achieved. During each etch treatment sub-step  406 , residual (unwanted) Au on the Si field is removed to leave a clean field (as shown) for the next plating sub-step  404 . The EAG-TSM method  400 , including the alternating cycles  402 , teiruinates once the desired Au via fill  408  has been achieved. 
     In some examples, an Au via-fill efficiency is defined during each deposition sub-cycle as a degree of via fill versus an unwanted Au-on-Si field deposition. Example fill efficiencies may be influenced by the gold chemistry (for example Sulphur trioxide SO 3  as opposed to cyanide (CN) based gold), the presence and/or type of additive and its concentration, processing conditions (for example, temperature, current density waveform, and convection to a lesser degree), and the deposition time of each sub-cycle. Deposition time may be a factor because spurious Au-on-Si field growth has been observed to hasten at longer deposition times (for example, &gt;2 hrs). Without wishing to be bound by theory, it is believed that Au deposition rates are faster after substantial nucleation has taken place, and that the Au surface area available for reaction also becomes progressively larger. It may be noted that although Au-on-Si deposition is spontaneous in some examples, under optimal chemistry and process conditions it is still substantially less than the Au via fill, i.e. which has a favorable fill efficiency. Thus, a significant amount of Au filling may have taken place before proceeding to an etch treatment step to remove the spurious Si field deposition. The plating cycle time is optimized to prevent excessive Si plating, maintaining good Au via-filling characteristics including uniformity. 
     Example chemistry options are available for etching the spurious Au-on-Si field deposition. Options may include I-/I2 (typically 6:1 molar ratio) and aqua-regia (formulary mix of nitric and hydrochloric, usually at 1:3 molar ratio), which are compatible with an Si mask. Etchant concentration, enhancing additives, temperature and processing conditions may be optimized to yield desired etch rates. Etching of the Au-on-Si field may occur at a much higher rate than the Au-via-fill. This may be due to the existence of large geometrical differences which may impose very different mass transfer conditions. The field readily has good access to etchant molecule supply, while the via on the other hand is always etchant supply limited due to diffusion limitations. As a result, the Au-on-Si field deposition is effectively removed with minimal reduction of the Au-via fill product. 
     Each deposition and etch treatment sub-cycle time can also be individually optimized. For example, while filling roughly the first half of an example 50 μm via, relatively small amounts field deposition may be observed. At thicker via fills, these spurious Au-on-Si depositions tend to occur more readily. In some examples, an optimal EAG-TSM process will generally be one in which the initial sub-deposition fill target is large, followed by higher frequency dep-etch cycles at later stages. 
       FIG. 5A  illustrates aspects of example formations created by an example EAG-TSM process  500 , or the method  400  described above. As shown at  502  and  504 , the spurious Au-on-Si field deposition is well removed after each etch treatment cycle in the alternating series of plate and step operations  500 . Process cycle times, especially on final stages thereof can be tuned to help minimize final overburden, which is defined as the amount of Au-on-Si field deposition. Respective sectional views of examples with slight and no overburden are shown in  FIGS. 5B-5C . 
     Some further examples include implementation of EAG-TSM from an equipment-machine design perspective. An example implementation design may he classified according to the following classes: class (I) ex-situ etch, or class (II) in-situ etch. 
     A class (I) example may include deposition and etch treatment phases in two separate process modules wherein the main function of each is directed to respective plate and etch treatment processes only. A wafer processed in this arrangement will typically be required to be transferred (usually facilitated by a robot) between the plate and etch modules during each cycle. An example plating cell may include certain elements optimized to produce good quality gold plating. Similarly, an etch module may also be optimized for quality etch treatment. Thus, for example, an etch treatment module may include (a) a dispense technique or (b) adopt an immersion approach. In option (a) employing the dispense technique, a set of nozzles delivers etchant liquid on to a wafer surface only during processing. Etchant effluent, if still sufficiently potent, can be reclaimed for later reuse. On the other hand, the option (h) immersion approach immerses the wafer into an etchant bath. This can be an attractive method from a process and design simplicity perspective. 
     The class (II) in-situ design approach may combine in some examples both the deposition and etch treatment in the same process module. An example of such a module  600  is depicted in  FIG. 6 . In some examples, the same module (or plating cell)  600  performs a gold plating step and an etch treatment step. The steps may alternate repeatedly in a series. One step may be repeated a number of times before the alternate step (or cycle) is performed. Thus, for example, two plating steps followed by an etching step, then one plating step followed by three etching steps, and so forth. Many variations are possible. 
     As shown, at the end of a plating cycle, a wafer holder removes (in the right-side view) a processed wafer out of the illustrated plating solution. One or more sets of nozzles (e.g. as illustrated) within the module  600  rinse the withdrawn wafer, initially with water and then with etchant in preparation for an etch treatment step. In some examples, high wafer rotation speeds are applied during the rinse and etch treatment steps to help divert rinsate and etch treatment effluent from entering the plating cell bath. As shown, a movable barrier such as a shutter may also be retracted or advanced to further minimize contaminating the plating solution bath. 
     Thus, in some embodiments, an EAG-TSM cyclical plate-etch treatment is provided as a combinatory process. Examples may provide certain differentiating characteristics and benefits. A resolution of problems associated with close-off related void defects may be provided. Due to the defect-inhibiting nature of EAG-TSM processes, some examples may be deployed successfully to form thick or tall via structures and achieve no-defect performance levels higher than conventional plating processes. Some examples address production issues relating to excessive spurious Au-on-Si field deposition. Example EAG-ISM processes can produce minimal Au-on-Si deposition at levels otherwise not achievable with conventional plating process. Some example EAG-TSM processes facilitate the minimization or even complete removal of Au overburden. Some examples allow independent control, tuning, and optimization of via fill deposition versus Au-on-Si field deposition. 
     Although an embodiment has been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may he practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
     Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will he apparent to those of skill in the art upon reviewing the above description.