Jim Connett, on May 22, 2024, 11:24 AM
Steps – The Foundation of End-To-End Traceability in Manufacturing
While all parts of an MES model are important in the manufacturing process, the definition of steps is a foundational element upon which all other modeled components are built. Steps serve as the natural boundary between one action and another action at the physical level. At the transactional level, steps help to clarify and specify data collection and KPI measurements that help manufacturing facilities perform at their optimal levels.
Many types of step and step implementations exist. A step could be implemented as a singular transaction, a group of ordered transactions, or a collection of non-sequential steps whereby all steps must be completed in any order before the material can move to the next step. Much could be said about each of the implementation types, but for now, we will focus on the overarching benefits of properly defining steps with an understanding that the benefits will be realized regardless of how steps are ultimately implemented.
We loosely define a step as “a unit of work performed on a material.” MESs might use the term “operation” to define the same (we will use the term “step”). No matter the term used, we are talking about an action performed on material that (in most cases):
- is singular in its purpose
- is automation- and equipment-agnostic
- does not have to result in a value-added action on the material (though it often does)
Navigating Complexities in MES Step Definition
Defining steps in the MES is usually straightforward, but equipment complexity or multifunction tools can result in necessary deviations from general best practices. These situations require clear analysis to understand intricacies, planning, and foresight for proper implementation, and a willingness to refine the model when conflicts emerge. The principles mentioned below will frame the step modeling but practical implementation should always take priority. With that now clear (maybe!), let’s briefly expand on each of these end goals.
Understanding the Singular Purpose of MES Steps
A step executes one action, one process, or one function, and is measured and quantified in isolation. For example, imagine a step that measures line structure dimensions using a scanning electron microscope (SEM). As part of this measurement, the SEM collects width, length, and density measurements across 60 points and reports an aggregated average, range, and standard deviation to a Statistical Process Control (SPC) application for collection and evaluation. The SEM is reporting a significant amount of data, but the step itself is singular in purpose: collect measurement data on a line structure.
When it comes to cluster tools – that is, tools linked together in some way, the “singular in purpose” motif remains, but it becomes a bit broader in scope. Take, for example, coaters and steppers in Photolithography. In automated facilities, these two independent tools are often joined together in some way to execute the coat-expose-develop process as one logical step – 1) the track coats the wafer, 2) the stepper exposes the image on the wafer, and 3) the track then develops the image. If these two pieces of equipment (performing three “steps”) were physically disconnected from each other, then separate steps for each “action” would be warranted. In our example scenario, the two tools are independent pieces of equipment (sometimes manufactured by different vendors) yet they are physically connected and therefore modeled as one piece of equipment (from a step point-of-view). The track depends on the attached stepper to expose the coated wafers sent to it by the track, and the stepper depends on the attached track to coat the wafers for exposure and develop the wafers after being exposed. Depending on the business requirements, these linked tools could be modeled as separate tools – and therefore separate steps – in the process or as one piece of equipment – and therefore, one step – to accomplish the task of coating, exposing, and developing. To be sure, this is a complex process with many nuances. The best approach depends on the desired complexity, data requirements, and available resources for implementation efforts.
Automation’s Role in Defining Manufacturing Steps
The existence or absence of automation is never a prerequisite for a step. A step can involve the use of multi-million-dollar tools or no tools at all. One step we often see in the semiconductor manufacturing space that does not involve high automation is a step defined at the beginning of the flow that marks the induction of bare silicon from storage into the fab. This step is sometimes required by ERP systems to cost (it’s no longer raw material, it’s now WIP). The transit of these wafers from raw material in storage to WIP in the fab may – but does not often – require equipment, but it can – and should – be modeled as a step.
Differentiating Value-Add and Non-Value-Add Process Steps
Many process steps involve physical changes to the wafer that add value, like cleaning, etching, curing, or component attachment. Other necessary steps don’t directly add value but could still be modeled separately, such as wafer movements between pods, alignments, or validations. Combining value-add and non-value-add actions into one step violates the previously stated goal of a single action definition and may cause unintended side effects with data collection, KPIs, scrap, and rework points. Keeping each step singularly focused, whether value-added or non-value-added, minimizes these complications.
In general, if there is any doubt as to whether an action performed should be a step or not, make it a step in the flow as the first course of action. Modeling in the MES is never a “one-size-fits-all” proposition. Exceptions can and do exist. A willingness to change the model when needed will help guide the modeling process to an effective solution. As stated earlier, the three characteristics we see in step definitions are just guidelines, not absolute requirements. Outlier situations and unique requirements will often expand the definition of a step well beyond the three considerations listed above. It's vital to have the insight of an experienced modeler in this discussion. SYSTEMA is positioned to partner with you to guide these discussions and help implement the best solution for your unique environment. For more information, please contact us.
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