Seminar on semiconductor technology







Semiconductor Laboratory

Seminar on semiconductor technology

The web page is based on a two-day seminar consisting of some lectures, demonstrations, and lab tours that provide an introduction to semiconductor technology.

Introduction

<div align="center">
<iframe allowfullscreen="" src="https://tube.tugraz.at/paella/ui/embed.html?id=0b41eba2-8f86-46a7-b00d-529e750ad92b" style="border:0px #FFFFFF none;" name="Paella Player" scrolling="no" marginheight="0px" marginwidth="0px" width="480" height="300" frameborder="0"></iframe>
<p style="text-align:center">Introduction - <a href="https://tube.tugraz.at/paella/ui/watch.html?id=0b41eba2-8f86-46a7-b00d-529e750ad92b">Video</a></p>
</div>

<p><a href="http://lampz.tugraz.at/~hadley/semi/books/books.php">Reference books and websites</a></p>
<p>The link to the original Intel video is:</p>

<p>This is the link to the original Wacker video that shows how silicon is purified.</p>

<p>This is the link to the original MicroChemicals video on wafer production. This video describes the Czochralski process.</p>

<p>The video of the float zone process was taken from <a href="https://pv-manufacturing.org/silicon-production/float-zone-silicon/">PV-Manufacturing.org</a>.</p>

<p>For more information on wafers, see: <a href="http://www.processpecialties.com/siliconp.htm">Silicon facts at Process Specialties</a>.</p>

<!-- don't  need this in the overview
<div align="center">
<iframe allowfullscreen="" src="https://tube.tugraz.at/paella/ui/embed.html?id=a51661b9-fc4d-4075-abc8-f575a7f8d232" style="border:0px #FFFFFF none;" name="Paella Player" scrolling="no" marginheight="0px" marginwidth="0px" width="480" height="300" frameborder="0"></iframe>
<p style="text-align:center">Resistivity and the Seebeck effect -  <a href="https://tube.tugraz.at/paella/ui/watch.html?id=a51661b9-fc4d-4075-abc8-f575a7f8d232">Video</a></p>
</div>

<ul>
<li><a href="http://lampz.tugraz.at/~hadley/sem/maxwell/maxwell.php">Maxwell contacts.</a></li>

<li><a href="http://lampz.tugraz.at/~hadley/semi/ch9/measurements/srp/srp.php">Spreading Resistance Profiling.</a></li>

<li><a href="http://lampz.tugraz.at/~hadley/semi/ch9/measurements/4pt/4pt.php">Four point resistance measurements.</a></li>

<li>   
<a href="https://www.pvlighthouse.com.au/">PV Lighthouse</a> provides calculators that can be used to determine the doping concentration is the resistivity and doping type (n-type or p-type) is known.</li>
</ul>

-->

Lithography

<p>Lithography is the process of making a pattern on a substrate. The most common form of lithography is photolithography (based on light). The key element is the photoresist. With help of a mask only parts of the photoresist-covered substrate are irradiated by light, 
leading to either cross-linking (negative resist) or scission (positive resist) of the resist. The photoresist is then cured and afterwards developed in a wet chemical step, removing the areas with short bonds in the resist layer.</p>
<p>If the exposed area is removed = positive resist</p>
<p>If the non-exposed area is removed = negative resist</p>

Picture taken from: <a href="http://lampz.tugraz.at/~hadley/memm/books/MKSHandBook.pdf#page=153&zoom=100,0,0">Chapter VI.A (p.143)</a> of the MKS Handbook.</p>
</center>

<p>Problematic is the not cost-effective production for small series or prototypes due to the expensive and time-consuming mask production. Resolution limiting factor is the diffraction limit.
Further methods of irradiation are x-rays, electrons and ions.<br>
For more information about Photolithography see the websites <a href="http://www.lithoguru.com/scientist/lithobasics.html">lithoguru.com</a> for an overview of the basic process for semiconductor industry.<br>
The website <a href="http://lampz.tugraz.at/~hadley/semi/lithography/lithography.php">lampz.tugraz.at</a> offers information about process conditions and resists.<br>
For further knowledge about scaling down with the use of deep UV (DUV) and extreme UV (EUV) look at <a href="http://lampz.tugraz.at/~hadley/memm/books/MKSHandBook.pdf#page=153&zoom=100,0,0">Chapter VI.A</a> of the MKS Handbook.</p>
<p>An overview of photolithography as well as considerations to take (resolution, DOF, wavelength, immersion,...) are presented in this video:</p>
<center>
<iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/HPyckggOa4U" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</center>
<p>A whole chapter about optical lithography containing the process flow (spin coating, baking, alignment, exposure, development, inspection), resist chemistry, resist application, alignment and overlay, exposure, resist profile, resolution, pattern shape, lithography practice and photoresist stripping
can be found in <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch9">Chapter 9 of Introduction to Microfabrication</a>.</p>

<h3>Electron beam lithography (EBL)</h3>

A video about EBL set in comparison with photolithography with basic explanation is the following one:
<center><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/PWV9pvdRBNY" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center>

<p>Electrons are used as a particle beam instead of light. With the de-Broglie wavelength:</p>

$$\lambda_B=\frac{h}{\sqrt{2mE}}$$

<p>One can see that the wavelength, which is the resolution limiting factor in photolithography (visible light, UV-light,...), can be reduced with increasing the acceleration voltage (higher E). This leads to the possibility of structuring down to the nanometer range. 
As the wavelength is not the limiting factor for resolution anymore the size of the electron beam due to electron repulsion starts to play a role. <br>
The main issue is the resulting excitation bulb of the electron beam on the substrate, causing a higher interaction volume than the actual beam diameter. 
EBL is very costly and time consuming. Main advantage is the flexibility and the maskless direct scribing of the wanted pattern. It is mainly used for mask production (in conjunction with photolithography), rapid prototyping and small batch production.</p>

<p>In the following video series (3 parts) the procedure of EBL in a lab is demonstrated in detail from preparing the Si-wafer, setting up the instruments up to developing the wafer and seeing the final product.</p>
<center><p><iframe width="280" height="157" src="https://www.youtube-nocookie.com/embed/bvgITKqYpuY" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<iframe width="280" height="157" src="https://www.youtube-nocookie.com/embed/fZmC5xeHHaw" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<iframe width="280" height="157" src="https://www.youtube-nocookie.com/embed/ktkDqofYfMY" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></p></center>
<p>To learn more about EBl there is a chapter in <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch8">Introduction to Microfabrication Chapter 8</a>.</p>

<h3>Laser direct writing</h3>
<p>Laser processes can be divided into 3 subsections, namely ablation, photolytic and photothermal.</p>
<p>Laser ablation uses a very short pulsed laser (up to femtoseconds) to heat up the surface of the wafer. This leads to an explosive evaporation of the material due to the high energy of the intense laser absorbed. 
Limitation of laser ablation is mainly the wavelength of the laser (energy) and a high absorbing material which needs to match the laser. With femtosecond pulses nearly all materials can be ablated. Higher machining speeds can be reached with nanosecond pulses as the longer pulse time offers more energy.<br>
A local facility that works with laser ablation is the <a href="http://www.iap.tuwien.ac.at/www/atomic/laser/index">TU Wien</a>.<br>
Also in the Institute of Solid State Physics of the TU Graz one can find a laser cutter (Universal laser cutter).</p>
<p>The following video gives detailed information about laser ablation:</p>
<p><center><iframe width="320" height="180" src="https://www.youtube-nocookie.com/embed/F9y8a0NWCXg" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center></p>
<p>For laser ablation in binary semiconductors see the paper <a href="file:///C:/Users/alexk/Downloads/QE-2010_Eng.pdf">Pulsed laser ablation of binary semiconductors: mechanisms of vaporisation and cluster formation</a>.</p>

<p>Photoytic laser writing is mainly used as laser assisted CVD where the energy of the laser produces a breakdown of a reactive gas.</p>
<p>A typical photothermal laser writing process is laser annealing. In <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch24">Chapter 24 of Introduction to Microfabrication</a> is more information about laser direct writing.</p>

<h3>Screen printing</h3>

<p>The video of Mekoprint Electronics shows the screen printing process in a roll-to-roll fashion on a flexible substrate for production of flexible printed circuits:<br>
<center><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/q_RYZEWRA1I" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center></p>
<p>Screen printing is an old technique which is used to mainly produce patterned, thick layers of conductive or dielectric materials (also sometimes for OFETs) but also presents the major technique for metallization in solar cells. For screen printing a screen mask made of mesh (synthetic or metal fibers) and a squeegee are used. 
The squeegee is used to squeeze the ink through the screen mask onto the substrate to coat it in a pattern like the mask. This process is at low cost, does not need vacuum and yields high output-volume but mostly 2D structuring is possible. Another limitation is the need of a mask, which does not allow e.g. complete circles to be printed with this technique.</p>

<p style="text-align:center"><img src="lithography/screen_printing.PNG" alt="pscreenprinting"><br />Picture taken from: <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch23">Chapter 23 (p.291)</a> of Introduction to Microfabrication.</center></p>

<h3>Ink Jet printing</h3>
<p>Ink jet printing is an additive manufacturing technology, giving the possibility of printing functional materials with dielectric, conductive, chemical, optical or mechanical properties. 
Same as conventional ink jet printing a piezo-driven or heated small volume can be contracted to squeeze out a small droplet (~ pico-liter) of ink at a given position on the substrate with the main difference that a functional material is used as ink. 
To counter a post-ejection movement of the droplet which produces satellite drops, different pulses for the piezo element are used to minimize this and to improve resolution. With this technique it is possible to create features down to 20 microns. 
This method is very versatile, fast and even 3D structuring is possible. With varying the used ink even polymer transistors on Si-wafers can be printed. More about ink jet printing can be read up in Introduction to Microfabrication <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch24">Chapter 24.5</a>.<br> 
The following video shows the working principle of a common ink jet printer based on resistive heating to produce a gas bubble that pushes the ink out of the reservoir. The same is used in semiconductor processing:</p>
<center><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/9yeZSaigBj4" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center>
<p>Using semiconductor-based inks is relatively new (2020). For more information about the recent research developments in this field see the <a href="https://arxiv.org/abs/2011.12359">article of the Cornell University</a> or the <a href="https://onlinelibrary.wiley.com/doi/full/10.1002/advs.201801445">article of Advanced Science</a>.</p>

Etching

<p>Etching is a group of processes used in microfabrication and semiconductor processing for removing layers from the surface of wafers or semiconductor structures.<br>
It is a common process used in semiconductor industry for removing material. Controllability, Selectivity and Uniformity of the process play a high role for suitability. 
Etching can be separated into dry and wet etching. The following video gives an insight into wet and dry etching:</p>

<p>For an overview of etching in microelectronics I recommend <a href="http://onlinelibrary.wiley.com.wiley.ftubhan.tugraz.at/doi/10.1002/9781119990413.ch11/pdf">Chapter 11 of Introduction to Microfabrication</a>. For additional and more in-depth information go to <a href="http://lampz.tugraz.at/~hadley/memm/books/MKSHandBook.pdf">Chapter VII of the MKS Handbook</a>.</p>
<p>As cleaning is a big part of etching you can read more about wet and dry cleaning for etch preparation in <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch12">Chapter 12</a> of Introduction to Microfabrication.</p>

<h3>Wet etching</h3>

<p>Wet etching is the chemical removal of material by radical reactions. Liquid chemicals are used which lead to high etching rates at low costs. Due to the chemical nature of this process it yields good selectivity and is mostly used for applications in the micron range. Problematic is the critical etching distance control which is done by either time control, etch stop layers (material layers or dopant-specific) or self-limitation (removed material hinders further etching). Wet etching is most of the time an isotropic etching process.</p>
<p>In the following video a typical isotropic wet etching process in a lab can be seen:</p>

<center>
<iframe width="560" height="315" src="https://www.youtube.com/embed/1gyTZwwCZM8" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</center>
<p>Some wet etching recipes can be found at <a href="https://cleanroom.byu.edu/wet_etch">cleanroom.byu.edu</a> or the book <a href="https://www.sciencedirect.com/book/9780127282503/thin-film-processes#book-info">Thin film Processes</a> by Vossen and Kern. </p>

<p>It is also possible to perform wet etching in an anisotropic fashion by having different etching rates for different crystallographic directions. There is a chapter on <a href="http://onlinelibrary.wiley.com.wiley.ftubhan.tugraz.at/doi/10.1002/9781119990413.ch20/pdf">Anisotropic wet etching</a> in Introduction to Microfabrication</a>.</p>

<h3>Dry etching</h3>
<p>For producing even smaller structures than the micron scale often dry etching is used as well as for chemically resistant materials. Dry etching shows higher etching anisotropy which results in higher aspect ratios of etched structures. In comparison to wet etching, the dry etching process must be performed under vacuum. Dry etching can be divided into physical and chemical dry etching.</p>
<p>A good overview about dry etching processes in a bit more detail with schematic descriptions can be found in the video of the Stanford Nanofabrication Institute:
<center>
<iframe width="280" height="157" src="https://www.youtube.com/embed/UMvRXF3ZZr4" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</center>

<h4>Physical dry etching</h4>

<h5>Ion beam etching</h5>
<p>Similar setup as in <a href="https://en.wikipedia.org/wiki/Plasma-enhanced_chemical_vapor_deposition">PE-CVD</a> can be used for physical sputter etching. With the help of a plasma ions (mostly Ar) are produced and accelerated towards the substrate where they knock out atoms of the substrate. The big advantage is the non-material-dependent high anisotropy. However, due to this non-material dependency this process is non-selective. Furthermore, disadvantages are a low etch rate, implantation of sputtering ions into substrate, trenching and broadening (change of wanted geometry) and material redeposition. </p>

<p>In <a href="http://lampz.tugraz.at/~hadley/memm/books/MKSHandBook.pdf#page=174&zoom=100,0,0">Chapter VII.D</a> of the MKS Handbook is a good description of Sputtering and Ion milling as well as a schematic drawing of an ion beam etching reactor. 
On the website <a href="https://www.azom.com/article.aspx?ArticleID=7533">azom.com</a> one can find more detailed descriptions and especially also microscope images of etched surfaces. 
An extremely detailed but not via the TUG library accessible book about this technique is the Handbook of Advanced Plasma Processing Techniques <a href="https://link.springer.com/book/10.1007/978-3-642-56989-0">Chapter "Ion beam etching of Compound Semiconductors"</a> (free preview).</p>

<h4>Chemical dry etching</h4>

<h5>Isotropic radial etching</h5>
<p>This dry etching process is based on chemical interaction between the substrate and the reactant gas to form a volatile product. There are plasma-less methods which use gases like XeF2 or ClF3 (from sublimation) that can dissociate spontaneously and form fluorine. Fluorine then aggressively attacks silicon and etches away the wafer. These processes are problematic for industrial use due to the safety problems as fluorine is hazardous and can form HF when in contact with water. Due to the character of this process it is fully isotropic and selective.
Another plasma-free option is oxide etching with HF. The HF vapor attacks SiO2 and etches it away. Same as before this process is isotropic and selective as it is based on non-directional vapor diffusion.</p>

<p>Furthermore, there are methods that use plasma (mostly refered as chemical dry etching or Plasma etching) for changing the surface chemistry of the substrate by glow discharge without destroying the surface with accelerated ions. 
Then the reactive and volatile gas is led into the vacuum chamber where it reacts with the modified parts of the substrate to a volatile reaction product. <br>
The following video explains basic knowledge about plasma and sheath and is recommended to start with:</p>
<center><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/kBGA8XfmGPg" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center>

<p>This process offers high selectivity, isotropic behavior and fast etch rates and is mostly used for cleaning wafers or removing complete layers. </p>
<p>More about plasma etching can be found at <a href="https://www.halbleiter.org/en/dryetching/etchprocesses/#Plasma_Etching">halbleiter.org/plasma_etching</a>.<br>
For information about oxide etching go to the website <a href="https://tantec.com/the-process-of-plasma-etching/">tantec.com</a>.<br>
The website <a href="https://en.wikipedia.org/wiki/Etching_%28microfabrication%29">wikipedia.org/Etching</a> offers some plasma etching recipies.<br>
A local facility that uses a plasma etcher is the Institute of Solid State Physics of the TU Graz. For further information about the used plasma etcher: <a href="http://lampz.tugraz.at/~hadley/semi/ch9/instruments/Diener/femto.php">Diener Femto Plasma Etcher</a><br>
The next video shows a quick demonstration of the Diener femto plasma etcher:</p> 
<center><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/gpoN_u7970Y" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center>

<p>For a detailed description of different setups for plasma etchers watch this video:</p>
<center><iframe width="280" height="157" src="https://www.youtube-nocookie.com/embed/5LZtsVI_Rg0" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center>

<h4>Combined physical and chemical dry etching</h4>
<h5>Reactive ion etching (RIE)</h5>
<p>For a first overview about RIE see the following video.</p>
<center><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/MC2yFdTqqbU" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center>
<p>This process is a combination of physical and chemical dry etching with a tunable ratio. One of the most important etching processes in semiconductor industry.  Same principal as in plasma etching where a volatile reactant should be formed. Ions formed by the plasma are accelerated towards the substrate to lower the activation energy (weakening of bonds, higher local temperature) for the following chemical etching step with a reactive gas. 
Due to the directional ions trajectory a high anisotropy in contrast to plasma etching is possible. Further advantage is a cleaner surface due to ion bombardment. More about RIE in <a href="http://lampz.tugraz.at/~hadley/memm/books/MKSHandBook.pdf#page=172&zoom=100,0,0">Chapter VII.C</a> in the MKS Handbook or the website <a href="https://www.halbleiter.org/en/dryetching/etchprocesses/#Reactive_Ion_Etching">halbleiter.org/reactive_ion_etching</a>.</p>
<p>A little "lab tour" of a RIE device is shown here:</p>
<center><iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/dkMeB3CE7L8" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></center>

<p>Deep reactive ion etching is an advanced technique based on reactive ion etching (RIE). It is highly anisotropic, capable of producing high aspect ratios (about 50:1) with structural depth up to some hundred microns. More about DRIE in <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch21">Chapter 21 of Introduction to Microfabrication</a>.</p>

<p>An overview of gases used in dry etching is given at the website <a href="https://www.halbleiter.org/en/dryetching/etchprocesses/#Overview_of_gases_used_in_dry_etching">halbleiter.org/gases</a>. More about vacuum and plasma, including plasma etching and sputtering can be found in <a href="https://onlinelibrary.wiley.com/doi/pdf/10.1002/9781119990413.ch33">Chapter 33</a> of Introduction to Microfabrication.</p>
A video that describes the theoretical basics of RIE instead of reading the book chapters is this one:
<center>
<iframe width="280" height="157" src="https://www.youtube-nocookie.com/embed/vodWDIEfmok?start=130" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</center>

<h3>Comparison and Overview</h3>

<p>In the following table some important characteristics of the presented processes are compared and summarized.</p>
<center>
<table>
  <tr>
    <th></th>
    <th>Wet etching</th>
    <th>Dry physical etching</th>
    <th>Dry chemical etching</th>
    <th>Dry combined etching</th>
  </tr>
  <tr>
    <th>Isotropy</th>
    <td>Isotropic</td>
    <td>Anisotropic</td>
    <td>Isotropic</td>
    <td>Tunable anisotropy</td>
   </tr>
   <tr>
    <th>Selectivity</th>
    <td>High</td>
    <td>(nearly) no</td>
    <td>High</td>
    <td>Medium</td>
   </tr>
   <tr>
    <th>Etch rate</th>
    <td>High</td>
    <td>Low</td>
    <td>High</td>
    <td>Medium</td>
   </tr>
   <tr>
    <th>Etching distance control</th>
    <td>Hard</td>
    <td>Good</td>
    <td>Hard</td>
    <td>Very good</td>
   </tr>
   <tr>
    <th>Costs</th>
    <td>Low</td>
    <td>High</td>
    <td>High</td>
    <td>Medium</td>
   </tr>
</table>
</center>

Wafer dicing

<p>The process of cutting a finished wafer into individual chips is typically called <em>wafer cutting</em>, but names like <em>wafer sawing</em>, <em>chip separation</em> and <em>die singulation</em> can also be found in literature.
It is the last step before chips are shipped off to be encapsulated in packages, and ultimately being sold to customers.
Dicing methods are selected based on the material composition of the wafer and the product requirements.
Typical materials that have to be diced include silicon, glass, metals (e.g. Al, Cu, Au, Ag,...), composite substrates (e.g. silicon carbide, gallium arsenide, gallium nitride,...), epoxies, imides and other layers.
The two most prevalent dicing methods in the semiconductor industry are mechanical dicing and ablative laser dicing.
</p>

<h3>Pre-dicing</h3>
<p>Before any dicing can be done, the wafer has to be attached to a polymer tape.
For handling purposes, the tape is attached to a ring-like plastic frame.
Since the dicing step has to fully separate the chips, a bit of the tape is damaged or removed while dicing and the tape is discarded after die picking is finished.
An adhesion layer is applied onto the tape before wafer mounting.  
This is done to keep the separated chips in place, preventing them from getting lost or damaged.
After dicing, the adhesion layer is cured with either heat or UV radiation. 
This reduced the stickiness of the tape, which is necessary for dies to be picked from the tape. 
To make the die picking easier, the polymer tape is mechanically expanded as a last step. 
This separates the dies without damaging chip sidewalls.</p>

<h3>Mechanical dicing</h3>
<p>In mechanical dicing, a thin rotating blade is used to grind away material in the scribe line, the electrically inactive material between the chips.
The blade has embedded diamond pieces to shear away material at high speeds (30 000 - 60 000 rpm).
To cool the blade and wafer and to transport the debris away from the dicing channel, the dicing setup includes a water jet that is guided toward the blade. </p>
<p style="text-align:center">
<img src="dicing/mechanical_dicing_sketch.png" alt="Mechanical dicing sktech">
<br>
Schematic cross section of the mechanical dicing process:<br> The dicing blade (green) grinds away material in the scribe line between chips. <br>
The chips are attached to a polymer dicing tape (blue) via an adhesion layer (yellow). <br>
Mechanical dicing can induce chipping (black), which is divided into frontside chipping, sidewall chipping and backside chipping.</p>
<p>While mechanical dicing has been the workhorse of the semiconductor industry since the 1970s, there are some innate disadvantages to this method.</p>
<p>First off, mechanical dicing works well for brittle materials like typical semiconducting substrates (Si, GaAs, GaN, SiC, ...).
But for ductile or soft materials, the voids and spaces between the abrasive are filled up, making it harder for the blade to transport material out of the dicing channel.
This process is called <em>blade clogging</em> or <em>blade overloading</em>, and can lead of increased chipping on the chip sidewalls and can destroy the blade.</p>
<p> Another problem with mechanical dicing is chipping. Depending on the amount and size of the chipping defects, the chip might have to be discarded.
This  becomes an even bigger problem for thinner wafers, because the mechanical stress on the thin substrate can warp the wafer or in some cases can even cause catastrophic wafer breakage. 
Therefore mechanical dicing is ill-suited for thin wafers below a thickness of 100 �m.</p>

<p>For more information, this video goes into details about the mechanical cutting process:</p>
<center>
<iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/klhaPz_bgaY" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen=""></iframe>
</center>

<h3>Ablative laser dicing</h3>
<p>In ablative laser dicing, a laser beam with nanosecond pulses is focused onto a very small spot on the wafer surface.
The fluence (energy per area) of the laser spot on the wafer becomes so high that the material melts and vaporizes. This opens up a so called <em>kerf</em>, separating the two chips.</p>
<p> For thin wafers, ablative laser dicing is the go-to method since chipping and catastrophic wafer breakage do not occur.</p>
<p style="text-align:center">
<img src="dicing/ablative_laser_dicing_sketch.png" alt="Ablative laser dicing sketch">
<br>
Schematic cross section of the ablative laser dicing process:<br> A laser beam (red) is focused onto the wafer surface, material melts and vaporizes and is ejected out which creates a kerf between chips. <br> Resolidified material is deposited as recast (orange). The heat introduced by the laser creates a Heat Affected Zone (purple).</p>
<p>Since laser dicing is a thermally driven process, there is some thermal damage in the surrounding substrate. This volume is called <em>Heat Affected Zone</em> or HAZ for short and introduces defects in the lattice. This reduces the break strength of the chip and can lead to chip breakage when stress is applied.</p>
<p>Another issue that occurs with laser dicing is the generation of recast. The material that is molten by the laser cools down on the chip sidewall and creates stress and microcracks, further reducing die strength.</p>
<p>There are several approaches to minimize the damage introduced by the laser while still allowing fast processing time. The most prominent methods increase the area of the laser while keeping the power steady, thus reducing the fluence. This keeps the processing time short while allowing a good sidewall quality.</p>

<p>Another method that is rising in popularity is dicing with pico- or femtosecond lasers. 
Using ultrashort pulses, there is not enough time for the laser to transfer energy to the lattice. 
This <em>cold ablation</em> leads to direct sublimation of the material and creates less thermal damage compared to nanosecond pulsed lasers. 
The biggest issue is that processing time is considerably slower than other dicing methods, making this method relatively expensive. 
But ultrashort pulse laser dicing is looking like the next obvious step to improve laser cutting.</p>

<p style="text-align:center">
<img src="dicing/ukp_sketch.png" alt="Ultrashort laser dicing sketch">
<br>
Schematic cross section of the ultrashort pulse ablative laser dicing process:<br> A laser beam (red) with pico- or femtosecond pulses is focused onto the wafer surface, material sublimates and creates a kerf between chips. <br> The amount of resolidified material (orange) and the size of the Heat Affected Zone (purple) are reduced.</p>

<p>A good overview of the ablative laser dicing process with both nanosecond and pico- and femtosecond methods can be found in this video:</p>
<center>
<iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/uDTMJksf5W8" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen=""></iframe>
</center>

<h3>Non-ablative laser dicing</h3>
<p>Non-ablative laser dicing, often called <em>stealth dicing</em>, can enable chip separation without generating some of the downsides of ablative laser dicing. Since the method does not remove material, there is no recast generated.
Stealth dicing is a two-step process. In the first step,  a setup with a wide numerical aperture is used. In other words, the setup has a very small depth of focus, where moving out of focus quickly reduces the laser fluence. This allows the laser to pass through the surface without generating enough fluence to damage the material. But in the middle of the material stack the fluence becomes so large that material melts and expands, creating defect zones with small voids.</p>

<p style="text-align:center">
<img src="dicing/stealth_dicing_1_sketch.png" alt="Laser induced defect zones in middle of material">
<br>
Schematic cross section of the first step in stealth dicing:<br> A laser beam setup with a small depth of focus (red) passes through the surface of the wafer and introduces a defect zone (violet) inside the bulk material.</p>
<p>The second step introduces stress into the wafer by expanding the tape. This leads to the creation of cracks, which then travel along the defect zones generated by the laser. The end result is a separated chip without any surface damage and relatively clean chip sidewalls.</p>
<p style="text-align:center">
<img src="dicing/stealth_dicing_2_sketch.png" alt="Separation of chips through tape expansion">
<br>
Schematic cross section of the second step in stealth dicing:<br> By expanding the tape, cracks start to form and travel along the laser modified volume.<br> This separates the dies without damaging the surface.</p>
<p>Stealth dicing has some major downsides compared to other methods. The wafer can not be doped heavily or have opaque structures in the scribe line, since this prevents the laser from properly coupling in. Also since the opening angle of the laser beam is so large, the scribe line has to be relatively broad. And the method does not work for very thin wafers.</p>

<p>An overview of stealth dicing is shown in this video:</p>
<center>
<iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/MhY8RPREI_c" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen=""></iframe>
</center>

<h3>Other separation methods</h3>
<p>There are many other separation methods which are not typically used in production for one reason or another, but can have some advantages or features which are required for certain problems.</p>
<p>Plasma dicing involves a lithography step. Then a Bosch plasma etching process is used to anisotropically etch the area that is not covered by the photoresist. This allows the separtion of chips that are not rectangular but requires Bosch etch tools. However, since plasma dicing is a parallel process it can be cost-efficient if the chips are very small. For more detail, watch the following video:</p>
<center>
<iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/019kdZku1L0" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen=""></iframe>
</center>
<p>Dicing-before-grinding or dicing-before-thinning involves a mechanical dicing step that does not fully separate the wafer. After mounting the diced side to a carrier wafer, the backside is grinded down until only the separated chips remain. This method enables mechanical cutting of very thin wafers, but is more expensive and requires post-grinding treatment to remove residual stress of the thinning process.</p>
<input type="button" value="-" onclick="document.getElementById('dicing_div').innerHTML=document.getElementById('blank').value;">