Tag Archives: Semiconductor

Knowledge Base Fact Sheet

Lithography Basics

14th December 2019

Introduction to the basics of photolithography

Lithography at its most general is the process of patterning the surface of a substrate to build up a complex design or structure.

Photo (UV) Lithography

Photo (or UV) lithography is the technique of pattering through the use of a light sensitive polymer (called photoresist) and a stencil (called a photomask)

The first step in any photolithographic process is to prepare the substrate – in most cases the substrate is a silicon wafer but can in theory be any material. Substrate preparation is undertaken to improve the adhesion of the photoresist to the substrate. Typical steps include substrate cleaning – to remove any dirt/contaminates, dehydration bake – to remove any water and then the addition of an adhesion promoter. Going through these steps will reduce the number of contaminates both organic and inorganic which will ensure the best results as we move through the lithography process.

Once the substrate has been prepared the photoresist can be applied to the surface. To get the best results a thin uniform coating is required; this thickness is controlled through a process called spin-coating. The photoresist is deposited onto the substrate. This is then spun on a turntable to 1000s of rpm spreading out the viscous photoresist into a thin layer. This thin resist is then soft-baked on a hotplate to remove excess solvent and to stabilise the resist film.

The next step in the photolithography process is to align the resist covered substrate to the photomask and to expose to UV light. The critical principle in photolithography is that the solubility of the photoresist is changed once exposed to UV light. Photoresists come in two categories: positive or negative.

In a positive resist, the part of the film that is exposed to the resist becomes more soluble and can be removed with a developer. In a negative the opposite happens, where the resist has been exposed to light the resist becomes harder and can’t be removed by the developer (see Figure 1d). There are many different compositions and versions of resist, allowing for different heights, temperatures, exposure settings and structures to be manufactured.

Photolithography Basics
Figure 1.
a) Cleaned and prepared substrate
b) Photoresist spun onto substrate
c) Align to photomask and expose to UV light
d) Develop photoresist.
• Positive removes exposed resist
• Negative removes unexposed resist

When aligning the substrate to the mask we use a mask aligner or a stepper to control where the pattern on the mask is projected onto the substrate below. A mask aligner is a faster process taking a pattern the same size as the wafer and projecting it onto the wafer. A stepper takes a small pattern and exposes that onto the wafer before moving the substrate a small way and exposing the same pattern onto a different part of the wafer – replicating the same patter over and over again.

The crucial aspect of the align and expose step is the exposure of the photoresist with UV light. The UV source can either be a traditional broadband mercury light or the more recent advance using a UV LED array.  The UV source defines the resolution (smallest features) of the lithography process. A rough rule of thumb is that the resolution achievable is slightly more than half the wavelength of light. UV light has a wavelength of ~465nm, so the resolution achieved will be ~250 nm.

Once exposed the next step is to develop the resist. The developer will wash away either the exposed or unexposed parts of the resist film depending on the nature of the photoresist. Development is either done as a spray, where a fine mist of developer is sprayed onto the exposed substrate or as a puddle, where a pool of developer is poured onto the exposed substrate. In both cases it will remove the unwanted photoresist leaving the desired pattern.

Now the substrate is ready for the next step of the processing, either the deposition of a thin film onto the surface of the substrate or the etching and removal of the substrate. These techniques can be combined together with multiple iterations of photolithography to make complex designs and patterns.

Author

Date

Version

Author

Chris Valentine

Date

25 October 2019

Version

IKB038 Rev. 1

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Knowledge Base Fact Sheet

Wafer Selection Guide

What the parameters mean in the wafer selection guide nomenclature charts.

Silicon Wafers

Silicon wafers are a thin slice of semiconducting material that is widely used in the production of electronic and micromechanical devices. They are characterised by a number of parameters, which affect their suitability and performance for a chosen tasks.

1. Wafer diameter

Diameter of the wafer listed in mm. Typically wafers are talked about in inches; typical sizes are 2”,3”,4”,5”,6”,8”& 12” – with 4”,6” and 8” the most commonly used in industry and academia.

2. Type

Type refers to the electrical behaviour of the wafer. Intrinsic (I), behaves as pure silicon. N-type, dominate charge carriers are electrons. P-type, dominate charge carriers are holes. Whether a wafer is P-type or N-type will affect the electrical response of  any device manufactured.

3. Crystallographic Orientation

Wafers are grown as single crystals that have an ordered, regular and repeating structure.  When they are sliced from the ingot the flat surface is aligned along one of several relative directions, known as the orientation. The orientation is classified by Miller indices, typical indices are (100), (110) and (111). Orientation affects the physical properties of the silicon wafers – how it is etched, ion implantation and how it integrates with other materials

4. Dopant

The dopant is a material that has been deliberately implanted in the silicon to change the TYPE of the silicon. TYPE and DOPANT are linked.

Typical N-type dopants are Phosphorus, Arsenic, and Antimony. These all provide an extra electron to the silicon which is then free to carry current.

Typical P-type dopants are Boron, Gallium. These have one less electron and so leave a ‘hole’ in the silicon lattice which is free to carry current.

5. Growth Method

The growth method refers to the process by which the silicon ingot is grown. There are two main techniques: Czochralski Zone (CZ) and Float Zone (FZ).

CZ: This is the dominant method used to grow commercial silicon wafers due to the better resistance to thermal stress, speed of production and low cost. CZ involves the heating a crucible of polycrystalline silicon until it melts; then dipping a seed of single crystal silicon in and withdrawing slowly to produce an ingot of crystalline silicon.

FZ: This is a high purity alternative to the CZ method. A polycrystalline rod of silicon and a single crystal seed are held face to face and rotated. The rod is then heated by a thin ring and the seed brought in to contact with the tip. The molten silicon orders itself into the single crystal and the heating zone is slowly moved up and the ingot of silicon extends. FZ produces high purity and high resistivity Si than typically possible in CZ processes.

6. Grade

Grade refers to the variety in the quality of the wafers. Typically these are PRIME, TEST and RECLAIMED.

  • Prime are the highest quality and produced to the highest tolerances on flatness, cleanliness and polish
  • Test are similar to prime, except with less rigourous specifications to flatness and cleanliness.
  • Reclaimed are wafers that have been stripped and polished of any previous patterning or processing.

There are sometimes other grades of Si wafer mentioned but these are either synonyms of the above or have a specific tolerance on a certain parameter.

7. Material

The material is the bulk material of the wafer, typically silicon, but this may vary, some transparent substrates such as glass or quartz are needed for optical devices and more exotic compound materials such as GaAs or InP for specific band gaps.

8. Resistivity

Resistivity is the measure of the resistance to current flow and the movement of the charge carriers (either holes or electrons) through the silicon. Resistivity is measured in Ohm-cm. The dopant level can be adjusted to reach target resistivities, with higher doping lowering the resistivity.

9. Thickness

The thickness of the silicon wafer affects the mechanical properties and is typically expressed in µm (microns) and with a tolerance (± ? µm), The tolerance is measured through a total thickness variation (TTV).

10. Polish

Wafer polishing is the final step in the manufacture of silicon wafers, which allows the production of a smooth, super-flat mirrored surface. There are two options for polishing: single side polish (SSP) and double side polish (DSP)

SSP: Only one face is polished, the second (the backside) is etched.

DSP: Both faces are polished, giving a high flatness to the wafer.

11. Alignment Fiducial

Alignment fiducial refers to the flats or notches used to identify the wafer. Originally flats were used to identify TYPE and well as ORIENTATION but now there is less convention about what the flats mean and notches are quite common on 8” (200mm) wafers.

12. Other

At Inseto, we use other to indicate if the wafers are laser marked with a unique identifier or if they have been stacked in a particular manner.

Cassette of Prime Silicon Wafers

Coated Wafers

Coated wafers are a subset of silicon wafers where either one or both surfaces have been coated with an additional material. In the Inseto naming convention they are characterised by COATING – the material the wafer is coated with; and COATING THICKNESS – the thickness of that coating, typically µm, nm or Å.

a. Oxide wafers

One typical coating requested is an Oxide coating. This can be a thermal oxide coating (ATOx) which always coats both sides of the wafer. ATOx stands for atmospheric thermal oxide. Other oxide coating methods include:

Dry Oxide – which produces a thinner oxide layer but with a higher uniformity film.

PECVD Oxide – produces a coating on a single side of the wafer

b. Nitride wafers

A second typical coating requested is a Nitride coating. Silicon Nitride (SiN) offers different mechanical and chemical properties to oxide layers. The nitride can be depositied by PECVD, LPCVD or low stress LPCVD. These variants are changes in the method of deposition and alter the final physical and mechanical properties of the film.

Oxide Coated Silicon Wafers

Glass Wafers

Glass wafers are generally used where the substrate is required to be transparent. At Inseto we separate out these from silicon and coated silicon wafers as they have some distinct parameters which inform your selection.

1. Wafer diameter

As with Silicon wafers, the diameter is typically listed in mm but may be referred to in inches. 

2. Material

This lists the material the glass is made from, typical options include Borosilicate, Fused Quartz, Fused Silica and Crystal Quartz. Some people use these terms interchangeably or will drop the ‘fused’ and ‘crystal’ terms

3. Crystallographic Orientation

Fused Quartz and Fused Silica have no orientation as they are not crystalline materials. Crystal quartz however does and can be X-Cut, Y-Cut, AT-Cut and ST-Cut depending on how the wafer is removed from the larger crystal.

4. Grade

The grade of the glass wafer listed here refers to the manufacturers specifications. Each has its own specific chemical, mechanical and optical properties.

5. Thickness

As with Silicon wafers this refers to the thickness and tolerance of the wafer, typically listed in µm.

6. Polish

As with Silicon wafers this refers to the finish on the surface of the glass and can be either SSP or DSP. Alongside this there is a rating X/Y. where both X and Y are numbers. X refers to the width of a scratch in µm and Y the diameter of a dig, pit or bubble in hundreths of a mm.

7. Edge Shape

This denotes how the edge of the glass wafer has been shaped. Most commonly a C shape, but chamfered and square cut are also options.

8. Alignment fiducial, Coating type, Coating thickness

The final 3 parameters we listed are alignment fiducial, coating type and coating thickness and contain the same information as in the Silicon wafers

Silica Wafers for Semiconductor Research

SOI Wafers

The last wafer type we separate out is SOI (Silicon on Insulator). SOI wafers make use of a silicon – insulator –silicon substrate and are used for specific applications where reducing parasitic capacity in the device is crucial. The choice of insulator within the silicon sandwich is highly specific to the application but silicon dioxide and sapphire are typical choices for microelectronics and radio frequency applications respectively.  The top layer of silicon is referred to as the ‘device’, the bottom layer the ‘handle’.

There are some standard parameters listed as with silicon wafers, these are Diameter, Type and crystallographic orientation. We then list the parameters specific to SOI wafers.

1. Device thickness

This is the thickness of the top layer of silicon, typically in µm

2. Growth Method

This is listed twice in an SOI wafer. First is the growth method of the device layer and can be CZ or FZ.

3. Device Resistivity

Measured as with a standard silicon wafer, this is the resistivity of the top layer of silicon in Ohm-cm

4. BOx

This is the thickness of the insulator layer or ‘buried oxide’ layer, hence BOx.  As with all thicknesses typically µm but can be nm or Å.    

5. Handle Thickness

The thickness of the bottom layer of silicon, typically in µm.

6. Growth Method

This second listing of growth method relates to the handle layer of silicon.

7. Handle Resistivity

Measured as with a standard silicon wafer, this is the resistivity of the bottom layer of silicon in Ohm-cm

8. Backside

This relates to how the backside of the handle layer has been treated and can be a variety of finishes including: Polished, etched, oxide, no oxide and laser marked.

SOI Prime Wafers

Author

Date

Version

Author

Chris Valentine

Date

24 October 2019

Version

IKB046 Rev. 1

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Lithography Conference – Making things better

30th November 2019

With some 90 visitors attending from industry and academia, the inaugural SUSS MicroTec sponsored “UK Lithography Conference”, held on 4th July 2019, had an over-arching theme of ‘productivity and efficiency’ and was a resounding success.

Hosted by Rutherford Appleton Laboratory, the conference was split into three sessions, the first of which was entitled ‘Surface Preparation’, which began with sound advice on substrate selection. For this, Ian Burnett of Inseto, had a clear message: quality lithography depends heavily on quality wafers. Though SEMI standards exist for wafer thickness tolerances, flatness, surface roughness etc. for repeat runs, and to ensure consistent results, there is no substitute for using wafers from the same ingot, supplied in the order in which they were sliced.

Next in the session, Joost Driven and Dominique Bouwes of Micronit Mictrotechnologies discussed material structures, focussing on the benefits of polymer and the associated challenges of processing it; challenges that include ensuring a crack-free surface, structural accuracy (i.e. dimensions of features), cleanliness (of channels, trenches and holes) and adhesion. Figure 1 shows how cracks can form.

Figure 1 – Cracks can form due to a release in tensile stress in the resist layer.

Driven and Bouwes than gave examples of polymer-based devices; a bio chip for life sciences and a MEMS-based hair flow sensor.

The first session concluded with Tim Bruchmueller, Product Manager 200mm coaters, of SUSS MicroTec discussing recent developments with coating technologies in the SUSS camp. Using the just-launched SUSS ACS 200 GEN3 LabCluster coater and developer as an example, and without being overly sales-pitchy, Bruchmueller explained how, for example, developments around the resist bottle (which is inverted) reduce the risk of getting air into the dispensers.

Figure 2 – Greater energy efficiency plus other benefits using the Peltier Effect

Also, by taking advantage of the Peltier-Effect (as exhibited by some semiconductors) means positive or negative temperature differentials can be created for heating or cooling purposes. Where cooling is concerned this means no need for compressors (and their associated vibrations) and refrigerant liquids. Perhaps the biggest benefit is a lower electricity bill, as reflected in figure 2.

Second Session

The theme for this session was Exposure, and Dr. Marc Hennemeyer, Director of SUSS MicroTec’s Application Centre for Lithography, started proceedings by giving a presentation on MEMS processing challenges. These include needing consistent processing for different types of substrate material (Si, SiO2, LiTa, ceramics etc.) and the popularity of material stacks (for example, Si-based CMOS device wafers on top of mechanical device wafers).

When forming relatively large features on substrates with high topographies, large exposure gaps are caused. However, this can lead to reduced side wall angles. Also, steppers cannot achieve sufficient process results due to their limited depth of focus (DOF). In this respect, Dr. Hennemeyer proposed Fresnel Zone Plate (FZP) processing as a solution, describing FZP as, in essence, a diffractive lens. The process, which is generally for feature sizes greater than 2um, is somewhat removed from traditional proximity lithography.

A diagram Dr. Hennemeyer talked around is reproduced here as figure 3. It compares the DOF of a traditional mask used to make a hole versus an FZP.

Figure 3 – Above, a comparison of traditional mask and the Fresnel Zone Plate used to create identical features (etched holes).

With reference to the above diagram, in the top left we have a round hole in a mask. To the right, we have a prediction of the light intensity when projected through the mask. The DOF is relatively close to the mask. Below that, on the left, we have an FZP mask. The pattern is larger and more complex, requiring polarity changes in the rings, but the DOF is greater (and further away from the mask).

Dr. Hennemeyer held the floor to give a follow-on presentation about improving proximity imaging quality using diffractive elements. A key point made concerned the use of optical proximity correction (OPC) and the inclusion of features on the source mask that are not meant to be printed. Rather they are present to ‘influence’ the shape that will be printed. For example, the rounding effect means that a square in the mask might produce something closer to a circle. However, the presence of features (smaller squares) to ‘re-enforce’ the corners can result in the printing of a much better square; subject to the size of the smaller squares and their distance from the main square on the mask. See figure 4.

Figure 4 – Optical proximity correction can be used to combat rounding effects and print shapes much closer to requirements.

However, OPC on mask aligners is more challenging than on steppers. Simulation provides considerable benefits though according to Dr. Hennemeyer, who went on to recount the developments of a joint SUSS/GenISys project. Findings to date reveal that light source stability and reliable gap settings are key to implementing OPC, which helps in the creation of ‘sharper’ features and steep side walls.

SUSS MicroTec kept the floor for the next presentation, as Christof Kronseder gave an overview of UV LED light sources and recounted a number of developments that have taken place during recent years. The advantages of LED over traditional mercury lamps include lower running costs (during use and by virtue of not requiring a warm-up) and reduced cooling requirements. Kronseder recounted that SUSS began its LED journey with an alternative for 350W mercury lamps and is currently working on a 1kW system.

Third Session

This session was themed ‘Imprint / Applications’ and began with a presentation from Dr. Simon Drieschner, an Applications Engineer with SUSS MicroTec, on solutions for micro and nano imprinting, using substrate conformal imprint lithography (SCIL) and SUSS’s proprietary SUSS MicroTec imprint lithography equipment (SMILE) respectively (see figure 5).

Figure 5 – Above, the SUSS MicroTec imprint lithography equipment (SMILE) process steps.

The presentation included a comparison of stamp materials from a total-cost-of-ownership perspective and factored in curing times, which are often overlooked but essential for volume manufacturing scenarios as they can vary from circa 15 minutes to more than three days. Two main materials were compared, epoxides (which are proven in the field) and hybrid acrylates (which are a relatively new development but watch this space as the benefits are considerable). See figure 6.

Figure 6 – New hybrid acrylate stamps require only a short (UV) curing time and have a long life.

The session concluded with a report from academia, in the form of a presentation from Swansea University, given by Dominic Chung Man Fung and which provided an example of the SCIL process as part of an Innovate UK funded project.

The project was to develop a low cost, volume fabrication process for a wafer scale distributed feedback (DFB) laser. Challenges included feature size and shape (plus achieving steep sidewall angles), stamp [soft master] curing time, the hard master having an anti-stick layer (ASL) and, of course, attaining high yield.

Incredible results have been achieved so far on 3” wafers. For instance, an ASL for the hard master has been created using FDTS (a.k.a. Perfluorodecyltrichlorosilane – an anti-sticking process used in other technology fields) and soft stamps are exhibiting high reproducibility. The most impressive achievement however is how rapidly high yield has been attained – see figure 7.

Figure 7 – Swansea University’s wafer on wafer yield improvements awed all present at the UK Lithography Conference.

Future goals include attaining 100% yield (far from unachievable considering the results to date), scaling to 4”, 6” and 8” wafers and performing studies into the lifetime of the soft master.

The inaugural UK Lithography conference provided attendees with a wealth of advice on how to achieve productivity and efficiency through tool developments and new methodologies.

The inaugural UK Lithography Conference concluded with a note of thanks from Matt Brown of Inseto, the organiser of the event. Thanks went to Rutherford Appleton Laboratory for hosting the conference, to SUSS MicroTec for their sponsorship and to the speakers (most of whom had travelled in from outside the UK).

To repeat a few words from the intro of this report, the event was a resounding success. The lithography community is facing (and as the conference proved, is solving) a whole host of technical challenges amidst a backdrop of commercial pressures.

Productivity and efficiency are being realised through developments in tools and methodologies and, through networking events like the conference, ideas are being shared and further developed – which is all great news for this exciting industry.