इलेक्ट्रॉन किरण अश्मलेखन

इलेक्ट्रॉन किरण अश्मलेखन तंत्र

वाणिज्यिक रूप में प्रयोग होने वाले इलेक्ट्रॉन किरण अश्मलेखन तंत्र काफी महंगे होते हैं (रु. १६ करोड से ज्यादा)। अधिकांश अनुसंधान प्रयोगों के लिये इलैक्ट्रॉन सूक्ष्मदर्शी को बदल कर इस कार्य के लिये उपयोगित किया जाता है (करीब रु. ४० लाख)। ऐसे तंत्र १० नैनोमीटर या उससे कम तक की रेखाएं बना पाती हैं।
इलेक्ट्रॉन किरण अश्मलेखन तंत्र को उनके किरण पुंज के आकार और पुंज के विक्षेपण या मोड़ने की शैली के अनुसार वर्गीकृत किया जाता है। पुराने तंत्रों में रेखापुंज प्रणाली का प्रयोग होता था, पर अब सदिश पद्वति को अपनाया जा रहा है।

इलेक्ट्रॉन स्रोत

निम्न विभेदन तंत्रों में तापायनिक स्रोतों का प्रयोग होता है। ये स्रोत LaB₆ से बनाये जाते हैं। पर उच्च विभेदन के लिये क्षेत्र उद्गार स्रोत, जैसे तप्त W/ZrO₂, जिसमें निम्न ऊर्जा फैलाव और अधिक द्युति होती है, का प्रयोग होता है।

लेंस

स्थिरविद्युत और चुम्बकीय लेंसों, दोनो का ही प्रयोग होता है। परंतु स्थिरविद्युत लेंसों में त्रुटी अधिक होती है, इसलिये इनका प्रयोग सूक्ष्म फोकस/केंद्रकरण के लिये नही होता।

वेदिका को चलाना

सामान्यतः, निम्न विक्षेपण के लिये स्थिरविद्युत लैन्सों का प्रयोग होता है और बड़े विक्षेपण के लिये विद्युत चुम्बकीय लैन्सों का प्रयोग होता है। त्रुटियों  के कारण छापे मारने की सूक्ष्मता १०० से १ मैक्रॉन तक ही होती है। इसलिये बडे ढांचों की बनावट में वेदिका को चलाया जाता है। छोटे-छोटे वर्गों में ढांचा बनाकर उन्हे जोड दिया जाता है। इस प्रणाली में अत्यन्त ही सूक्ष्म नियन्त्रण की जरूरत पडती है।

इलेक्ट्रॉन किरण लेखन समय

इलेक्ट्रॉन किरण लेखन की दृष्टि से वाणिज्यिक उत्पादन के लिये बहुत धीमा पडता है। उदाहरण के लिये, 700 cm² के सिलिकॉन वेफर के अभिमुखीकरण के लिये 7000 सैकंड तक लग जाता है। इसका मुख्य कारण है इस प्रणाली का क्रमिक होना। प्रकाशिक अश्मलेखन इसकी तुलना में कई गुना तेज होता है, क्योंकि यह सारे सतह पर समांतर कार्य करता है।

Defects in electron-beam lithography

Despite the high resolution of electron-beam lithography, the generation of defects during electron-beam lithography is often not considered by users. Defects may be classified into two categories: data-related defects, and physical defects.
Data-related defects may be classified further into two sub-categories. Blanking or deflection errors occur when the electron beam is not deflected properly when it is supposed to, while shaping errors occur in variable-shaped beam systems when the wrong shape is projected onto the sample. These errors can originate either from the electron optical control hardware or the input data that was taped out. As might be expected, larger data files are more susceptible to data-related defects.
Physical defects are more varied, and can include sample charging (either negative or positive), backscattering calculation errors, dose errors, fogging (long-range reflection of backscattered electrons), outgassing, contamination, beam drift and particles. Since the write time for electron beam lithography can easily exceed a day, "randomly occurring" defects are more likely to occur. Here again, larger data files can present more opportunities for defects.

Electron energy deposition in matter

The primary electrons in the incident beam lose energy upon entering a material through inelastic scattering or collisions with other electrons. In such a collision the momentum transfer from the incident electron to an atomic electron can be expressed as , where  is the distance of closest approach between the electrons, and  is the incident electron velocity. The energy transferred by the collision is given by , where  is the electron mass and  is the incident electron energy, given by . By integrating over all values of  between the lowest binding energy,  and the incident energy, one obtains the result that the total cross section for collision is inversely proportional to the incident energy , and proportional to . Generally, , so the result is essentially inversely proportional to the binding energy.
By using the same integration approach, but over the range  to , one obtains by comparing cross-sections that half of the inelastic collisions of the incident electrons produce electrons with kinetic energy greater than . These secondary electrons are capable of breaking bonds (with binding energy ) at some distance away from the original collision. Additionally, they can generate additional, lower energy electrons, resulting in an electron cascade. Hence, it is important to recognize the significant contribution of secondary electrons to the spread of the energy deposition.

Resolution capability

With today's electron optics, electron beam widths can routinely go down to a few nm. This is limited mainly by aberrations and space charge. However, the practical resolution limit is determined not by the beam size but by forward scattering in the photoresist and secondary electron travel in the photoresist. The forward scattering can be decreased by using higher energy electrons or thinner photoresist, but the generation of secondary electrons is inevitable. The travel distance of secondary electrons is not a fundamentally derived physical value, but a statistical parameter often determined from many experiments or Monte Carlo simulations down to < 1 eV. This is necessary since the energy distribution of secondary electrons peaks well below 10 eV. Hence, the resolution limit is not usually cited as a well-fixed number as with an optical diffraction-limited system. Repeatability and control at the practical resolution limit often require considerations not related to image formation, e.g., photoresist development and intermolecular forces.
In addition to secondary electrons, primary electrons from the incident beam with sufficient energy to penetrate the photoresist can be multiply scattered over large distances from underlying films and/or the substrate. This leads to exposure of areas at a significant distance from the desired exposure location. These electrons are called backscattered electrons and have the same effect as long-range flare in optical projection systems. A large enough dose of backscattered electrons can lead to complete removal of photoresist in the desired pattern area.

Proximity effect

The smallest features produced by electron beam lithography have generally been isolated features, as nested features exacerbate the proximity effect, whereby electrons from exposure of an adjacent region spill over into the exposure of the currently written feature, effectively enlarging its image, and reducing its contrast, i.e., difference between maximum and minimum intensity. Hence, nested feature resolution is harder to control. For most resists, it is difficult to go below 25 nm lines and spaces, and a limit of 20 nm lines and spaces has been found. 
Proximity effects (due to electron scattering) can be reduced by solving the inverse problem and calculating the exposure function  that leads to a dose distribution as close as possible to the desired dose  when convolved by the scattering distribution point spread function

Charging

Since electrons are charged particles, they tend to charge the substrate negatively unless they can quickly gain access to a path to ground. For a high-energy beam incident on a silicon wafer, virtually all the electrons stop in the wafer where they can follow a path to ground. However, for a quartz substrate such as a photomask, the embedded electrons will take a much longer time to move to ground. Often the negative charge acquired by a substrate can be compensated or even exceeded by a positive charge on the surface due to secondary electron emission into the vacuum. The presence of a thin conducting layer above or below the resist is generally of limited use for high energy (50 keV or more) electron beams, since most electrons pass through the layer into the substrate. The charge dissipation layer is generally useful only around or below 10 keV, since the resist is thinner and most of the electrons either stop in the resist or close to the conducting layer.
The range of low-energy secondary electrons (the largest component of the free electron population in the resist-substrate system) which can contribute to charging is not a fixed number but can vary from 0 to as high as 50 nm. Hence, resist-substrate charging is not repeatable and is difficult to compensate consistently. Positive charging is more tolerable than negative charging, because the latter can deflect the electron beam away from the desired exposure location.

New frontiers in electron-beam lithography

To get around the secondary electron generation, it will be imperative to use low-energy electrons as the primary radiation to expose photoresist. Ideally, these electrons should have energies on the order of not much more than several eV in order to expose the photoresist without generating any secondary electrons, since they will not have sufficient excess energy. Such exposure has been demonstrated using a scanning tunneling microscope as the electron beam source. The data suggest that electrons with energies as low as 12 eV can penetrate 50 nm thick polymer photoresist. The drawback to using low energy electrons is that it is hard to prevent spreading of the electron beam in the photoresist. Low energy electron optical systems are also hard to design for high resolution. Coulomb inter-electron repulsion always becomes more severe for lower electron energy.
Another alternative in electron-beam lithography is to use extremely high electron energies (at least 100 keV) to essentially "drill" or sputter the material. This phenomenon has been observed frequently in transmission electron microscopy. However, this is a very inefficient process, due to the inefficient transfer of momentum from the electron beam to the material. As a result it is a slow process, requiring much longer exposure times than conventional electron beam lithography. Also high energy beams always bring up the concern of substrate damage.
Interference lithography using electron beams is another possible path for patterning arrays with nanometer-scale periods. A key advantage of using electrons over photons in interferometry is the much shorter wavelength for the same energy.
Despite the various intricacies and subtleties of electron beam lithography at different energies, it remains the most practical way to concentrate the most energy into the smallest area.

See also

Photolithography