Basic&Laboratory& Materials&Science&and&Engineering& Etching&of&Semiconductors&

Transcription

1 ! Basic&Laboratory&! Materials&Science&and&Engineering& Etching&of&Semiconductors& M104&!!!as!of:! !! Aim: To gain a basic understanding of etching techniques, characterization, and structuring of surfaces. Contents 1 Introduction What is etching? Basic semiconductor properties Etching of silicon Structural properties of silicon Direct and indirect dissolution of silicon Example for anisotropic silicon etching Experiment... 6 Stand

2 1 Introduction Etching of semiconductors is one of the basic production techniques in our electronic world. All integrated circuits typically have several etching steps in their processing. As an example, etching is used to flatten sample surfaces after mechanical treatment by removing several micrometers with low precision. It is also used for cleaning surfaces before entering a processing stage. For this, a few nanometers are removed with high accuracy. And of course, it is used for structuring semiconductors with a wide range of etching speeds for different anisotropic aspects. There is a large variety of applications for etching under varying conditions and with different goals. In fact, etching is an important technique in general, not just for semiconductors. To understand etching, chemical properties as well as the properties of semiconductors must be combined. 2 What is etching? Etching in its origin means "eating" or "grazing", so from the viewpoint of the etching agent it means to take away something. Today we normally discuss from the etched media s point of view. While silicon is dissolved by hydrofluoric acid, we speak of the etching of silicon, but not the etching of hydrofluoric acid. Etching may have several results: the etched material will change its aggregate state the etched material and/or the etching agent can change color there can be a shift in temperature there will be a change in volume and weight of etched material and etching agent Reviewing these points from bottom to top in the list above, science today gives the following explanations for these results: Etching is the dissolving of the etched material by the etching agent. Therefore, the volume and weight of the etched material is reduced while those of the etching agent will increase. To describe this process with the correct reaction path, a linear correlation between these two results must be found. If the dissolution does not take place spontaneously under normal conditions, additional energy is needed to start and/or maintain the etching. The additional energy is provided by the etching agent and sometimes by some activating energy. But depending on the energy state of the products, there can be a positive (exothermic) or negative (endothermic) energy balance for the complete reaction. This leads to a change in temperature. Dissolution of the etched material means there is reaction between the material and the etching agent. The product of this reaction can have a different color than the initial species. Often the etching of the material by the etching agent is not a single step reaction. So there might be some intermediates on the surface of the etched material. This can change the color of the surface as well. But also a second mechanism can lead to a color change of the surface, 1

3 since etching may lead to a rough surface. This roughness can also lead to a colored, shiny surface. Think of a CD; it appears colorful because of fine structures on the surface. When the atoms of the etched material are dissolved, they are still near the surface. This might cause a so-called steric hindrance for a new attack of the etching agent on the surface. This is a change in the aggregate state of the dissolved, etched material. The dissolved atoms become a liquid or gas in the etching agent or its solvent. Thus, they gain higher mobility and can leave the surface area of the solid; the next etching step can then take place. This is a necessary step for continuous etching. A typical example is the oxidation of surfaces. Aluminium is oxidized in air very quickly to aluminium oxide. This oxide is not removed from the surface because it sticks to the underlying aluminium atoms. Therefore, the reaction automatically stops when a certain thickness of the oxide layer (normally a few nanometers) has built up. For iron, the situation is completely different. Iron is also oxidized in contact with air but the reaction product, iron oxide, is quite easily removed from the surface. The reaction can start again and will work through the whole material, leading to the typical corrosion of iron, rust. The aluminium is passivated by the oxidation, and by this is more stable in air, whereas iron rusts and is unstable. Corrosion of iron is a kind of iron etching. The etching of solid materials is the most common type of etching. But of course, by definition one can also "etch" a liquid or a gas. But normally these reactions are referred to differently. 3 Basic semiconductor properties A semiconductor is defined by having a Fermi energy within a band gap of around 1 ev, typically. Therefore an intrinsic semiconductor has two mobile species, electrons and holes. A further distinction between semiconductors are direct or indirect. These are parameters which depend on the band structure of the semiconductors and influence the interaction of the semiconductor with light. Sometimes this interaction with light is a very important feature for etching, but this is not the subject of this lab course. Since etching takes place at the surface, the crystallography of the semiconductor surface is an important parameter for etching, as well as structural defects near the surface since they might be preferentially etched. Doping shifts the Fermi energy towards one of the band edges and one of the mobile species is drastically increased at the expense of the other one. For example, n-doping drastically increases the number of free electrons and decreases the number of free holes. As a consequence, at the semiconductor-electrolyte interface a space charge region can develop, in other words the majority carriers are driven away from the interface by an electrical field. Corresponding to the width of the space charge region, an electrical potential drop occurs across the space charge region. Since differences in the Fermi energies in the electrolyte and in the semiconductor are the driving forces for the chemical reactions, the potential drop in the space charge region reduces the etching of the semiconductor. 2

4 4 Etching of silicon Silicon is one of the few elemental semiconductors. It is the most important material of today's microelectronics industry. In the periodic table, silicon is below carbon. Carbon-based diamond is chemically the most inert material. Correspondingly, the silicon lattice, having the same diamond-like structure, is one of the most chemically stable materials as well. In addition, silicon dioxide is extremely stable. Nearly all rocks, sand, and minerals are formed from silicon dioxide because this is the most stable end product of all chemical reactions. Silicon dioxide is one of the best dielectrics, in other words large potentials can drop across very thin silicon diode layers. The combination of the potential drop across the space charge region within the silicon and across a silicon dioxide layer that may cover the silicon surface dominates the chemical dissolution of silicon. 4.1 Structural properties of silicon The silicon lattice is represented by a periodical arrangement of atoms in three dimensions. Every atom inside this solid is connected to its neighbors by four bonds in a tetrahedral coordination. Figure 1. Diamond-like structure of the silicon lattice At the silicon-electrolyte interface the situation is, of course, different. Bonds pointing into the silicon lattice will be connected to silicon atoms but bonds pointing into the electrolyte will be broken and are substituted by the atoms from the environment of the solid, for example oxygen. Therefore, surface atoms will have different properties from the inner atoms. Since inner atoms are simply not accessible, the etching will start and take place at these surface atoms. 3

5 The first essential step for etching is the wetting of the surface by the etching agent. Only if there is real contact between both species can a reaction take place. The wetability depends on the surface tension or hydrophobicity. Often adding tensides may improve the wettability, but this topic is not in the focus of the lab course. Since the surface atoms are fixed to the lattice by the inward-pointing back bonds, etching means breaking these back bonds. Breaking the first back bond is typically the decisive, energy-consuming step. The energy of this excited state (two broken bonds) is lowered by forming bonds with the species of the etching agent. The reaction of more back bonds typically follows quickly, and often releases energy so that the overall dissolution process may be exothermic. Since the number of bonds which point into the electrolyte differs for different crystallographic planes of the silicon surface, the crystallographic orientation of the silicon surface in contact with the electrolyte strongly influences the etching process. 4.2 Direct and indirect dissolution of silicon The dissolution of silicon requires holes from the silicon bulk. One dissolution mechanism directly dissolves silicon according to the following reaction: Si + 2h + + 6HF SiF H + + H 2 A second dissolution mechanism first transforms silicon into silicon dioxide according the reaction: Si + 2H 2 O + 4h + SiO 2 + 4H + and afterwards the silicon dioxide is dissolved. This two-step dissolution is called indirect silicon dissolution. When a dielectric silicon dioxide layer is formed at the interface, all chemical reactions stop. Silicon dioxide is one of the rare materials which can absorb 1 V over a length of a few nanometers. Therefore, a silicon dioxide layer several nanometers thick has the same effect as a space charge region layer with a thickness on the order of a micrometer. Taking away 1 ev of driving force for the chemical dissolution has a tremendous effect. According to E = kt, 1eV corresponds to a temperature of 10,000 K. Since silicon dioxide is extremely stable, only very few electrolytes can be used for etching silicon: hydrofluoric acid containing electrolytes and some alkali hydroxide containing electrolytes at higher temperatures. If the indirect silicon dissolution is dominant, flat surfaces will form. The reason for this is the amorphous structure of silicon dioxide. Neither the formation nor the dissolution of silicon dioxide shows significant anisotropy. If direct silicon dissolution is dominant, strong crystallographic dependencies can be expected as discussed before. By appropriately designing the electrolyte, any fraction of direct and indirect dissolution can be chosen which allows for very different applications: 4

6 1. Anisotropic etching, leading to surfaces which are composed of certain crystallographic planes (in silicon, mostly the (111) surface). 2. Defect etching, which preferentially etches areas with surfaces near defects (e.g. dislocations); often used for characterizing the material quality. 3. Polishing (smoothing) of surfaces. 4.3 Example for anisotropic silicon etching Figure 2 illustrates the properties of the (111) and the (110) planes of the silicon lattice. For the (111) plane, only one bond points into the electrolyte and three bonds are connected to neighboring silicon atoms. This is the highest possible number of bonds to the lattice and therefore the (111) surface is the most stable crystallographic plane. In contrast to this, the (110) plane shows the largest possible opening to the electrolyte and thus is preferentially dissolved. Exactly these features are found when etching silicon with hydroxide ions (OH - ) containing solutions at temperatures above 80 C. The silicon surface will be covered by (111) surfaces after etching since these planes are the most inert to dissolution. (110) (111) Figure 2. Crystallographic orientation of the (111) and (110) surface planes of the silicon lattice 5

7 5 Experiment You will now etch silicon using different solutions. The resulting surfaces will be analyzed by the naked eye, binoculars, or a light microscope. The solutions are corrosive and toxic! Work only with protective equipment and under the control of your supervisor. Protective equipment consists of: Lab coat Eye protection glasses or goggles Thick (ultra)nitrile gloves (not the thin ones!), penetration time 450 min You ll learn prior to the experiment where the first aid kits & materials (especially for HF) are stored. After each etching, take out the sample with forceps, rinse it with deionized water, and blow it dry. Collect the rinse water because it contains chemicals that are harmful to the environment. The experiment is divided into 3 parts: 1. Polishing: Inspect one of the four sawn silicon pieces with the optical microscope. Afterwards, polish all four pieces using the following etching solution: 60 ml HF (48% aq. solution) 100 ml HNO 3 (65% aq. solution) 60 ml CH 3 COOH (96% aq. solution) Place the four samples on the Teflon holder without overlapping them, and carefully place them into the temperature controlled bath containing the 220 ml of prepared solution for approximately 5 to 8 minutes. Record the temperature of the solution before and after the experiment. Is the reaction exothermic or endothermic? Explain. Record your observations. Investigate one of the polished silicon pieces with the optical microscope and describe your observations. 2. Defect etching: Prepare 100 ml of the defined "SECCO" solution below: K 2 Cr 2 O 7 : H 2 O : HF (48%) g : 33 ml : 67 ml Make a slight scratch on one of the polished samples. Etch the sample by placing it in the prepared SECCO solution for 5 minutes at room temperature. 6

8 Again, use the optical microscope to investigate the scratch on the sample before and after etching. 3. Pyramid structuring: Prepare the following solution: 70 g KOH pellets 190 ml deionized water Mix on warm surface (max. 80 C) until KOH has completely dissolved Add 60 ml of isopropanol. Divide the solution into two parts. Place the remaining two samples in the Teflon holders provided. Etch one of the samples at 80 C and the other at room temperature for 30 minutes. Describe what you observe. A. Give your observations and interpretations of each experiment. B. Discuss the composition of the three solutions with respect to the reactions necessary to allow for the different etching results. 7