WO2015006217A1

WO2015006217A1 - Solid phase analysis of layers deposited by chemical vapor deposition

Info

Publication number: WO2015006217A1
Application number: PCT/US2014/045577
Authority: WO
Inventors: Richard A. Hogle
Original assignee: Linde Aktiengesellschaft
Priority date: 2013-07-09
Filing date: 2014-07-07
Publication date: 2015-01-15

Abstract

Techniques for the analysis of source materials used to deposit and clean Chemical Vapor Deposited (CVD) layers. A quartz tube is used for the deposition of the layer to be analyzed, resulting in significantly reduced costs and reduced complexity of the analysis. The methods of the invention do not require crystalline epitaxy, Rather, amorphous films created at CVD growth tenl perntures and pressures are adequate for the analysis techniques of the invention. By using solid phase CVD layers like those formed in the user's deposition process, dopant and trace impurities can be trapped and analyzed, therefore assuring that all impurities that may have an impact on final device performance will be captured and detected.

Description

SOLID PHASE ANALYSIS OF LAYERS DEPOSITED BY CHEMICAL VAPOR

DEPOSITION

FIELD OF _.TjhjE Y IQN

(001) The invention relates to analysis of gases used for chemical vapor deposition by analyzing a controlled deposited layer.

B A C K 3R UND_OF JHE VEOTJON

(002) The growth of various semiconducting and insu lating layers by chemical vapor deposition (CVD) is wide spread in the semiconductor and electronics field. For example, si licon, germanium and tungsten layers may be deposited by CVD techniques wherein the layers are grown from gases from a compressed or liquefied source or from vapor from a volatile liquid source entrained in a carrier gas or delivered by direct liquid injection, wherein the source material may be trichlorosilane (TCS), disi!ane (Si:,H₆), dichiorosilane (DCS), silane (Sf H.;), trisiiane (SisHg), hexachlorodisilane (SiaCl*,), germane (Gel L), digermane (Ge-)H_¾), tungstenhexa.fi uoride (WF_o), etc.

(003) Qual ity control of the deposited layers is very important, both for consistency of products produced as wel l as to remain alert to impurities that may occur in the deposited layers. Therefore, it is common to analyze the deposited layers to determine their composition. Impurities at concentrations as low as parts per tri l lion, such as dopant elements like boron, phosphorus and arsenic, can severely alter the desired performance of si licon or germanium semiconductor devices, such as those used in computers or photovoltaic energy production, because group 111 and V elements can be electron acceptors or donors to group IV semiconductor materials. In addition to doping impurities, other trace metal and non-metal contaminants may contribute to defects in electronic devices. These trace level impurities are often difficult to capture and to detect ί for purposes of Quality Assurance (QA) analysis from the gas phase. Such impurities might not be accurately sensed by the gas supplier QA methods and the impact on device performance may not be known until the device CVD layer is fabricated . By growing and testing CVD layers similar to those to be grown by the user prior to delivery of the gas, there is a better chance that the impurities important to the device performance can be sensed before the gas is provided to the user.

(004) Current gas phase analysis often provides incomplete results. For example, the gas to be tested is either condensed to a liquid state and evaporated leaving a residue to be analyzed or is hydroiyzed in deionized (Dl) water and then evaporated to leave a dry residue. The residue remaining after evaporation is mixed with nitric acid and then injected its an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) for analysis. However, this may give incomplete results because some impurities in the original gas may be lost in the evaporation of the trapped liquid or may not dissolve in the DI water. These lost impurities may be important to the user in CVD or Low Pressure CVD (LPCVD) techniques. In other words, some critical impurities may not be detected by conventional analysis technology. These methods may not trap the important impurities and therefore analysis may be incomplete and damaging impurities or compounds may he missed by the QA process.

(005) Another analysis technique is based on solid phase analysis that requires expensive equipment and complex processes. This analysis technique requ ires carefully controlled growth of epitaxial layers using sil icon or germanium precursors. This technique requires a high degree of technical skill and very expensive vacuum equipment to crow the crystalline layers needed. The evaluation is then carried out using cryogenic FTIR (Fourier Transform Infrared Spectroscopy) analysis or FTP!., (Fourier Transform Photolumineseence) of the grown crystall ine layer in a liquid helium cryostat. The equipment and clean room; requirements needed for such growth and analysis can cost in excess of a mi llion dol lars and requires special ski lls to conduct as wel l as interpret the results. Further, the reflected spectrum from the Fl^'iR or FTPL must be correlated to specific elemental impurities in the epitaxial layer, inferring that the impurities came from the original sample. This is therefore an indirect measurement based on past calibrations of the equipment using pure elements arid not a direct measurement and identification of the element atomic mass as in mass spectroscopy,

(006) For al l of the above reasons, there remains a need in the art for improvements to analysis techniques that will capture deleterious impurities in a manner similar to the ultimate CVD process but without the high cost and inherent inaccuracies and capture problems of existing techniques.

SUM MA R Y OF n il . i rsi;: f | ν ι : ιΠ0

(007) The invention provides improved techniques for the analysis of source materials used for creating a layer deposited by CVD, The methods of the invention do not require crystalline epitaxy, Rather, amorphous films created at CVD growth temperatures and pressures are adequate for the analysis techniques of the invention. By using solid phase CVD layers like those formed in the user's deposition process, dopant and trace impurities can be trapped and analyzed, therefore assuring that all impurities that may have an impact on final device performance will he captured and detected. According to the invention, a quartz tube is used for the deposition of the layer to be analyzed, resulting in significantly reduced costs and complexity of the analysis. The chemically clean quartz tube avoids the potential for contam ination during analysis without requiring clean rooms or advanced LPCVD equipment. tt^Jll - l ^i f- ilii . !N V 1 : ΝΓΠΟΝ

(008) Fig. 1 is a schematic diagram of stage J of the analysis technique according to the invention. (009) Fig, 2 is a schematic diagram of stage 2 of the analysis technique according to the invention.

(010) Fig. 3 is a schematic diagram of stage 3 of the analysis technique according to the invention.

DETAILED DESCRIPTION OF THE I VENTION

(011) The present invention provides improved techniques for the analysis of layers deposited by chemical vapor deposition as wi ll be more fully explained with reference to the d awing figures.

(012) Figure 1 is a schematic diagram of the deposition stage of the analysis technique according to the invention, wherein a pre-cleaned and pre -weighed quartz tube 10 is mounted between end caps 30, 35 within a furnace 20. The quartz tube 10 is cleaned with nitric acid to remove any surface metallic contamination and then dried, A layer of solid materia; 22 is deposited on the interior of the quartz tube 10 by CVD methods at pressures from 1 to 760 Ton^* and temperatures from 400° C to 1200° C. The deposition gas and, in some cases another reactant gas like hydrogen (H ?) gas, are del ivered to the interior of the quartz tube 10. While continuing delivery of the deposition gas (and any other reactant gas), the quartz tube 10, is heated to deposition temperature (400° C to 1200° C) and held at deposition pressure ( 1 to 760 Torr) to deposit the layer 22 on the interior wall of the center portion of the quartz, tube 1 0. The deposition conditions can be adjusted to best replicate the cond itions the gas end user will employ, The quartz tube will also protect the deposited layer from contamination from the surroundings.

Therefore, a "clean room" environment m ight not be needed, which results in significant cost reductions. (013) A second stage, the dissolution stage of the analysis technique according to the invention is shown in. Figure 2. in particular, after the layer has been deposited, the quartz tube 10 is cooied by introducing clean, filtered cooling gas, such as nitrogen ( ₂), argon (Ar), helium (He) or other inert gas. Once cooled, a drop of acid or selective removal solution 40 is introduced to the interior of the quartz tube 30. The drop 40 is added in the amount of 50 to 200 micro liters and may be a high purity acid or base solution, e.g. hydrofluoric acid mixed with hydrogen peroxide. This drop 40 acts to dissolve the deposited layer. The drop 40 location is controlled by moving the drop 40 back and forth within the quartz tube 10 using slight inert gas (such as N₂) pressure to fully dissolve the deposited layer 22. The drop 40 dissolves components of the deposited layer 22, such as si licon or other deposits like germanium, tungsten, boron, phosphorous, aluminum, etc., and the drop 40 is then delivered to analysis equipment 50 using inert gas (such as N₂) gas pressure as shown in Figure 3. Many possible layers can be grown this way and a suitable acid or solution blend can be chosen for that material for ful l dissolution of the layer with the impurities included.

(014) As an alternative, a highly selective extraction solution may be used to dissolve the deposited amorphous silicon or germanium layer and extract impurities originating from the source material without removing any quartz (Si0₂) material. The selective extraction solution may be a basic solution of tetra methyl ammonium hydroxide (TMAH) and potassium hydroxide (KOH). Isopropyi alcohol (IPA) may be added to the basic solution.

(015) Figure 3 shows the sample extraction stage of the technique according to the invention wherein the drop 40 is removed to a vial, or by direct injection, for analysis in an 1CP-MS. After drying the acid solution, the quartz tube is weighed to determine how much of the deposit has been removed by the acid and this weight is used in the calculation of the concentration related to the original source sample deposited. (016) The quartz tube can be a standard quartz, tube having an outside diameter of a quarter to a half inch, Heating to deposition temperature includes heating from 400°C to I200°C with the deposited layer reaching a typical thickness of 10 to 250 microns but may be up to 2000 microns on the interior wail of the center portion of the quartz tube. The analysis equipment can be any standard analysis equipment, including ICP-MS equipment configured to accept the specific acid used.

(017) A more specific example of the invention comprises delivering TCS as the deposition gas along with H? gas to the quartz tube. The tube is then heated to a temperature of 90Q°C to 1 100°C to deposit a layer of pojysiHcon to a thickness of 10 to 2000 microns on the interior of the center portion of the quartz tube. The exact weight of polysiiicon deposited can be determ ined by weighing the tube before and after the deposition. Along with the silicon deposit contaminants from the TCS gas supply, such as B, P, As, Sb, Fe, etc, will also be deposited. When the desired thickness of amorphous or polycrystal line silicon is obtained, the tube is cooled and the tube is then weighed. The tube is then reinstalled in the same rig used for the deposition or in a rig with only a N₂ supply and no heater. A drop of acid or solution, such as H O¾, HF/H₂O2, TMAH or KOH/IPA is then introduced to the quartz tube and moved back and forth over the deposited layer using N2 gas pressure applies at each end of the tube alternately so the drop contacts the entire deposited surface. The acid or solution dissolves the silicon and contaminants, up to 2,000 pprn silicon in acid. This dissolved solution is then delivered from the quartz tube to analysis equipment, such as ICP-MS equipment. The quartz tube is dried of residual acid solution then weighed to determine the amount of silicon that was dissolved in the acid. This amount of S i is then used as the reference matrix amount to determine the ppbw (parts per bill ion by weight) of impurities in the bulk silicon deposited. The quartz tube may be discarded or possi bly cleaned and used in further analysis. A quartz tube of the same type with the same preparation is run through the same steps and thai analysis wi thout introducing tested chemical precursor materia! is used as a background measurement to compensate for contri butions from the quartz itself. it. is important to obtain the background contamination level in quartz tubes since bulk impurities within the quartz wali may be enriched on the quartz tube surface through thermal diffusion paths at deposition temperatures.

(018) Since the condition used in the quartz tube can be identical to the conditions the users employ to grow layers, the same impurities that would be trapped by the CVD user's process will also be trapped by the analysis process of the invention. Known methods used to analyze epitaxy layers like Vapor Phase Decomposition (VPD) and other bulk si licon analysis methods have been used by end users to evaluate trapped impurities using acid or solution drops and 1CP-MS and can be conducted on the precursor gas before shipment according to the invention.

(019) While the above example relates to the use of TCS as the deposition gas, the invention is also useful for the analysis of other gases delivered from a compressed or liquefied source or of vapor from a volatile liquid source either entrained in a carrier gas or delivered by direct liquid injection. This includes disilane, DCS, S 1FI4, Si₂C]6, Gel : , WF₆ or any gas that can deposit solid materia! by CVD, Dopant gases such as phosphine (PHj), diborane ( I l h. and arsine (AsFK) and metal or organometailic precursors such as trimethyl gallium can also be analyzed using the method of the invention or can be added to si l icon precursors as a dopant or calibration measurement. Carrier and reducing gases such as hydrogen gas as well as cleaning gases like HC1, BCK, Ci₂, HF and F? can be analyzed with the help of previously characterized layers deposited by CVD that are then removed with the cleaning gas. impurities in the cleaning gas wil l be deposited in the tube during the cleaning process as the deposited layer is removed and those impurities can be analyzed by the same method. The deposition according to the invention using quartz tubes can be carried out at any location, includ ing remote locations, or at multiple locations with the deposited layers being sent to central ized analysis equipment, in this case, it may be necessary to remove sil icon native oxides wi h HF/H₂0 vapor prior to completing the analysis. W hen using HF7H₂0₂ 'Ή₂0 and H 0₃ /1 1 F H₂0 as the dissolving solution, degassing of Sif₄ can be al lowed to occur before injection into the analysis equipment to improve the Detection Limits (DL) of the specific impurities. The result of the impurities that were found in the silicon layer can then be converted to the concentration (ppbw) in the original gas like TCS (SiHCl. by multiplying the ratio of the molecular weight of Si and the molecular weight of SiHCb or (molecular weight of Si) / (molecular weight of SiHCh) ^:::: 28/135.5^™ 1 / 4.8. The concentration of an impurity "i" can be found using standard K^'P -MS methods as x. in parts per billion by weight (ppbw) in the dissolved silicon. This can be related to the original TCS by the equation Xj/4.8.

(020) The invention provides many advantages. In particular, the invention does not. require clean room conditions, but rather can use standard lab protocols to clean the tube with analytical grade acid. The process of the invention does not require special skills to perform. F urther, the equipment needed for the quartz tube deposition of the invention can be obtained or built for a small fraction of the mi llion dollars or more capital required for current solid phase methods. Al l of these advantages represent a significant cost savings over the currently used analysis techniques. Moreover, the deposited tubes of the invention can be sent to an outside Sab for final analysis or can use analysis equipment already in place at the gas production plant that is used for other products as well. Again, this provides a significant cost savings over current techn iques.

(021 ) It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the l ight of the foregoing

description, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the appended claims.

Claims

What is claimed:

1 . A method of analyzing source material for use in a deposition process, the method comprising:

deli vering source material to the interior of a quartz tube mounted in a furnace; heating the quartz tube to deposition temperature;

depositing a layer from, the source material on the interior wall of the quartz tube; cooling the quartz tube;

introducing a dissolving solution to the interior of the quartz tube;

ful ly dissolving the deposited layer in the dissolving solution;

delivering the dissolving solution with the dissolved deposited layer to analysis equipment.

2. The method according to claim 1 , wherein the source material is gas from a compressed or liquefied source, vapor from a volatile liquid source entrained in a carrier gas or vapor from a liquid delivered by direct liquid injection,

3. The method according to claim 1. wherein the dissolving solution is chosen to accomplish complete dissolution of the deposited layer.

4. The method according to claim 1 , wherein the dissolving solution is an acid solution or a base solution depending on the type of deposit being analyzed.

5. The method according to claim 1 , wherein the dissolving solution is HN0₃ or HF/H2O2 in aqueous solution.

6. The method according to claim 1 , wherein the dissolving solution is a selective etching .solution that removes the deposited layer without etching the quartz tube,

7. The method according to claim 6, vvheretn the selective etching solution is TMAH or KOH/1PA aqueous solution.

8. The method according to claim 1 , wherein the source material is diehlorosilane, silane, hexachiorodisllane, germane, digermane, tungsten hexafluoride, phospine, diborane, arsine, trichlorosilane, disilane, trisi!ane, metal precursors, organometailic precursors, trimethyl gallium, or any material that can deposit a CVD solid phase layer.

9. The method according to claim 1 , further comprising delivering a reactant, carrier or cleaning gas with the source material.

10. The method according to claim 9, wherein the reactant, carrier or cleaning gas is H₂, HQ, BQi, Cl₂, HF, or F₂. j I , The method according to claim 9, wherein the reactant gas includes hydrogen gas.

12. The method according to claim 1 , wherein the quartz tube has an outside diameter of ¼ to ½ inch.

1 3, The method according to claim 1 , wherein heating comprises heating the quartz tube to a temperature of 400°C to 1200°C.

34. The method according to claim 1 , wherein the quartz tube is held to a pressure of 3 to 760 Torr.

15. The method according io claim 1. wherein the deposited layer has a thickness of 10 to 2000 microns,

16. The method according to claim 1 , wherein cooling comprises introducing a cooling gas ΐο the interior of die quartz tube.

17. The method according to claim 16, wherein the cooling gas is N₂, Ar, He or other inert gas.

18. The method according to claim 1 , wherein fully dissolving the deposited layer comprises moving the dissolving solution o ver the entire surface of the deposited layer using inert gas pressure.

19. The method according to claim 1 , wherein delivering the dissolving solution comprises using inert gas pressure.

20. The method according to claim 1 , wherein the analysis equipment is an inductively coupled plasma mass spectronieiry unit.