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Background 

   The following contains a brief description of isolation technologies used for radiation hardened integrated circuits.  The technologies mentioned are junction isolation, Dielectric Isolation (DI), Silicon on Sapphire (SOS), and Silicon on Insulator (SOI).  The report is intended to be informational.  The results of Internet searches and interviews with Code 605 personnel involved with the Strategic Systems Program were the primary sources of information.  The Strategic Systems Program relies on these non conventional isolation techniques for radiation hardened integrated circuits. 

Executive Summary 

  For over 40 years junction isolation has been the primary technology used to isolate the individual components on integrated circuits.  Early alternate technologies such as DI, SOI, and SOS were driven by their ability to tolerate radiation.  There are relatively few applications for radiation hardened parts therefore the demand for DI, SOI, and SOS has been quite small.  DI, SOI, and SOS wafers accounted for a very small portion of the worldwide Silicon wafer production prior to 2000.  In the late 90’s the demand for high speed VLSI Integrated Circuits was pushing junction isolation to its technological limits.  The performance advantages of thin film Silicon on Insulator (SOI) integrated circuits brought about improvements in SOI.  This resulted in an increased demand for SOI wafers.  Semiconductor Equipment and Materials International (SEMI) estimates that SOI wafer sales doubled to $405 million in 2005.1  Yet SEMI estimates that was less than 5% of Silicon wafers sales worldwide.  SEMI predicts that sales will reach $911 million by 2009.  The worldwide demand for high performance integrated circuits has produced a reliable source for a commercially available radiation tolerant semiconductor manufacturing process.  

Introduction  

   Monolithic Integrated Circuits (ICs) have traditionally utilized junction isolation as a means of electrically isolating individual active and passive devices on the IC.  Individual devices are diffused into a substrate.  A reversed biased P-N junction between the substrate and individual devices provides the isolation between devices. The process is well established, reliable, and economical.   

   DI, SOI, and SOS make use of an engineered wafer.  The IC manufacturer buys a wafer that has been manufactured with an insulating layer beneath the active Silicon.  These wafers can be engineered to replace the standard bulk Silicon wafers in a conventional junction isolation IC manufacturing process.  An IC isolated by a dielectric such as Silicon Dioxide (SiO2) or Sapphire (Al2O3) has advantages over its junction isolation counterpart.  Three of the main advantages are immunity to radiation, lower leakage currents, and higher operating frequency all due to the elimination of junction isolation.   

    Photocurrents generated by ionizing radiation in junction isolation can impact an integrated circuit in many ways.  Photocurrents are reduced by dielectric isolation.  The main benefit of dielectric isolation is the elimination of LATCHUP (LATCHUP is the inadvertent creation of a parasitic Thyristor structure in an Integrated Circuit).   A switch to DI, SOI, or SOS can dramatically improve an IC’s immunity to Prompt Dose radiation or Single Event Effect radiation.  There are many factors to consider when designing a radiation tolerant integrated circuit.  Switching a process to DI, SOI, or SOS is not a guarantee that the IC will be radiation tolerant.  The dielectric that improves Prompt Dose and Single Event response may trap charge and degrade the Total Dose response unless additional processing steps are taken.  Several processing and design techniques must be utilized to guarantee a radiation tolerant IC.  Some of these can also be used to improve the radiation tolerance of junction isolation technologies.   

   Dielectric isolation doesn’t have the leakage current to substrate that junction isolation does. The leakage current of a reverse biased P-N junction increases with temperature.  This can limit the maximum operating temperature of IC’s utilizing junction isolation. 

      Reversed biased P-N junctions possess an inherent capacitance.  This parasitic capacitance limits the operating frequency of an IC.  The parasitic capacitance of a dielectrically isolated part is lower than that of one utilizing junction isolation.  This gives dielectric isolation a performance advantage versus its junction isolation counterpart.  It enables a dielectrically isolated part to operate at higher frequencies and lower power.  This performance advantage is currently the main driver for commercial SOI technology.   

Technologies

   DI
     Technically DI, SOS, and SOI are all dielectric isolation.  In this discussion DI refers to a specific isolation technique that is different from SOI and SOS.  DI processes, often referred to as poly DI, have been used for over 30 years in radiation environments.

   DI processing typically begins with an N+ type wafer.  An N type Epitaxial layer is grown on the wafer.  Masked N and P diffusions form islands for the individual devices.  The individual devices are isolated by etching moats around the islands.  The moats extend down to the N+ starting wafer.  A thick oxide is grown that forms the isolation on the sides and bottoms of the devices.  A thick layer of Poly-Silicon is deposited to fill in voids and add strength.  The Poly-Silicon will be the substrate for the finished device. 

   The N+ starting wafer is removed by abrasive polishing.  Modern DI processes use a final electrochemical etch that automatically stops at the lower doped Epitaxial layer; this allows the thickness of the final islands to be determined by the Epitaxial process.  The remaining dielectrically isolated islands can be processed into integrated circuits.

   Figure (1) is a simple diagram showing a cross section of a DI wafer.  DI is a mature process.  It is well suited for high voltage applications (MOS Vgs to150V, BJT BV_ceo to 35V).  It is a relatively low frequency technology due to lack of additional development.  The process does not produce a uniform thin film of Silicon needed for high performance integrated circuits.  Wafer warping has limited the size of the wafers to 4 inches.  It is not certain how long IC manufacturers will continue to produce DI.
 

 Figure (1) Diagram of  Dielectric Isolation Cross Section

SOS
   SOS refers to Silicon on Sapphire.  The active Silicon device is formed on an electrically insulating crystalline Al2O3 (Sapphire) substrate. An Epitaxial layer of Silicon is grown on a crystalline Sapphire wafer.  Subsequent Ion implantation and heat treatment produce a thin silicon film suitable for high performance IC fabrication.   

   During IC fabrication islands of Silicon are formed by patterned oxidation of the Silicon film.  The islands of Silicon are isolated laterally by Silicon Dioxide that extends to the Sapphire substrate.  Sapphire is one of the few insulating substrates that single crystal Silicon can be grown on.  Figure (2) is a diagram showing a cross section  of an SOS wafer.    

    SOS technology has been developing since the 1960s.  In the beginning it was plagued by the lattice unconformity between the crystalline Silicon and crystalline Sapphire.  The unconformity created lattice defects in the Silicon.  Diffusion of Aluminum from the Sapphire into the Silicon was also a problem.  Improvements in technology have made SOS more reliable and accelerated the development since the 1990’s.  SOS can have excellent Prompt Dose and Single Event Effect radiation immunity.  High quality passive components such as inductors can be constructed on it.   An additional advantage is that the translucent sapphire wafer allows for implementation of high performance optical interconnect communication.  This can accomplished using Vertical Cavity Surface Emitting Lasers (VCSEL) and coupling through the substrate.  Flatness of Sapphire wafers is somewhat inferior to Silicon wafers.2  This has discouraged large area high integration circuits from being fabricated on SOS.
 

  Figure (2) Diagram of  Silicon on Sapphire Cross Section

SOI
  Technically DI could be considered Silicon on Insulator (SOI).  In this discussion SOI refers to integrated circuits that consist of a thin film of single crystal Silicon separated from the bulk substrate by a layer of insulating Silicon dioxide.  Several vendors, including IBM, National Semiconductor, Honeywell, Advanced Micro Devices and Intel, are currently supporting SOI processes. The performance of an SOI device relies on the ability to fabricate a thin film of defect free single crystal Silicon.   Currently there are two approaches that have been widely used for manufacturing SOI starting wafers.  One method is known as Separation by Implanted Oxygen (SIMOX) and the other is wafer bonding.   Oxygen ions are implanted into Silicon to form the Silicon dioxide layer in the SIMOX process.  The energy and dose rate of the implant control the depth and thickness of the oxide layer.  The oxide layer forms after a high temperature annealing.  The annealing does not remove all of the lattice damage from the implanted Oxygen.  Silicon pipes from the active silicon layer through the oxide to the substrate are also a problem.  Current processes have reduced defect levels to acceptable levels.  Lateral devices fabricated on SIMOX wafers are less affected by Silicon defects than vertical devices.  SIMOX is well suited for high performance low voltage CMOS integrated circuits.

   Figure (3) is a diagram showing a cross section of a SIMOX wafer.  Patterned local oxidation of the Silicon film during IC fabrication defines the lateral isolation of the individual IC components. 
 

 Figure (3) Diagram of SIMOX Cross Section

   Wafer bonding avoids the damage caused by implanted Oxygen. The oxidized surfaces of two Silicon wafers are bonded.  Part of one of the wafers is selectively removed to leave a thin uniform layer of crystalline Silicon on top of an insulating layer of Silicon dioxide.  Patterned local oxidation of the single crystal Silicon film forms the lateral isolation.  Wafer bonding is well suited for high performance CMOS, BiCMOS and higher power devices.  Figure (4) is a diagram showing a cross section of wafer bonded  SOI.

   A patented process called Smart Cutâ is a very promising wafer bonding technique.3 Smart Cutâ starts with an oxidized seed wafer.  Hydrogen is implanted into the Silicon wafer through the oxide.  The depth of the implant is controlled by the implant energy.  The seed wafer is aligned and bonded to a handle wafer.  The bonded wafers are heated in an oven.  The heating causes the wafers to split along the Hydrogen implanted plane.  The result is a thin film of crystalline Silicon from the seed wafer resting on an insulating Silicon dioxide layer.  The handle wafer provides strength and support for the finished product.  The remaining seed wafer can be used again.