Data Remanence in Flash Memory Devices

Data Remanence in Flash Memory Devices
Data Remanence in Flash Memory Devices
Sergei Skorobogatov
University of Cambridge, Computer Laboratory,
15 JJ Thomson Avenue, Cambridge CB3 0FD, United Kingdom
[email protected]
Abstract. Data remanence is the residual physical representation of
data that has been erased or overwritten. In non-volatile programmable
devices, such as UV EPROM, EEPROM or Flash, bits are stored as
charge in the floating gate of a transistor. After each erase operation,
some of this charge remains. Security protection in microcontrollers and
smartcards with EEPROM/Flash memories is based on the assumption
that information from the memory disappears completely after erasing.
While microcontroller manufacturers successfully hardened already their
designs against a range of attacks, they still have a common problem
with data remanence in floating-gate transistors. Even after an erase
operation, the transistor does not return fully to its initial state, thereby
allowing the attacker to distinguish between previously programmed and
not programmed transistors, and thus restore information from erased
memory. The research in this direction is summarised here and it is shown
how much information can be extracted from some microcontrollers after
their memory has been ‘erased’.
Data remanence as a problem was first discovered in magnetic media [1,2]. Even
if the information is overwritten several times on disks and tapes, it can still
be possible to extract the initial data. This led to the development of special
methods for reliably removing confidential information from magnetic media.
Semiconductor memory in security modules was found to have similar problems with reliable data deletion [3,4].
Data remanence affects not only SRAM, but also memory types like DRAM,
UV EPROM, EEPROM and Flash [5]. As a result, there is possibility that some
information still can be extracted from memory that has been erased. This could
create problems with secure devices where designers assumed that all sensitive
information is gone once the memory is erased.
In some smartcards and microcontrollers, a password-protected boot-loader
restricts firmware updates and data access to authorised users only. Usually, the
on-chip operating system erases both code and data memory before uploading
new code, thus preventing any new application from accessing previously stored
secrets. If the passwords or secret keys can be extracted afterwards, it could create serious problems for confidentiality of the previously encrypted information.
Chip manufacturers do not publish data about remanence effects for their
memory chips. The only parameter they specify is data retention – the time
during which the memory content is preserved. This is almost the opposite of
data remanence. Data retention time can be used roughly to estimate the data
remanence effect, but this works only for devices within the same family [4].
Therefore, a series of experiments was performed to check whether it is feasible to extract information from erased EPROM, EEPROM and Flash memory
devices using low-cost methods. The results should be of considerable concern
to designers of embedded security applications.
Unlike SRAM, which has only two stable logic states, EPROM, EEPROM and
Flash cells store analog values in the form of a charge on the floating gate of a
MOS transistor. The floating-gate charge shifts the threshold voltage of the cell
transistor and this is detected with a sense amplifier when the cell is read. The
maximum charge the floating gate can accumulate varies from one technology
to another and normally is between 103 and 105 electrons. For standard 5 V
EEPROM cells, programming causes about a 3.5 V shift in the threshold level.
Some modern Flash memory devices employ multiple level detection, thus increasing the capacity of the memory [6]. There are also memory devices with a
fully analog design, which store charges proportional to the input voltage [7].
Read time
≈ 10 ns
≈ 5 ns
≈ 50 ns
≈ 50 ns
≈ 50 ns
≈ 20 ns
≈ 50 ns
Write time
≈ 10 ns
≈ 10 ms
≈ 10 ms
≈ 1 ms
≈ 10 µs
≈ 50 ns
> 5 years
> 10
> 10
> 40
> 100
> 40
Cell size
103 –106
104 –106
Table 1. Characteristics of different memory types used in microcontrollers
There are two basic processes that allow placing electrons on the floating
gate – Fowler-Nordheim tunnelling and channel hot electron (CHE) injection
[8]. Both processes are destructive to the very thin dielectric insulation layer
between the floating gate and the channel of a transistor. This oxide layer is responsible for preserving the charge on the floating gate. As a result, the number
of possible write/erase cycles is limited, because the floating gate slowly accumulates electrons, causing a gradual increase in the storage transistor’s threshold
voltage and programming time. After a certain amount of program/erase cycles
(typical values are represented in Table 1) it is no longer possible to erase or
program the cell. Another negative effect (which is the main failure mode for
Flash memory) is negative charge trapping in the gate oxide. It inhibits CHE
injection and tunnelling, changes the write and erase times of the cell, and shifts
its threshold voltage.
The amount of trapped charge can be detected by measuring the gate-induced
drain leakage current of the cell, or its effect can be observed indirectly by
measuring the threshold voltage of the cell. In older devices, which had the
reference voltage for the sense amplifier tied to the device supply voltage, it
was often possible to do this by varying the device supply voltage. In newer
devices, it is necessary to change the parameters of the reference cell used in
the read process, either by re-wiring portions of the cell circuitry or by using
undocumented test modes built into the device by manufacturers.
Another relevant phenomenon is overerasing. If the erase cycle is applied to
an already-erased cell, it leaves the floating gate positively charged, thus turning
the memory transistor into a depletion-mode transistor. To avoid this problem,
some devices, for example Intel’s original ETOX [9], first program all cells to 0
before erasing them to 1. In later devices, this problem was solved by redesigning
the cell to avoid excessive overerasing. However, even with this protection, there
is still a noticeable threshold shift when a virgin cell is programmed and erased.
The changes in the cell threshold voltage caused by write/erase cycles are
particularly apparent in virgin and freshly-programmed cells. It is possible to
differentiate between programmed-and-erased and never-programmed cells, especially if the cells have only been programmed and erased once, since virgin
cell characteristics will differ from the erased cell characteristics. The changes
become less noticeable after ten program/erase cycles.
Programmed floating-gate memories cannot store information forever. Various processes (such as field-assisted electron emission and ionic contamination)
cause the floating gate to lose the charge, and these go faster at higher temperatures. Another failure mode in the very thin tunnel oxides used in Flash
memories is programming disturb, where unselected erased cells adjacent to selected cells gain charge when the selected cell is written. This is not enough to
change the cell threshold sufficiently to upset a normal read operation, but could
cause problems to the data retention time and should be considered during measurement of the threshold voltage of the cells for data analysis and information
recovery. Typical guaranteed data retention time for EPROM, EEPROM and
Flash memories are 10, 40 and 100 years, respectively.
Experimental Method
Obviously, in a floating gate memory cell, the floating gate itself cannot be accessed. Its voltage is controlled through capacitive coupling with the external
nodes of the device. Often, the floating-gate transistor is modelled by a capacitor equivalent circuit called the capacitor model [10]. In practice, write/erase
characteristics for many EEPROM/Flash memories are close to that of a charge/
discharge of a capacitor. Meanwhile there are some differences in how the charge/
discharge process takes place in real memory cells. There is an initial delay between the time the voltages are applied to the cell, and the charge starting to
be removed or injected. This delay is caused by the need for very high electric
fields to be created inside the floating-gate transistor to start the injection or
tunnelling process. Some EEPROM cells have been reported to have nonuniformity during the erase operation [11]. As a result, it might take longer to erase
a half-charged cell than a fully-charged cell. In addition, an ideal capacitor discharges exponentially: q = q0 · e−t/τ . Applied to the floating gate, that would
mean that after t = 10τ the charge is totally removed from the cell. In practice
this does not happen, because the parameters of the cell’s transistor change as
the charge is removed from its floating gate. All the above-mentioned problems
could seriously affect data remanence in floating-gate memories.
The main difficulty with analysis of the floating-gate memory devices, especially EEPROM and Flash, is the variety of different designs and implementations from many semiconductor manufacturers. There are hundreds of different
types of floating-gate transistor, each with its own characteristics and peculiarities. It means that for security applications where data remanence could cause
problems, careful testing should be applied to the specific non-volatile memory
device used in the system.
Fig. 1. The test board for data remanence evaluation
Fig. 2. Test setup for semi-invasive analysis
Some microcontrollers with different memory types to investigate the possible
influence of data remanence on EPROM, EEPROM and Flash memories were
tested. For that purpose I built a special test board controlled by a PC via a
parallel interface (Figure 1). The board has two programmable power supplies
for generating VDD and VPP voltages, a programming interface with bidirectional
voltage level converters, and sockets for microcontroller chips. That allowed me
to control the voltages applied to the chip under test with 100 µV precision and
apply any signals within a 1 µs time frame.
Recently introduced semi-invasive attack methods [15] might also be helpful
for testing data remanence effect in floating-gate memory devices. These methods
are more effective in some applications as they do not require physical access to
the internal wires inside the chip thus reducing the preparation time. However,
partial or full decapsulation of the sample is required [16]. For such analysis,
a low cost laser diode pointer with external power control was mounted on
the autofocus module optical port of a Mitutoyo FS60Y microscope. Computer
controlled Newport PM500-XYZ motorised stage was used for moving the sample
under test (Figure 2). Using 100× objective on the microscope it was possible
to focus the red laser beam (650 nm) down to 0.5 µm (Figure 3). Although the
Fig. 3. Focusing the laser with a 100× objective
laser used was classified as a class 2M laser device, an ordinary digital camera
mounted on the microscope was used for navigation to avoid necessity of looking
at the laser beam with unprotected eyes.
Non-Invasive Results
The first experiment was performed on the Microchip PIC12C509 microcontroller [12] with UV EPROM. The chip was programmed with all 0’s (charged
cell state) and exposed to UV light for different periods of time. Then it was
read in the test board at different power supply voltages to estimate the threshold level for each EPROM cell in the memory array. The reference voltage was
assumed to be tied to the power supply line and therefore the threshold level of
the transistor is proportional to the power supply voltage VTH = K · VDD . The
fact that the exact threshold voltage of the transistor is not measured does not
affect the results because an attacker is normally interested in the relative erase
timing between the memory and the security protection. Once the security fuse
is erased, the memory can be easily read. The same test was applied to a chip
with a programmed security fuse. The results are presented in Figure 4. As can
be seen from the graph, the memory gets fully erased before the security fuse is
erased. However some security flaws still could exist. Although nothing could be
extracted directly by reading the memory when the fuse is erased, power glitch
tricks could work. For example, after seven minutes of exposure to the UV light
(253 nm peak, 12 mW/cm2 ) the memory content can be read non-corrupted at
VDD below 2.2 V, but the security fuse remains active up to 4.8 V. If the attacker works out the exact time when the data from memory is latched into the
output shift register and the time when the state of the security fuse is checked,
he might be able to extract the memory contents by reducing the power supply
down to 2 V for the data latching and increasing it to 5 V to make the security
fuse inactive.
There is another trick that makes recovery of memory contents possible, even
when there is no overlap between the erased security fuse and non-corrupted
memory content at the time of erasure. For example, I found that newer samples
of the same chip will start to corrupt the memory before the security fuse is
erased (Figure 4). In this case a power glitch cannot be used to recover information from the memory. What can be done instead is a careful adjustment of the
threshold voltage in the cell’s transistor. It is possible to inject a certain portion
of charge into the floating gate by carefully controlling the memory programming time. Normally, the programming of an EPROM memory is controlled
by external signals and all the timings should be supplied by a programmer
unit. This gives an opportunity for the attacker to inject charge into the floating gate thus shifting the threshold level enough to read the memory contents
when the security fuse is inactive. Such a trick is virtually impossible to apply
to modern EEPROM and Flash memory devices for several reasons. Firstly, the
programming is fully controlled by the on-chip hardware circuit. Secondly, the
programming of EEPROM and Flash cells is normally performed by using much
faster Fowler-Nordheim tunnelling rather than CHE injection. As a result it is
very hard to control the exact amount of charge being placed into the cell. Also,
the temperature and the supply voltage affect this process making it even harder
to control.
UV Erasure of PIC12C509
EPROM erased
old fuse
new fuse
Fig. 4. Memory contents of PIC12C509 tested at different power-supply voltages
after UV erasure
Electrical Erasure of PIC16F84A
FLASH erased
Fig. 5. Memory contents of PIC16F84A tested at different power-supply voltages
after electrical erasure
The next experiment was done to the Microchip PIC16F84A microcontroller
[13] which has Flash program memory and EEPROM data memory. A similar
test sequence was applied with the only difference that electrical erasing was
used (Figure 5). A huge difference in the memory behaviour can be observed.
The memory erase starts 65 µs after the ‘chip erase’ command was received and
by 75 µs the memory is erased. However, this time changes if the temperature
or the supply voltage is changed. For example, if the chip is heated to 35 ◦ C
the memory erase starts at 60 µs and is finished by 70 µs. The security fuse
requires at least 125 µs to be erased giving at least five times excess for reliable
memory erase. Reducing the power supply voltage increases the erase time for
both the memory and the fuse erase, so that the ratio remains practically the
same. It should be mentioned that unless terminated by the hardware reset, the
chip erase operation lasts for at least 1 ms. Both this fact and the fast erase time
give an impression that EEPROM and Flash memories have fewer problems with
data remanence and therefore should offer better security protection. I decided
to investigate whether this is true or not.
In my early experiments with the security protection in PIC microcontrollers,
I noticed that the same PIC16F84 chip behaves differently if it is tested right
after the erase operation was completed. As this microcontroller is no longer in
use and has been replaced by the PIC16F84A, the testing was applied to the
new chip.
As can be seen from Figure 5, the memory is completely erased and read as
all 1’s well before the end of the standard 10 ms erase cycle. The threshold of
the cell’s transistors becomes very low after the erase and cannot be measured
the same way as with UV EPROM because the chip stops functioning if the
power supply drops below 1.5 V. With the power glitch technique, it is possible
to reduce the supply voltage down to 1 V for a short period of time – enough
for the information from memory to be read and latched into the internal buffer.
But this is still not enough to shift the reference voltage of the sense amplifier
low enough to detect the threshold of the erased cells. To achieve the result
another trick was used in addition to the power glitch. The threshold voltage of
all the floating gate transistors inside the memory array was shifted temporarily
by V∆ = 0.6–0.9 V, so that VTH = K · VDD − V∆ . As a result it became possible
to measure the threshold voltage of an erased cell which is close to 0 V. This
was achieved by precisely controlling the memory erase operation, thus allowing
the substrate and control gates to be precharged and terminating the process
before the tunnelling is started. As a result, the excess charge is trapped in
the substrate below the floating gate, and shifts the threshold of the transistor.
The process of recombination of the trapped excess charge could take up to one
second, which is enough to read the whole memory from the device. This can be
repeated for different supply voltages combined with power glitches, in order to
estimate the threshold of all the transistors in the memory array.
Threshold Voltage Distribution
Threshold Voltage Change During Erase Cycles
first erasure
second erasure
fully erased
V TH / V
V TH / V
Number of Erase Cycles
Memory Address
Fig. 6. Change of the threshold voltage during erasure for programmed and
previously erased cells (left) and for previously programmed cells after the second
erase cycle (right) in PIC16F84A
Applying the above test to differently programmed and erased chips, the
diagrams for threshold voltage dependence in the Flash program memory from
different factors such as the number of erased cycles (Figure 6, left) and memory
address (Figure 6, right) were built. As can be seen, the charge is not entirely
removed from the floating gate even after one hundred erase cycles thus making it
possible for the information to be extracted from the memory. This was measured
on a sample after 100 program/erase cycles to eliminate the effect of the threshold
shift taking place in a virgin cell. At the same time the memory analysis and
extraction is complicated by the fact that the difference in threshold voltages
between the memory cells is larger than within the same cell after single erase
cycle. The practical way to avoid this problem is to use the same cell as a
reference and compare the measured threshold level with itself after the extra
erase operation is applied to the chip. Very similar results were received for the
EEPROM data memory inside the same PIC16F84A chip. The only difference
was that the threshold voltage after ten erase cycles was very close to that of the
fully erased cell, thus making it almost impossible to recover the information if
the erase operation was applied more than ten times.
In the next test, the chip was programmed with all 0’s before applying the
erase operation. As a result it was practically impossible to distinguish between previously programmed and non-programmed cells. That means that preprogramming the cells before the erase operation could be a reasonably good
solution to increase the security of the on-chip memory.
One more thing should be mentioned in connection with hardware security. Some microcontrollers have an incorrectly designed security protection fuse,
which gets erased earlier than the memory. As a result, if the ‘chip erase’ operation is terminated prematurely, information could be read from the on-chip
memory in a normal way. That was the case, for example, for the Atmel AT89C51
microcontroller. When this became known in the late nineties, Atmel redesigned
the chip layout and improved security to prevent this attack, so that chips manufactured since 1999 do not have this problem. Nowadays, most microcontroller
manufacturers design their products so that the security fuses cannot be erased
before the main memory is entirely cleared, thus preventing this low cost attack
on their devices.
Semi-Invasive Results
The first experiment was performed on the PIC16F84A microcontroller to check
whether it would be possible to extract any information from previously erased
memory using semi-invasive methods with the setup mentioned in Section 3.
The location of the memory was initially found under a normal optical microscope. Then, using a proprietary laser scanning setup [16], areas sensitive to
the ionisation with laser radiation (bright areas) were found (Figure 7).
A standard Flash memory array consists of the current source, memory cells,
row and column selectors and a sense amplifier consisting of an amplifier and a
comparator to the reference cell signal which will distinguish between 0 and 1
[8]. Obviously, if we are interested in restoring the state of previously erased or
discharged cell we have to either reduce the current flowing through the cell, or
increase the reference voltage of the read sense amplifier, or reduce the coefficient
of the amplification itself.
Because the laser can only generate the current in p-n junctions, it is not
possible to manipulate the transistor in all of the desired ways. However, for
most memories built with NMOS technology this will work quite well as the
laser will inject current with the opposite polarity to the current sent through
the memory cells.
Sensitivity image [mV]
Fig. 7. Optical and laser-scanned images of the PIC16F84A EEPROM area
In my experiments I erased the data EEPROM memory for the time necessary
for the memory to be read back fully erased at minimum and maximum power
supply voltages. Then the sample was placed under a microscope and several
areas were tested with a laser pointer beam with powers ranging from 10 µW
to 5 mW. Better results were received when either the area close to the column
selector or the area close to the input of the sense amplifier was exposed to the
laser beam. For each memory bit the value of the laser power corresponding to the
change of its value from 1 to 0 was stored in the file. Due to the reason mentioned
in the previous chapter it was not possible to extract the memory contents
directly by adjusting the reference voltage of the sense amplifier. Therefore,
after the first measurement an extra memory erase operation was performed
and the next measurement was done. Comparing the results for each memory
cell revealed its content because a previously programmed cell had changed its
threshold value while a non-programmed cell had not.
Going back to Figure 5 it can be noticed that when more than 75 µs has
elapsed since the erase command the contents of the memory cannot be read
directly. Using the above technique I was able to reliably extract the information
from the memory after a 150 µs erase pulse. This is still well below the standard
10 ms erase operation but is sufficient to erase the security fuse so that the
attacker can perform a ‘chip erase’ operation and then extract the information
from the memory.
The most important advantage of the semi-invasive technique is that it is
independent of the power supply voltage and uses only laser power alteration to
measure the threshold voltage of the memory transistors. This overcomes certain
protections used in modern secure chips where either voltage monitors or voltage
stabilisers are used.
The next step in my research was to test whether such a semi-invasive technique would work for modern submicron chips. As a target for my next experiments I chose the Atmel ATmega8 microcontroller [17] which employs 0.35 µm
Fig. 8. Optical image of EEPROM area in the ATmega8 microcontroller before
and after wet chemical etching
technology (Figure 8). It has three metal layers and as a result there is very little
information that can be gained from direct optical observation of the chip under
a microscope. To solve this problem and find the memory components on the die
it was deprocessed using a wet chemical etching technique. The same die with
the top metal layer removed is shown in Figure 8. As a result of this operation
all of the memory arrays located on the chip die were recognised.
Sensitivity image [mV]
Fig. 9. Laser-scanned image of the ATmega8 EEPROM area
To find the active areas for the laser injections, the previously mentioned
laser scanning technique was used. However, as the chip was built with smaller
Fig. 10. Focusing the laser on the ATmega8 die using a 100× objective
technology and a large part of its surface is covered with metal wires, only a
small part of the die was sensitive to the laser beam (Figure 9) and the injected
current was significantly smaller than in case of PIC16F84A chip which has
0.9 µm technology. In addition, the chemical-mechanical polishing used in the
production of ATmega8 die reduces the transparency of the layers and only a
small fraction of light reaches the active area on the chip (Figure 10). All these
facts made the analysis and further testing of this chip more difficult.
The ATmega8 microcontroller employs a very reliable security protection
feature which ensures that the memory is erased well before the security fuse
that prevents external access to the memory. In my experiments, I was able
to extract information from the erased memory only if the erase pulse was less
than 100 µs long, whereas the standard ‘chip erase’ operation takes 10 ms. It was
still impossible to read the memory contents even after a 70 µs long erase pulse
at both minimum and maximum power supply voltages, but this is still not
enough to overcome the security protection. However, semi-invasive methods
again showed their advantages, especially because I was not able to find any
non-invasive approach for extracting the information from an erased ATmega8
To avoid data remanence attacks in secure applications, the developer should
follow some general design rules that help to make data recovery from semiconductor memories harder [5]:
– Cycle EEPROM/Flash cells 10–100 times with random data before writing
anything sensitive to them, to eliminate any noticeable effects arising from
the use of fresh cells.
– Program all EEPROM/Flash cells before erasing them to eliminate detectable effects of residual charge.
– Remember that some non-volatile memories are too intelligent, and may
leave copies of sensitive data in mapped-out memory blocks after the active
copy has been erased. That also applies to file systems, which normally
remove the pointer to the file rather than erasing the file itself.
– Use the latest highest-density storage devices, as the newest technologies
generally make data recovery more difficult.
– Using memories covered with top metal layer or built with modern deep submicron technologies helps against semi-invasive attacks because such attacks
require the laser beam to reach the transistor active areas.
Using encryption, where applicable, also helps to make data recovery from
erased memory more difficult. Ideally, for secure applications, each semiconductor memory device should be evaluated for data remanence.
Floating-gate memory devices, such as UV EPROM, EEPROM and Flash, have
data remanence problems. From some samples, information can still be recovered
after 100 erase cycles. Even if the residual charge cannot be detected with existing methods, this might be possible in the future with new technologies. Hardware designers should pay attention to the evaluation of components planned to
be used in systems sensitive to data remanence.
Fortunately, the presented techniques for extracting erased memory can be
applied only to a limited number of chips with EEPROM or Flash memory.
Firstly, some microcontrollers, such as the Texas Instruments MSP430 family
[14], have an internally stabilised supply voltage for the on-chip memory. Changing the power supply from 1.8 V to 3.6 V does not affect a memory read operation
from partially erased cells. Secondly, most microcontrollers fully reset and discharge the memory control circuit if the chip is reset or the programming mode
is re-entered. But still, if the memory contents do not disappear completely, this
can represent a serious threat to any security based on an assumption that the
information is irrecoverable after one memory erase cycle. Where non-invasive
methods fail, invasive methods could still succeed. For example, the memory
control circuit can be modified using a focused ion-beam workstation to directly
access the reference voltage, the current source or the control gate voltage. Finally, some chips program all the memory locations before applying the erase
operation. This makes it almost impossible to extract any useful information
from the erased memory.
Semi-invasive methods have once again shown their use in hardware security
analysis. However, they have some limitations, especially for modern deep submicron technologies, where multiple metal layers and small transistor size prevent
easy and precise analysis. Further improvements to these methods might involve approaching the die from its reverse side but this requires the use of more
expensive equipment.
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